TRANSFERENCE OF  See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

HEAT 25 . Modes of Transference.—There are three See also:

• PRINCIPAL

principal modes of transference of heat, namely (r) convection, (2) See also:

• CONDUCTION, ELECTRIC

conduction,. and (3) See also:

• RADIATION, THEORY OF

radiation . (r) In convection, heat is carried or conveyed by the See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion of heated masses of See also:

• MATTER

matter . The most See also:

• FAMILIAR (through the Fr. familier, from Lat. familiaris, of or belonging to the familia, family)

familiar illustrations of this method of transference are the See also:

• HEATING

heating of buildings by the circulation of See also:

• STEAM
• STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam; the origin is unknown)

steam or hot See also:

• WATER

water, or the equalization of temperature of a See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass of unequally heated liquid or See also:

• GAS

gas by convection currents, produced by natural changes of See also:

• DENSITY (Lat. densus, thick)

density or by artificial stirring . (2) In conduction, heat is transferred by contact between contiguous particles of matter and is passed on from one particle to the next without visible relative motion of the parts of the See also:

• BODY

body . A familiar See also:

• ILLUSTRATION

illustration of conduction is the passage of heat through the See also:

• METAL
• METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. µATaXAov, in the same sense, probably connected with ,ueraAAdv, to search after, explore, µeTa, after, aAAos, other)

metal plates of a See also:

• BOILER

boiler from the See also:

• FIRE (in O. Eng. f j'r; the word is common to West German languages, cf. Dutch vuur, Ger. Feuer; the pre-Teutonic form is seen in Sanskrit pu, pavaka, and Gr. irup; the ultimate origin is usually taken to be a root meaning to purify, cf. Lat. purus)

fire to the water inside, or the transference of heat from a soldering See also:

• BOLT

bolt to the See also:

• SOLDER (derived through the French from Lat. soldare, to make solidus, firm)

solder and the metal with which it is placed in contact . (3) In radiation, the heated body gives rise to a motion of vibration in the See also:

• AETHER, or ETHER (Gr. deli p, probably from caeca, I burn, though Plato in his Cratylus (410 B)

aether, which is propagated equally in all directions, and is reconverted into heat when it encounters any obstacle capable of absorbing it . Thus radiation differs from conduction and convection in taking See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place most perfectly in the See also:

• ABSENCE (Lat. absentia)

absence of matter, whereas conduction and convection require material communication between the bodies concerned . In the See also:

• MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of cases of transference of heat all three modes of transference are simultaneously operative in a greater or less degree, and the combined effect is generally of See also:

• GREAT

great complexity . The different modes of transference are subject to widely different See also:

• LAWS

laws, and the difficulty of disentangling their effects and subjecting them to calculation is often one of the most serious obstacles in the experimental investigation of heat . In space void of matter, we should have pure radiation, but it is difficult to obtain so perfect a vacuum that the effects of the residual gas in transferring heat by conduction or convection are inappreciable . In the interior of an opaque solid we should have pure conduction, but if the solid is sensibly transparent in thin layers there must also be an See also:

• INTERNAL

internal radiation, while in a liquid or a gas it is very difficult to eliminate the effects of convection .

These difficulties are well illustrated in the See also:

• HISTORICAL

historical development of the subject by the experimental investigations which have been made to determine the laws of heat-transference, such as the laws of cooling, of radiation and of conduction . 26 . See also:

• NEWTON
• NEWTON, ALFRED (1829–1907)
• NEWTON, JOHN (1725-1807)
• NEWTON, JOHN (1823-1895)
• NEWTON, SIR CHARLES THOMAS (1816-1894)
• NEWTON, SIR ISAAC (1642-1727)

Newton's See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

Law of Cooling.—There is one essential See also:

• CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition See also:

• COMMON

common to all three modes of heat-transference, namely, that they depend on difference of temperature, that the direction of the See also:

• TRANSFER (from Lat. transferre, to bear across, carry over)

transfer of heat is always from hot to See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

cold, and that the See also:

• RATE

rate of transference is, for small See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences, directly proportional to the difference of temperature . Without difference of temperature there is no transfer of heat . When two bodies have been brought to the same temperature by conduction, they are also in See also:

• EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium as regards radiation, and See also:

• VICE

vice versa . If this were not the See also:

• CASE
• CASE, JOHN (d. 1600)

case, there could be no equilibrium of heat defined by equality of temperature . A hot body placed in an enclosure of See also:

• LOWER

lower temperature, e.g. a calorimeter in its containing See also:

• VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel, generally loses heat by all three modes simultaneously in different degrees . The loss by each mode will depend in different ways on the See also:

• FORM (Lat. forma)

form, extent and nature of its See also:

• SURFACE

surface and on that of the 2nciosure, on the manner in which it is supported, on its relative position and distance from the enclosure, and on the nature of the intervening See also:

• MEDIUM

medium . But provided that the difference of temperature is small, the rate of loss of heat by all modes will be approximately proportional to the difference of temperature, the other conditions remaining See also:

• CONSTANT, BENJAMIN (1845-1902)

constant . The rate of cooling or the rate of fall of temperature will also be nearly proportional to the rate of loss of heat, if the specific heat of the cooling body is constant, or the rate of cooling at any moment will be proportional to the difference of temperature . This See also:

• SIMPLE

simple relation is commonly known as Newton's law of cooling, but is limited in its application to comparatively simple cases such as the foregoing . Newton himself applied it to estimate the temperature of a red-hot See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron See also:

• BALL (in Mid. Eng. bal; the word is probably cognate with " bale," Teutonic in origin, cf. also Lat. falls, and Gr. 7raXXa)
• BALL, JOHN (1585-1640)
• BALL, JOHN (1818-1889)
• BALL, JOHN (d. 1381)
• BALL, SIR ALEXANDER JOHN, BART
• BALL, THOMAS (1819- )

ball, by observing the See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time which it took to cool from a red heat to a known temperature, and comparing this with the time taken to cool through a known range at See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary temperatures .

