GROUP X  . Carbonic Acid.—Carbonic acid gas, carbonic oxide (CO) and some other irrespirable gases produce their effects practically by asphyxiation . When dissolved in water, however, carbonic acid gas is a gentle stimulant to the mouth, stomach and bowel, the mixture being absorbed more rapidly than plain water; hence its greater value in assuaging thirst . Nitrous oxide (laughing gas) was at one time believed to act simply by cutting off the supply of oxygen to the tissues, but it also has a specific effect in producing paralysis of certain parts of the central nervous system, and hence Its value as an anaesthetic; when given in small amounts mixed with air it produces a condition of exhilaration .