ON WATKi; AM) ITS Co.MI'orNliS 



79 



26'6 cubic centimetres (instead of 36) ; and under 30 atmospheres, 33'7 

 cubic centimetres. 31 However, as the researches of Sechenoff show, 

 the absorption of carbonic anhydride within certain limits of change 

 of pressure, and at the ordinary temperature, by water and even by 

 solutions of salts which are not chemically changed by it, or do not 

 form compounds with it very closely follows the law of Henry and 

 Dalton, so that the chemical bond between this gas and water is so 

 feeble that the breaking up of the solution with separation of the gas 

 is accomplished by a decrease of pressure alone. 32 The case is different 

 if a considerable affinity exists between the dissolved gas and water. 

 Then it might even be expected that the gas would not be entirely 

 separated from water in a vacuum, as should be the case with gases 

 according to the law of Henry and Dalton. Such gases and, in 

 general, all which are very soluble exhibit a distinct deviation from 

 the law of Henry and Dalton. As examples, ammonia and hydro- 

 chloric acid gas may be taken. The former is separated by boiling and 

 decrease of pressure, while the latter is not, but they both deviate dis- 

 tinctly from the law. 



It will be remarked, for instance, from this table that whilst the pres- 



31 These figures show that the co-efficient of solubility decreases with an increase of 

 pressure, notwithstanding that the carbonic anhydride approaches a liquid state. And, 

 indeed, liquefied carbonic anhydride does not intermix with water, and does not exhibit a 

 rapid increase in solubility at its temperature of liquefaction. This indicates, in the first 

 place, that solution does not consist in liquefaction, and in the second place that the solu- 

 liility of a substance is determined by a peculiar attraction of water for the substance 

 dissolving. Wroblewski even considers it possible to admit that a dissolved gas retains 

 its properties as a gas. This he deduces from his experiments, which showed that the 

 rate of diffusion of gases in a solvent is, for gases of different densities, inversely propor- 

 tional to the square roots of their densities, just as the velocities of movement of gaseous 

 molecules (see Note 34 on p. 80). Wroblewski showed the affinity of water, H 2 O, for carbonib 

 anhydride, COo, from the fact that on expanding moist compressed carbonic anhydride 

 (compressed atO under a pressure of 10 atmospheres) he obtained (a fall in temperature 

 takes place from the expansion) a very unstable definite crystalline compound, COo + 8H 2 O. 



32 As, according to the researches of Roscoe and his collaborators, ammonia exhibits 

 a considerable deviation at low temperatures from the law of Henry and Dalton, whilst 

 at 100 the deviation is small, it would appear that the dissociating influence of tem- 

 perature tells on all gaseous solutions ; that is, at high temperatures, the solutions of 

 all gases will follow the law, and at lower temperatures there will in all cases be a 

 deviation from it. 