According to this law if the excess of temperature of the body above its surroundings is observed at equal intervals of time, the observed values will form a geometrical progression with a common ratio . Supposing, for instance, that the surrounding temperature were o° C., that the red-hot ball took 25 minutes to cool from its See also:

• ORIGINAL

original temperature to 20° C., and 5 minutes to cool from 20° C. to 10° C., the original temperature is easily calculated on the See also:

• ASSUMPTION, FEAST OF

assumption that the excess of temperature above o° C. falls to See also:

• HALF

half its value in each See also:

• INTERVAL

interval of 5 minutes . Doubling the value 200 at 25 minutes five times, we arrive at 640° C. as the original temperature . No other method of estimation of such temperatures was available in the time of Newton, but, as we now know, the simple law of proportionality to the temperature difference is inapplicable over such large ranges of temperature . The rate of loss of heat by radiation,and also by convection and conduction to the surrounding See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air, increases much more rapidly than in simple proportion to the temperature difference, and the rate of increase of each follows a different law . At a later date See also:

• SIR

Sir See also:

• JOHN
• JOHN (1167–1216)
• JOHN (1290-c. 1320)
• JOHN (1296-1346)
• JOHN (1371–1419)
• JOHN (1468-1532)
• JOHN (1801-1873)
• JOHN (Heb. llni')
• JOHN (ZAPOLYA) (1487-1540)
• JOHN, 4TH MARQUESS OF TWEEDDALE (c. 1695-1762)
• JOHN, DON (1545-1578)
• JOHN, DON (1629–1679)
• JOHN, GOSPEL OF ST
• JOHN, or HAYS (1513–1571)
• JOHN, THE APOSTLE
• JOHN, THE EPISTLES OF

John See also:

• HERSCHEL, CAROLINE LUCRETIA (1750-1848)
• HERSCHEL, SIR F
• HERSCHEL, SIR FREDERICK WILLIAM (1738-1822)
• HERSCHEL, SIR JOHN FREDERICK WILLIAM, BART

Herschel measured the intensity of the See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar radiation at the surface of the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth, and endeavoured to form an estimate of the temperature of the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun by comparison with terrestrial See also:

• SOURCES

sources on the assumption that the intensity of radiation was simply proportional to the temperature difference . He thus arrived at an estimate of several million degrees, which we now know would be about a thousand times too great . The application of Newton's law necessarily leads to absurd results when the difference of temperature is very large, but the See also:

• ERROR (Lat. error, from errare, to wander, to err)

error will not in See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general exceed 2 to 3 0 /to if the temperature difference does not exceed ro° C., and the percentage error is proportionately much smaller for smaller differences . 27 . See also:

• DULONG, PIERRE LOUIS (1785—1838)

Dulong and See also:

• PETIT, SIR DINSHAW MANECKJI, BART

Petit's Empirical Laws of Cooling.—One of the most elaborate experimental investigations of the law of cooling was that of Dulong and Petit (See also:

• ANN

Ann . Claim . Phys., 1817, 7, pp .

225 and 337), who observed the rate of cooling of a See also:

• MERCURY
• MERCURY (MERCU1uus)
• MERCURY (symbol Hg, atomic weight = 2oo)

mercury thermometer from 300° C. in a water-jacketed enclosure at various temperatures from 6° C. to 8o° C . In See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order to obtain the rate of cooling by radiation alone, they exhausted the enclosure as perfectly as possible after the introduction of the thermometer, but with the imperfect appliances available at that time they were not able to obtain a vacuum better than about 3 or 4 mm. of mercury . They found that the velocity of cooling V in a vacuum could be represented by a See also:

• FORMULA
• FORMULA (Lat. diminutive of forma, shape, pattern, &c., especially used of rules of judicial procedure)

formula of the type V=A(aa–a'o) (5) in which t is the temperature of the thermometer, and to that of the enclosure, a is a constant having the value 1.0075, and the coefficient A depends on the form of the bulb and the nature of its surface . For the ranges of temperature they employed, this formula gives much better results than Newton's, but it must be remembered that the temperatures were expressed on the arbitrary See also:

• SCALE
• SCALE (1) A

scale of the mercury thermometer, and were not corrected for the large and uncertain errors of See also:

• STEM (O. Eng. staefn, stemn, cf. Du. stain, Ger. Stamm, &c., probably related to " staff ")

stem-exposure (see See also:

• THERMOMETRY (Gr. Bepµos, warm; µerpov, a measure)

THERMOMETRY) . Moreover, although the effects of cooling by convection currents are practically eliminated by exhausting to 3 or 4 mm . (since the density of the gas is reduced to 1/2ooth while its viscosity is not appreciably affected), the rate of cooling by conduction is not materially diminished, since the conductivity, like the viscosity, is nearly See also:

• INDEPENDENT

independent of pressure . It has since been shown by Sir See also:

• WILLIAM
• WILLIAM (1143-1214)
• WILLIAM (1227-1256)
• WILLIAM (1J33-1584)
• WILLIAM (A.S. Wilhelm, O. Norse Vilhidlmr; O. H. Ger. Willahelm, Willahalm, M. H. Ger. Willehelm, Willehalm, Mod.Ger. Wilhelm; Du. Willem; O. Fr. Villalme, Mod. Fr. Guillaume; from " will," Goth. vilja, and " helm," Goth. hilms, Old Norse hidlmr, meaning
• WILLIAM (c. 1130-C. 1190)
• WILLIAM, 13TH

William See also:

• CROOKES, SIR WILLIAM (1832– )

Crookes (Prot . See also:

• ROY, WILLIAM (1726-1790)

Roy . See also:

• SOC

Soc., 1881, 21, p . 239) that the rate of cooling of a mercury thermometer in a vacuum suffers a very great diminution when the pressure is reduced from i mm. to •oor mm., at which pressure the effect of conduction by the residual gas has practically disappeared . Dulong and Petit also observed the rate of cooling under the same conditions with the enclosure filled with various gases . They found that the cooling effect of the gas could be represented by adding to the See also:

• TERM

term already given as representing radiation, an expression of the form V'=Bp°(t-to)I.233 .

(6) They found that the cooling effect of convection, unlike that of radiation, was independent of the nature of the surface of the thermometer, whether silvered or blackened, that it varied as some See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power c of the pressure p, and that it was independent of the See also:

• ABSOLUTE (Lat. absolvere, to loose, set free)

absolute temperature of the enclosure, but varied as the excess temperature (t–to) raised to the power I'233,„ ,This highly artificial result undoubtedly contains some elements of truth, but could only be applied to experiments similar to those from which it was derived . F . Herve de la Provostaye and P . Q . Desains (Ann . Chico . Phys., 1846, 16, p . 337), in repeating these experiments under various conditions, found that the coefficients A and B were to some extent dependent on the temperature, and that the manner in which the cooling effect varied with the pressure depended on the form and See also:

• SIZE

size of the enclosure . It is evident that this should be the case, since the cooling effect of the gas depends partly on convective currents . which are necessarily greatly modified by the form of the enclosure in a manner which it would appear hopeless to See also:

• ATTEMPT (Lat. adtemptare, attentare, to try)

attempt to represent by any general formula . 28 . Surface Emissivity.—The same remark applies to many attempts which have since been made to determine the general value of the constant termed by See also:

• FOURIER, F
• FOURIER, FRANCOIS CHARLES MARIE (1772-1837)
• FOURIER, JEAN BAPTISTE JOSEPH (1768-1830)

Fourier and See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

early writers the " exterior conductibility," but now called the surface emissivity .

