84 Royal Society. 



constant pressure. The value of a increases with the pressure 

 and it is greater at lower than at higher temperatures. But a 

 remarkable relation exists between the coefficients in the present 

 case which does not exist between the coefficients obtained from 

 the expansion of the gas. The values of a, deduced for the same 

 range of temperature from the elastic forces at different pressures, 

 are directly proportional to one another. We have, in short, 

 0-004367 ^.^ 0-04804 481> 



0-004604 ' 0-05067 



How far this relation will be found to exist under other condi- 

 tions of temperature and pressure will appear when experiments 

 now in progress are brought to a conclusion. 



Law of Dalton.— This law, as originally enunciated by its author, 

 is, that the particles of one gas possess no repulsive or attractive 

 power with regard to the particles of another. " Oxygen gas, 

 he states, " azotic gas, hydrogenous gas, carbonic acid gas, aqueous 

 vapour, and probably several other elastic fluids may exist in com- 

 pany under any pressure and at any temperature without any regard 

 to their specific gravities, and without any pressure upon one 

 another." The experiments which I have made on mixtures of 

 carbonic acid and nitrogen have occupied a larger portion of time 

 than all I have yet referred to. They have been carried to the 

 great pressure of 283-9 atmospheres, as measured in glass tubes 

 by a hydrogen manometer, at which pressure a mixture of^3 volumes 

 carbonic acid and 4 volumes nitrogen was reduced at 7°'6 to -^j 

 of its volume without liquefaction of the carbonic acid. As this 

 note has already extended to an unusual length, I will not now 

 attempt to give* an analysis of these experiments, but shall briefly 

 state their general results. The most important of these results 

 is the lowering of the critical point by admixture with a non- 

 condensable gas. Thus in the mixture mentioned above of carbonic 

 acid and nitrogen, no liquid was formed at any pressure till the 

 temperature was reduced below -20° C. Even the addition of 

 only T l - of its volume of air or nitrogen to carbonic acid gas will 

 lower the critical point several degrees. Finally, these experi- 

 ments leave no doubt that the law of Dalton entirely fails under 

 high pressures, where one of the gases is at a temperature not 

 greatly above its critical point. The anomalies observed in the 

 tension of the vapour of water when alone and when mixed with 

 air find their real explanation in the fact that the law of Dalton is 

 only approximately true in the case of mixtures of air and aqueous 

 vapour at the ordinary pressure and temperature of the atmo- 

 sphere, and do not depend, as has been alleged, on any disturbing 

 influence produced by a hygroscopic action of the sides of the 

 containing vessel. The law of Dalton, in short, like the laws of 

 Boyle and Gray-Lussac, only holds good in the case of gaseous 

 bodies which are at feeble pressures and at temperatures greatly 

 above their critical points. Under other conditions these laws 

 are interfered with ; and in certain conditions (such as some of 

 those described in this note) the interfering causes become so 

 powerful as practically to efface them. 



