C6H5  .CH7 H . These facts may be explained in the same way as with

carbon, by admitting tetrahedral grouping . A special feature, however, wanting with carbon, is that compounds with one atom only of the element in question have been obtained as antipodes . A second observation of some interest is that the compounds in question are electrolytes and that, as in solutions, where they are split up into ions, activity must be due to the last, the ionic complex, for in-stance, R1R2R3S, must cause optical rotation . Optical antipodes have also been obtained with quinquevalent nitrogen in compounds of the type: RiRIR3RaNR6 . Le Bel observed these in methylethylpropyl-isobutylammonium chloride; since then Pope and Peachey and Wedekind studied the same question more thoroughly, and as a general result it is now stated that ammonium compounds with four different radicals behave as asymmetric carbon compounds . The explanation may be that the four radicals arrange themselves in the two possible tetrahedral configurations, and that the fifth element or group, e.g. chlorine or hydroxyl, more loosely linked, finds its fittest place, as shown in figs . 7 and 8 . 2 . Stereo-isomers Without Optical Activity.—The chief cases here belong to the derivatives of nitrogen with double linking and the metallic compounds which have been chiefly studied by Werner . The nitrogen compounds showing stereo-isomerism belong to two classes, according to the structural formulae, containing C:N or N :N ; in their general behaviour they seem related to the ethylene derivatives . The first group was detected by Victor Meyer and Goldschmidt in C6H6•C :NOH benzildioxime: CsHs• :NOH .

Later investigations, especially by Hantzsch, showed that a grouping R1•C •R2 R1•C•R2 X•N N X gives rise to stereo-isomerism, the supposed difference being that X is either more

close to R, or to R2 . This peculiarity is observed in the aldoximes and ketoximes, derived from aldehydes and ketones on treatment with hydroxylamine, and the two simplest examples are ethyl-aldoxime H3C•CH:NOH, and phenyl-benzyl-ketoxime, (C6H5) (C6H4CH2) C : NOH . As the behaviour of these stereo-isomers much resembles that of ethylene-compounds, they are often indicated as cis- and trans-forms . The second stereo-isomerism in nitrogen-compounds was detected by Schraube in potassium benzenediazotate, and may perhaps be reproduced by the following symbols: C6Hs•N and C6H6N KON NOK . The last group of stereo-isomers, in which insight is most difficult yet, is that of Werner's complex metallic compounds, observed with cobalt, platinum and chromium . No enantiomorphous character throws light here, and there is no relation to ethylene derivatives . With cobalt the fact is that in the hexammonic cobalt salts, e.g . Co(NH3)sC13, when NH3C1 is substituted by NO2 isomerism appears as soon as the number of substituents is two ; Jorgensen's flavo-salts Co(NHa)a(NO2)2C1, and Gibbs's isomeric croceo-salts offer examples . Werner puts forward that a grouping of (NH3)6 at the summits of a regular octahedron around Co may explain this . I00 48 14 o•8 42, Platinum compounds such as (H3N)i PtCl2 have been obtained in fig . 2) this point is seen simply, together with a number of other points which together form the so-called " horopter." According to Joh . Muller, Helmholtz, Hering, Volkmann and others, these are those points of the object-space (e.g .

Q and R in fig . 2), whose images fall on identical or corresponding spots on the retina, by which are meant those points on the retina whose

nerve filaments are united and which are equidistant in the same direction from the centre of the yellow spot (see EYE; VISION) . The horopter varies according to the position of the fixed spot in the object-space; for example, it is the ground two forms, Werner admitting here the following plane configurations ClPtz Cl Cl NH3 > and )Pt( HEN/ \NH3 H3N/ Cl shows a behaviour analogous to that of cobalt, and analogous space-formulae may be used here . But, in a general way, at present it is extremely difficult to decide upon their value . (annual since 1904) . (J .