This coefficient represents the rate of loss of heat from a body per unit See also:

• AREA

area of surface per degree excess of temperature, and includes the effects of radiation, convection and conduction . As already pointed out, the combined effect will be nearly proportional to the excess of temperature in any given case provided that the excess is small, but it is not necessarily proportional to the extent of surface exposed except in the case of pure radiation . The rate of loss by convection and conduction varies greatly with the form of the surface, and, unless the enclosure is very large compared with the cooling body, the effect depends also on the size and form of the enclosure . Heat is necessarily communicated from the cooling body to the layer of gas in contact with it by conduction . If the linear dimensions of the body are small, as in the case of a See also:

• FINE

fine See also:

• WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire, or if it is separated from the enclosure by a thin layer of gas, the rate of loss depends chiefly on conduction . For very fine metallic wires heated by an electric current, W . E . See also:

• AYRTON, WILLIAM EDWARD (1847-1908)

Ayrton and H . Kilgour (Phil . Trans., 1892) showed that the rate of loss is nearly independent of the surface, instead of being directly proportional to it . This should be the case, as See also:

• PORTER
• PORTER, BENJAMIN CURTIS (1843– )
• PORTER, DAVID (1780-1843)
• PORTER, DAVID DIXON (1813-1891)
• PORTER, ENDYMION (1587–1649)
• PORTER, HENRY (ft. 1596-1599)
• PORTER, HORACE (1837– )
• PORTER, JANE (1776–1850)
• PORTER, MARY (d. 1765)
• PORTER, NOAH (1811–1892)

Porter has shown (Phil . See also:

• HAG

Hag., See also:

• MARCH
• MARCH (1) (from Fr. marcher, to walk; the earliest sense in French appears to be " to trample," and the origin has usually been found in the Lat. marcus, hammer; Low Lat. marcare, to hammer; hence to beat the road with the regular tread of a soldier: cf.
• MARCH, AUZIAS (c. "1395-1458)
• MARCH, EARLS OF
• MARCH, FRANCIS ANDREW (1825– )

March 1895), since the effect depends tnainly on conduction .

The effects of conduction and radiation may be approximately estimated if the conductivity of the gas and the nature and forms of the surfaces of the body and enclosure are known, but the effect of convection in any case can be determined only by experiment . It has been found that the rate of cooling by a current of air is approximately proportional to the velocity of the current, other things being equal . It is obvious that this should be the case, but the result cannot generally be applied to convection currents . Values which are commonly given for the surface emissivity must therefore be accepted with great reserve . They can be regarded only as approximate, and as applicable only to cases precisely similar to those for which they were experimentally obtained . There cannot be said to be any general law of convection . The loss of heat is not necessarily proportional.to the area of the surface, and no general value of the coefficient can be given to suit all cases . The laws of conduction and radiation admit of being more precisely formulated, and their effects predicted, except in so far as they are complicated by convection . 29 . Conduction of Heat.—The laws of transference of heat in the interior of a solid body formed one of the earliest subjects of mathematical and experimental treatment in the theory of heat . The law assumed by Fourier was of the simplest possible type, but the mathematical application, except in the simplest cases, was so difficult as to require the development of a new mathematical method . Fourier succeeded in showing how, by his method of See also:

• ANALYSIS (Gr. avci and Meta, to break up into parts)

analysis, the See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution of any given problem with regard to the flow of heat by conduction in any material could be obtained in terms of a See also:

• PHYSICAL

physical constant, the thermal conductivity of the material, and that the results obtained by experiment agreed in a qualitative manner with those predicted by his theory .

But the experimental determination of the actual values of these constants presented formidable difficulties which were not surmounted till a later date The experimental methods and difficulties are discussed in a See also:

• SPECIAL

special See also:

• ARTICLE (from Lat. articulus, a joint)

article on CONDUCTION OF HEAT . It will suffice here to give a brief historical See also:

• SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

sketch, including a few of the more important results by way of illustration . 30 . Comparison of Conducting See also:

• POWERS, HIRAM (1805-1873)

Powers.—That the power of transmitting heat by conduction varied widely in different materials was probably known in a general way from prehistoric times . Empirical knowledge of this See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind is shown in the construction of many articles for heating, cooking, &c., such as the See also:

• COPPER (symbol Cu, atomic weight 63.1, H=1, or 63.6, O = 16)

copper soldering bolt, or the See also:

• NORWEGIAN

Norwegian cooking-See also:

• STOVE

stove . Oneof the earliest experiments for making an actual comparison of conducting powers was that suggested by See also:

• FRANKLIN
• FRANKLIN, BENJAMIN (1706-1790)
• FRANKLIN, SIR JOHN (1786-1847)
• FRANKLIN, WILLIAM BUEL (1823-1903)

Franklin, but carried out by See also:

• JAN

Jan Ingenhousz (Journ. de phys., 1789, 34, pp . 68 and 38o) . Exactly similar bars of different materials, See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

glass, See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

wood, metal, &c., thinly coated with See also:

• WAX

wax, were fixed in the See also:

• SIDE
• SIDE (mod. Eski Adalia)

side of a trough of boiling water so as to project for equal distances through the side of the trough into the See also:

• EXTERNAL

external air . The wax coating was observed to melt as the heat travelled along the bars, the distance from the trough to which the wax was melted along each affording an approximate indication of the See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

distribution of temperature . When the temperature of each See also:

• BAR
• BAR (0. Fr. barre, Late Lat. barra, origin unknown)
• BAR, CONFEDERATION OF
• BAR, FRANCOIS DE (1538-1606)
• BAR, THE

bar had become stationary the heat which it gained by conduction from the trough must be equal to the heat lost to the surrounding air, and must therefore be approximately proportional to the distance to which the wax had melted along the bar . But the temperature fall per unit length, or the temperature-gradient, in each bar at the point where it emerged from the trough would be inversely proportional to the same distance . For equal temperature-gradients the quantities of heat conducted (or the relative conducting powers of the bars) would therefore be proportional to the squares of the distances to which the wax finally melted on each bar .

This was shown by Fourier and Despretz (Ann. chins. phys., 1822, 19, p . 97) . 31 . See also:

• DIFFUSION (from the Lat. diffundere; dis-, asunder, and fundere, to pour out)

Diffusion of Temperature.—It was shown in connexion with this experiment by Sir H . See also:

• DAVY, SIR HUMPHRY

Davy, and the experiment was later popularized by John See also:

• TYNDALL, JOHN (182o-1893)

Tyndall, that the rate at which wax melted along the bar, or the rate of See also:

• PROPAGATION

propagation of a given temperature, during the first moments of heating, as distinguished from the melting-distance finally attained, depended on the specific heat as well as the conductivity . See also:

• SHORT, FRANCIS JOB (1857– )

Short prisms of iron and See also:

• BISMUTH

bismuth coated with wax were placed on a hot metal See also:

• PLATE

plate . The wax was observed to melt first on the bismuth, although its conductivity is less than that of iron . The See also:

• REASON (Lat. ratio, through French raison)

reason is that its specific heat is less than that of iron in the proportion of 3 to I1 . The densities of iron and bismuth being 7.8 and 9.8, the thermal capacities of equal prisms will be in the ratio •86 for iron to •29 for bismuth . If the prisms receive heat at equal rates, the bismuth will reach the temperature of melting wax nearly three times as quickly as the iron . It is often stated on the strength of this experiment that the rate of propagation of a temperature See also:

• WAVE

wave, which depends on the ratio of the conductivity to the specific heat per unit See also:

• VOLUME

volume, is greater in bismuth than in iron (e.g . See also:

• PRESTON
• PRESTON, JOHN (1587-1628)

Preston, Heat, p .

628) . This is quite incorrect, because the conductivity of iron is about six times that of bismuth, and the rate of propagation of a temperature wave is therefore twice as great in iron as in bismuth . The experiment in reality is misleading because the rates of reception of heat by the prisms are limited by the very imperfect contact with the hot metal plate, and are not proportional to the respective conductivities . If the iron and bismuth bars are properly faced and soldered to the See also:

• TOP (cf. Dan. top, Ger. Topf, also meaning pot)

top of a copper See also:

• BOX
• BOX (Gr. irl or, Lat. buxus, box-wood; cf. 2r6Eir, a pyx)

box (in order to ensure See also:

• GOOD, JOHN MASON (1764-1827)

good metallic contact, and exclude a non-conducting film of air), and the box is then heated by steam, the rates of reception of heat will be nearly proportional to the conductivities, and the wax will melt nearly twice as fast along the iron as along the bismuth . A bar of See also:

• LEAD
• LEAD (pronounced iced)

lead similarly treated will show a faster rate of propagation than iron, because, although its conductivity is only half that of iron, its specific heat per unit volume is 2.5 times smaller . 32 . See also:

• BAD, BCD

Bad Conductors . Liquids and Gases.—See also:

• COUNT (Lat. comes, gen. comitis, Fr. comte, Ital. conte, Span. conde)

Count See also:

• RUMFORD, BENJAMIN THOMPSON, COUNT (1753-1814)

Rumford (1792) compared the conducting powers of substances used in clothing, such as See also:

• WOOL, WORSTED AND WOOLLEN MANUFACTURES

wool and See also:

• COTTON
• COTTON (Fr. coton; from Arab. qutun)
• COTTON, CHARLES (163o–1687)
• COTTON, GEORGE EDWARD LYNCH (1813–1866)
• COTTON, JOHN (1585–1652)
• COTTON, SIR ROBERT BRUCE

cotton, See also:

• FUR (connected with O. Fr. forre, a sheath or case; so " an outer covering ")

fur and down, by observing the time which a thermometer took to cool when embedded in a- - globe filled successively with the different materials . The times of cooling observed for a given range varied from 1300 to 900 seconds for different materials . The See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

low conducting power of such materials is principally due to the presence of air in the interstices, which is prevented from forming convection currents by the presence of the fibrous material . Finely powdered See also:

• SILICA

silica is a very bad conductor, but in the compact form of See also:

• ROCK
• ROCK (O.Fr. rake, Sp. rota, Ital. rocca; possibly from a Lat. form rupica, from rupes, rock)
• ROCK, DANIEL (1799–187t)

rock crystal it is as good a conductor as some of the metals . According to the kinetic theory of gases, the conductivity of a gas depends on molecular diffusion .

See also:

• MAXWELL
• MAXWELL, JAMES CLERK (1831–1879)

Maxwell estimated the conductivity of air at ordinary temperatures at about 2o,000 times less than that of copper . This has been verified experimentally by See also:

• KUNDT, AUGUST ADOLPH EDUARD EBERHARD (1839-1894)

Kundt and Warburg, Stefan and See also:

• WINKELMANN, EDUARD (1838-1896)

Winkelmann, by taking special precautions to eliminate the effects of convection currents and radiation . It was for some time doubted whether a gas possessed any true conductivity for heat . The experiment of T . See also:

• ANDREWS, JAMES PETTIT (c. 1737–1797)
• ANDREWS, THOMAS (1813–1885)

Andrews, repeated by See also:

• GROVE (O.E. graf, cf. O.E. gr(efa, brushwood, later" greave "; the word does not appear in any other Teutonic language, and the New English Dictionary finds no Indo-European root to which it can be referred; Skeat considers it connected with " grave," to
• GROVE, SIR GEORGE (182o-1900)
• GROVE, SIR WILLIAM ROBERT (1811-1896)

Grove, and See also:

• MAGNUS, HEINRICH GUSTAV (1802-1870)

Magnus, showing that a wire heated by an electric current was raised to a higher temperature in air than in See also:

• HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen, was explained by Tyndall as being due to the greater mobility of hydrogen which gave rise to stronger convection currents . In reality the effect is due chiefly to the greater velocity of motion of the ultimate molecules of hydrogen, and is most marked if molar (as opposed to molecular) convection is eliminated . Molecular convection or diffusion, which cannot be distinguished experimentally from conduction, as it follows the same law, is also the See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main cause of conduction of heat in liquids . Both in liquids and gases the effects of convection currents are so much greater than those of diffusion or conduction that the latter are very difficult to measure, and, except in special cases, comparatively unimportant as affecting the transference of heat . Owing to the difficulty of eliminating the effects of radiation and convection, the results obtained for the conductivities of liquids are somewhat discordant, and there is in most cases-great uncertainty whether the conductivity increases or. diminishes with rise of temperature . It would appear, however, that liquids, such as water and See also:

• GLYCERIN, GLYCERINE

glycerin, differ remarkably little in conductivity in spite of enormous differences of viscosity . The viscosity of a liquid diminishes very rapidly with rise of temperature, without any marked See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change in the conductivity, whereas the viscosity of a gas increases with rise of temperature, and is always nearly proportional to the conductivity . 33 .

Difficulty of Quantitative Estimation of Heat Transmitted.—The conducting powers of different metals were compared by C . M . Despretz, and later by G . H . See also:

• WIEDEMANN, GUSTAV HEINRICH (1826-1899)

Wiedemann and R . See also:

• FRANZ, ROBERT (1815-1892)

Franz, employing an See also:

• EXTENSION (Lat. ex, out ; tendere, to stretch)

extension of the method of Jan Ingenhousz, in which the temperatures at different points along a bar heated at one end were measured by thermometers or thermocouples let into small holes in the bars, instead of being measured at one point only by means of melting wax . These experiments undoubtedly gave fairly accurate relative values, but did not permit the calculation of the absolute amounts of heat transmitted . This was first obtained by J . D . See also:

• FORBES
• FORBES, ALEXANDER PENROSE (1817-1875)
• FORBES, ARCHIBALD (1838-1900)
• FORBES, DAVID (1828--1876)
• FORBES, DUNCAN
• FORBES, EDWARD (1815—1854)
• FORBES, JAMES DAVID (1809—1868)
• FORBES, SIR JOHN (1787—1861)

Forbes (Brit . Assoc . See also:

• REP, REPP, or REPS

Rep., 1852; Trans .

Roy . Soc . Ed., 1862, 23, p . 133) by deducing the amount of heat lost to the surrounding air from a See also:

• SEPARATE

separate experiment in which the rate of cooling of the bar was observed (see CONDUCTIO.i of HEAT) . See also:

• CLEMENT (Lat. Clemens, i.e. merciful; Gr. K)4gs)
• CLEMENT, FRANCOIS (1714-1793)
• CLEMENT, JACQUES (1567-1589)

Clement (Ann. chim. phys., 1841) had previously attempted to determine the conductivities of metals by observing the amount of heat transmitted by a plate with one side exposed to steam at See also:

• IOO

ioo° C., and the other side cooled by water at z8° C . Employing a copper plate 3 mm. thick, and assuming that the two surfaces of the plate were at the same temperatures as the water and the steam to which they were exposed, or that the temperature-gradient in the metal was 92° in 3 mm., he had thus obtained a value which we now know to be nearly zoo times too small . The actual temperature difference in the metal itself was really about o•36° C . The See also:

• REMAINDER, REVERSION

remainder of the 92° drop was in the badly conducting films of water and steam See also:

• CLOSE
• CLOSE (from Lat. clausum, shut)
• CLOSE, MAXWELL HENRY (1822-1903)

close to the metal surface . Similarly in a boiler plate in contact with See also:

• FLAME (Lat. flamma; the root flag- appears in flagrare, to burn, blaze, and Gr. d, XEryew)

flame at 15oo° C. on one side and water at. say, 15o° C. on the other, the actual difference of temperature in the metal, even if it is an See also:

• INCH (O. Eng. ynce from Lat. uncia, a twelfthpart; cf. " ounce," and see As)

inch thick, is only a few degrees . The metal, unless badly furred with incrustation, is but little hotter than the water . It is immaterial so far as the transmission of heat is concerned, whether the plates are iron or copper, The greater See also:

• PART

part of the resistance to the passage of heat resides in a comparatively quiescent film of gas close to the surface, through which film the heat has to pass mainly by conduction . If a See also:

• BUNSEN, CHRISTIAN CHARLES JOSIAS, BARON VON (1791-186o,)
• BUNSEN, ROBERT WILHELM VON (1811-1899)

Bunsen flame, preferably coloured with See also:

• SODIUM [symbol Na, from Lat. natrium; atomic weight 23.00 (0=16)]

sodium, is observed impinging on a cold metal plate, it will be seen to be separated from the plate by a dark space of a millimetre or less, throughout which the temperature of the gas is lowered by its own conductivity below the temperature of incandescence .

There is no abrupt change of temperature in passing from the gas to the metal, but a continuous temperature-gradient from the temperature of the metal to that of the flame . It is true that this gradient may be upwards of r000° C. per mm., but there is no discontinuity . 34 . Resistance of a Gas Film to the Passage of Heat.—It is possible to make a rough estimate of the resistance of such a film to the passage of heat through it . Taking the See also:

• AVERAGE

average conductivity of the gas in the film as ro,000 times less than that of copper (about See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double the conductivity of air at ordinary temperatures) a millimetre film would be See also:

• EQUIVALENT

equivalent to a thickness of ro metres of copper, or about 1.2 metres of iron . Taking the temperature-gradient as r000° C. per mm. such a film would transmit r gramme-calorie per sq. cm. per sec., or 36,000 kilo-calories per sq. See also:

• METRE ((terpuc?, sc. rFxvf, from Gr. Or poi', measure)

metre per See also:

• HOUR

hour . With an area of 100 sq. See also:

• EMS

ems. the heat transmitted at this rate would raise a litre of water from 2o° C. to roo° C. in 800 secs . By experiment with a strong Bunsen flame it takes from 8 to ro minutes to do this, which would indicate that on the above assumptions the equivalent thickness of quiescent film should be rather less than i mm. in this case . The thickness of the film diminishes with the velocity of the burning gases impinging on the surface . This accounts for the rapidity of heating by a See also:

• BLOWPIPE

blowpipe flame, which is not due to any great increase in temperature of the flame as compared with a Bunsen . Similarly the efficiency of a boiler is but slightly reduced if half the tubes are stopped up, because the increase of See also:

• DRAUGHT (from the common Teutonic word " to draw "; cf. Ger. 7'racht, load; the pronunciation led to the variant form " draft," now confined to certain specific meanings)

draught through the remainder compensates partly for the diminished heating surface . Some resistance to the passage of heat into a boiler is also due to the water film on the inside .

But this is of less See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account, because the conductivity of water is much greater than that of air, and because the film is continually broken up by the formation of steam, which abstracts heat very rapidly . 35 . Heating by Condensation of Steam.—It is often stated that the rate at which steam will condense on a metal surface at a temperature below that corresponding to the saturation pressure of the steam is practically See also:

• INFINITE (from Lat. in, not, finis, end or limit; cf. findere, to deave)

infinite (e.g . See also:

• OSBORNE

Osborne See also:

• REYNOLDS, JOHN FULTON (182o-1863)
• REYNOLDS, SIR JOSHUA (1723-1792)
• REYNOLDS, WALTER (d. 1327)

Reynolds, Proc . Roy . Soc . Ed., 1893, p . 275), and conversely that the rate at which water will abstract heat from a metal surface by the formation of steam (if the metal is above the temperature of saturation of the steam) is limited only by the rate at which the metal can See also:

• SUPPLY (through Fr. from Lat. supplere, to fill up)

supply heat by conduction to its surface layer . The rate at which heat can be supplied by condensation of steam appears to be much greater than that at which heat can be supplied by a flame under ordinary conditions, but there is no reason to suppose that it is infinite, or that any discontinuity exists . Experiments by H . L . Callendar and J .

T . See also:

• NICOLSON, WILLIAM (1655-1727)

Nicolson by three independent methods (Proc . Inst . Civ . Eng., 1898, 131, p . 149; Brit . Assoc . Rep. p . 418) appear to show that the rate of See also:

• ABSTRACTION (Lat. abs and trahere)

abstraction of heat by evaporation, or that of communication of heat by condensation, depends chiefly on the difference of temperature between the metal surface and the saturated steam, and is nearly proportional to the temperature difference (not to the pressure difference, as suggested by Reynolds) for such ranges of pressure as are common in practice . The rate of heat transmission they observed was equivalent to about 8 calories per sq. cm. per sec., for a difference of 20 ° C. between the temperature of the metal surface and the saturation temperature of the steam . This would correspond to a condensation of 530 kilogrammes of steam at roo° C. per sq. metre per hour, or 109 lb per sq. ft. per hour for the same difference of temperature, values which are many times greater than those actually obtained in ordinary surface condensers . The reason for this is that there is generally some air mixed with the steam in a surface See also:

• CONDENSER

condenser, which greatly retards the condensation .

It is also difficult to keep the temperature of the metal as much as zo° C. below the temperature of the steam unless a very See also:

• FREE

free and copious circulation of cold water is available . For the same difference of temperature, steam can supply heat by condensation about a thousand times faster than hot air . This rate is not often approached in practice, but the facility of See also:

• GENERATION (from Lat. generare, to beget, procreate; genus, stock, race)

generation and transmission of steam, combined with its, high latent heat and the accuracy of See also:

• CONTROL (Fr. contrdle, older form contre rolle, from Med. Lat. contra-rotulus, a counter roll or copy of a document used to check the original; there is no instance in English of the use of " control " in this, its literal, meaning)

control and regulation of temperature afforded, render it one of the most convenient agents for the distribution of large quantities of heat in all kinds of manufacturing processes . 36 . Spheroidal See also:

• STATE
• STATE, GREAT OFFICERS OF

State.—An interesting contrast to the extreme rapidity with which heat is abstracted by the evaporation of a liquid in contact with a metal plate, is the so-called spheroidal state . A small drop of liquid thrown on a red-hot metal plate assumes a spheroidal form, and continues See also:

• SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming about for some time, while it slowly evaporates at a temperature somewhat below its boiling-point . The explanation is simply that the liquid itself cannot come in actual contact with the metal plate (especially if the latter is above the See also:

• CRITICAL (in alphabetical order of authors)

critical temperature), but is separated from it by a badly conducting film of vapour, through which, as we have seen, the heat is comparatively slowly transmitted even if the difference of temperature is several See also:

• HUNDRED

hundred degrees . If the metal plate is allowed to cool gradually, the drop remains suspended on its See also:

• CUSHION (from O. Fr. coisson, coussin; according to the New English Dict., from Lat. coxa, a hip; others say from Lat. culcita, a quilt)

cushion of vapour, until, in the case of water, a temperature of about 200° C. is reached, at which the liquid comes in contact with the plate and boils explosively, reducing the temperature of the plate, if thin, almost instantaneously to oo° C . The temperature of the metal is readily observed by a thermo-electric method, employing a See also:

• PLATINUM [symbol Pt, atomic weight 145.0 (0=16)]

platinum dish with a platinum-See also:

• RHODIUM [symbol Rh; atomic weight 102.9 (0=16)]

rhodium wire soldered with See also:

• GOLD
• GOLD [symbol Au, atomic weight 195.7(H = 1),197.2(0 =16)]

gold to its under side . The absence of contact between the liquid and the dish in the spheroidal state may also be shown by connecting one terminal of a See also:

• GALVANOMETER

galvanometer to the drop and the other through a See also:

• BATTERY (Fr. batterie, from battles, to beat)

battery to the dish, and observing that no current passes until the drop boils . 37 . Early Theories of Radiation..—It was at one time supposed that there were three distinct kinds of radiation—thermal, luminous and actinic, combined in the radiation from a luminous source such as the sun or a flame .

The first gave rise to heat, the second to See also:

• LIGHT

light and the third to chemical See also:

• ACTION

action . The three kinds were partially separated by a See also:

• PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism, the actinic rays being generally more refracted, and the thermal rays less refracted than the luminous . This conception arose very naturally from the observation that the feebly luminous See also:

• BLUE (common in different forms to most European languages)

blue and See also:

• VIOLET

violet rays produced the greatest photographic effects, which also showed the existence of dark rays beyond the violet, whereas the brilliant yellow and red were practically without action on the photographic plate . A thermometer placed in the blue or violet showed no appreciable rise of temperature, and even in the yellow the effect was hardly discernible . The effect increased rapidly as the light faded towards the extreme red, and reached a maximum beyond the extreme limits of the spectrum (Herschel), showing that the greater part of the thermal radiation was al-together non-luminous . It is now a See also:

• COMMONPLACE

commonplace that chemical action, See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour sensation and heat are merely different effects of one and the same kind of radiation, the particular effect produced in each case depending on the frequency and intensity of the vibration, and on the nature of the substance on which it falls . When radiation is completely absorbed by a See also:

• BLACK
• BLACK, ADAM (1784-1874)
• BLACK, JEREMIAH SULLIVAN (1810–1883)
• BLACK, WILLIAM (1841-1898)

black substance, it is converted into heat, the quantity of heat produced being equivalent to the See also:

• TOTAL

total See also:

• ENERGY (from the Gr. ivipyela; iv, in, ipyov, work)

energy of the radiation absorbed, irrespective of the colour or frequency of the different rays . The actinic or chemical effects, on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand, depend essentially on some relation between the See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period of the vibration and the properties of the substance acted on . The rays producing such effects are generally those which are most strongly absorbed . The spectrum of See also:

• CHLOROPHYLL (from Gr. XAwpos, green, 4bXXov, a leaf)

chlorophyll, the See also:

• GREEN, A
• GREEN, ALEXANDER HENRY (1832—1896)
• GREEN, DUFF (1791—1875)
• GREEN, JOHN RICHARD (1837—1883)
• GREEN, MATTHEW (1696-1737)
• GREEN, THOMAS HILL (1836-1882)
• GREEN, VALENTINE (1739–1813)
• GREEN, WILLIAM HENRY (.1825–1900)

green colouring matter of See also:

• PLANTS

plants, shows two very strong absorption bands in the red . The red rays of corresponding period are found to be the most active in promoting the growth of the plant . The chemically active rays are not necessarily the shortest .

Even photographic plates may be made to See also:

• RESPOND

respond to the red rays by staining them with pinachrome or some other suitable dye . The action of light rays on the retina is closely analogous to the action on a photographic plate . The retina, like the plate, is sensitive only to rays within certain restricted limits of frequency . The limits of sensitiveness of each colour sensation are not exactly defined, but vary slightly from one individualto another, especially in cases of partial colour-See also:

• BLINDNESS

blindness, and are modified by conditions of fatigue . We are not here concerned with these important physiological and chemical effects of radiation, but rather with the question of the See also:

• CONVERSION (Lat. conversio, from convertere, to turn or ,change)

conversion of energy of radiation into heat, and with the laws of emission and absorption of radiation in relation to temperature . We may here also assume the identity of visible and invisible radiations from a heated body in all their physical properties . It has been abundantly proved that the invisible rays, like the visible, (1) are propagated in straight lines in homogeneous See also:

• MEDIA

media; (2) are reflected and diffused from the surface of bodies according to the same law; (3) travel with the same velocity in free space, but with slightly different velocities in denser media, being subject to the same law of See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

refraction; (4) exhibit all the phenomena of diffraction and interference which are characteristic of wave-motion in general; (5) are capable of polarization and double refraction; (6) exhibit similar effects of selective absorption . These properties are more easily demonstrated in the case of visible rays on account of the great sensitiveness of the See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye . But with the aid of the thermopile or other sensitive See also:

• RADIOMETER

radiometer, they may be shown to belong equally to all the radiations from a heated body, even such as are See also:

• THIRTY

thirty to fifty times slower in frequency than the longest visible rays . The same physical properties have also been shown to belong to electromagnetic waves excited by an electric See also:

• DISCHARGE (adapted from the O. Fr. discharge, modern decharge, from a med. Lat. discargare, to unload, dis- and earn care, to load, cf. " charge ")

discharge, whatever the frequency, thus including all kinds of aetherial radiation in the same See also:

• CATEGORY (Gr. Karrlyopia, " accusation ")

category as light . 38 . Theory of Exchanges.—The apparent concentration of cold by a See also:

• CONCAVE

concave See also:

• MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror, observed by G .

B . Porta and rediscovered by M . A . Pictet, led to the enunciation of the theory of exchanges by See also:

• PIERRE

Pierre See also:

• PREVOST, ANTOINE FRANCOIS (1697–1763)
• PREVOST, CONSTANT (1787-1856)
• PREVOST, EUGENE MARCEL (1862- )
• PREVOST, PIERRE (1751-1839)

Prevost in 1791 . Prevost's leading See also:

• IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea was that all bodies, whether cold or hot, are constantly radiating heat . Heat equilibrium, he says, consists in an equality of ex-change . When equilibrium is interfered with, it is re-established by inequalities of See also:

• EXCHANGE

exchange . If into a locality at See also:

• UNIFORM

uniform temperature a refracting or reflecting body is introduced, it has no effect in the way of changing the temperature at any point of that locality . A reflecting body, heated or cooled in the interior of such an enclosure, will acquire the surrounding temperature more slowly than would a non-reflector, and will less affect another body placed at a little distance, but will not affect the final equality of temperature . Apparent radiation of cold, as from a See also:

• BLOCK
• BLOCK (from the Fr. bloc, and possibly connected with an Old Ger. Block, obstruction, cf. " baulk ")
• BLOCK, MAURICE (1816–19or)

block of See also:

• ICE (a word common to Teutonic languages; cf. Ger. Eis)

ice to a thermometer placed near it, is due to the fact that the thermometer being at a higher temperature sends more heat to the ice than it received back from it . Although Prevost does not make the statement in so many words, it is clear that he regards the radiation from a body as depending only on its own nature and temperature, and as independent of the nature and presence of any adjacent body . Heat equilibrium in an enclosure of constant temperature such as is here postulated by Prevost, has often been regarded as a consequence of See also:

• CARNOT, LAZARE EIPPOLYTE (18ot-1888)
• CARNOT, MARIE FRANCOIS SADI (1837-1894)
• CARNOT, SADI NICOLAS LEONHARD (1796-1832)

Carnot's principle .

Since difference of temperature is required for transforming heat into See also:

• WORK

work, no work could be obtained from heat in such a See also:

• SYSTEM

system, and no spontaneous changes of temperature can take place, as any such changes might be utilized for the See also:

• PRODUCTION (Lat. productionem, from producere, to pro-duce)

production of work . This See also:

• LINE

line of reasoning does not appear quite satisfactory, because it is tactitly assumed, in the reasoning by which Carnot's principle was established, as a result of universal experience, that a number of bodies within the same impervious enclosure, which contains no source of heat, will ultimately acquire the same temperature, and that difference of temperature is required to produce flow of heat . Thus although we may regard the equilibrium in such an enclosure as being due to equal exchanges of heat in all directions, the equal and opposite streams of radiation annul and neutralize each other in such a way that no actual transfer of energy in any direction takes place . The state of the medium is everywhere the same in such an enclosure, but its energy of-agitation per unit volume is a See also:

• FUNCTION

function of the temperature, and is such that it would not be in equilibrium with any body at a different temperature . 3o . " Full" and Selective Radiation . See also:

• CORRESPONDENCE
• CORRESPONDENCE (from med. scholastic Lat. correspondentia, corrcspondere, compounded of Lat. cum, with, and respond ere, to answer; cf. Fr. correspondance)

Correspondence of Emission and Absorption.—The most obvious difficulties in the way of this theory arise from the fact that nearly all radiation is more or less selective in See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character, as regards the quality and frequency of the rays emitted and absorbed . It was shown by J . See also:

• LESLIE
• LESLIE, CHARLES (1650-1722)
• LESLIE, CHARLES ROBERT (1794–1859)
• LESLIE, FRED [FREDERICK HOBSON] (1855–1892)
• LESLIE, SIR JOHN (1766-1832)
• LESLIE, THOMAS EDWARD CLIFFE (1827-1882)

Leslie, M . See also:

• MELLONI, MACEDONIO (1798-1854)

Melloni and other experimentalists that many substances such as glass and water, which are very transparent to visible rays, are extremely opaque to much of the invisible radiation of lower frequency; and that polished metals, which are perfect reflectors, are very feeble radiators as compared with dull or black bodies at the same temperature . If two bodies emit rays of different periods in different proportions, it is not at first sight easy to see how their radiations can See also:

• BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance each other at the same temperature . The See also:

• KEY
• KEY (in O. Eng. tang; the ultimate origin of the word is unknown; it appears only in Old Frisian kei of other Teutonic languages; until the end of the 17th century the pronunciation was kay, as in other words in O. Eng. ending in aeg; cf. daeg, day; clang
• KEY (the phraseograms being indicated by hyphens)
• KEY, SIR ASTLEY COOPER (1821-1888)
• KEY, THOMAS HEWITT (1799-1875)

key to all such difficulties lies in the fundamental conception, so strongly insisted on by See also:

• BALFOUR, FRANCIS MAITLAND (1851-1882)
• BALFOUR, ROBERT (known also as BALFOREUS) (1550?-1625?)
• BALFOUR, SIR JAMES (of Pittendreich) (d. 1583 Or 1584)
• BALFOUR, SIR JAMES, BART

Balfour See also:

• STEWART, ALEXANDER TURNEY (1803-1876)
• STEWART, BALFOUR (1828-1887)
• STEWART, CHARLES (1778–1869)
• STEWART, DUGALD (1753-1828)
• STEWART, J
• STEWART, JOHN (1749—1822)
• STEWART, JULIUS L
• STEWART, SIR DONALD MARTIN (1824–19o0)
• STEWART, SIR HERBERT (1843—1885)
• STEWART, SIR WILLIAM (c. 1540—c. 1605)
• STEWART, STUART
• STEWART, WILLIAM (c. 1480-c. 1550)

Stewart, of the absolute uniformity (qualitative as well as quantitative) of the full or See also:

• COMPLETE

complete radiation stream inside an impervious enclosure of uniform temperature .

It follows from this conception that the proportion of the full radiation stream absorbed by any body in such an enclosure must be exactly compensated in quality as well as quantity by the proportion emitted, or that the emissive and absorptive powers of any body at a given temperature must be precisely equal . A good reflector, like a polished metal, must also be a feeble radiator and absorber . Of the incident radiation it absorbs a small fraction and reflects the remainder, which together with the radiation emitted (being precisely equal to that absorbed) makes up the full radiation stream . A partly transparent material, like glass, absorbs part of the full radiation and transmits part . But it emits rays precisely equal in quality and intensity to those which it absorbs, which together with the transmitted portion make up the full stream . The ideal black body or perfect radiator is a body which absorbs all the radiation incident on it . The rays emitted from such a body at any temperature must be equal to the full radiation stream in an isothermal enclosure at the same temperature . Lampblack, which may absorb between 98 to 99 % of the incident radiation, is generally taken as the type of a black body . But a closer approximation to full radiation may be obtained by employing a hollow vessel the internal walls of which are blackened and maintained at a uniform temperature by a steam jacket or other suitable means . If a relatively small hole is made in the side of such a vessel, the radiation proceeding through the See also:

• APERTURE (from Lat. aperire, to open)

aperture will be the full radiation corresponding to the temperature . Such a vessel is also a perfect absorber . Of radiation entering through the aperture an infinitesimal fraction only could possibly emerge by successive reflection even if the sides were of polished metal internally .

A thin platinum See also:

• TUBE (Lat. tuba)

tube heated by an electric current appears feebly luminous as compared with a blackened tube at the same temperature . But if a small hole is made in the side of the polished tube, the light proceeding through the hole appears brighter than the blackened tube, as though the inside of the tube were much hotter than the outside, which is not the case to any appreciable extent if the tube is thin . The radiation proceeding through the hole is nearly that of a perfectly black body if the hole is small . If there were no hole the internal stream of radiation would be exactly that of a blackbody at the same temperature however perfect the reflecting power, or however low the emissive power of the walls, because the defect in emissive power would be exactly compensated by the internal reflection . Balfour Stewart gave a number of striking illustrations of the qualitative identity of emission and absorption of a substance . Pieces of coloured glass placed in a fire appear to lose their colour when at the same temperature as the coals behind them, because they compensate exactly for their selective absorption by radiating chiefly those See also:

• COLOURS, MILITARY

colours which they absorb . Rocksalt is remarkably transparent to thermal radiation of nearly all kinds, but it is extremely opaque to radiation from a heated plate of rocksalt, because it emits when heated precisely those rays which it absorbs . A plate of See also:

• TOURMALINE

tourmaline cut parallel to the See also:

• AXIS (Lat. for " axle ")

axis absorbs almost completely light polarized in a See also:

• PLANE

plane parallel to the axis, but transmits freely light polarized in a perpendicular plane . When heated its radiation is polarized in the same plane as the radiation which it absorbs . In the caseof incandescent vapours, the exact correspondence of emission and absorption as regards wave-length of frequency of the light emitted and absorbed forms the See also:

• FOUNDATION (Lat. fundatio, from fundare, to found)

foundation of the See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science of spectrum analysis . See also:

• FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer had noticed the coincidence of a pair of See also:

• BRIGHT, JOHN (1811-188g)
• BRIGHT, SIR CHARLES TILSTON

bright yellow lines seen in the spectrum of a See also:

• CANDLE (Lat. candela, from candere, to glow)

candle flame with the dark D lines in the solar spectrum, a coincidence which was afterwards more exactly verified by W . A .

See also:

• MILLER, HUGH (1802–1856)
• MILLER, JOAQUIN (CINCINNATUS HEINE) (1841- )
• MILLER, JOE (JosEPH or JosIAS( (1684-1738)
• MILLER, SAMUEL FREEMAN (1816-1890)
• MILLER, WILLIAM (1782-1849)
• MILLER, WILLIAM (1795-1861)
• MILLER, WILLIAM (1796–1882)
• MILLER, WILLIAM HALLOWES (18o1–188o)

Miller . See also:

• FOUCAULT, JEAN BERNARD LEON (1819—1868)

Foucault found that the flame of the electric arc showed the same lines bright in its spectrum, and proved that they appeared as dark lines in the otherwise continuous spectrum when the light from the See also:

• CARBON (symbol C, atomic weight 12)

carbon poles was transmitted through the arc . See also:

• STOKES, SIR G
• STOKES, SIR GEORGE GABRIEL, BART