CONDENSATION AND CRITICAL PHENO- 

 MENA. 



II. MIXTURES OF TWO SUBSTANCES. 



THE phenomena which are displayed by mixtures 

 during condensation and near the critical point are 

 more complicated than those of single substances, so much 

 so that a complete explanation of the behaviour of mixtures 

 was not obtained till the phenomena were unravelled by the 

 application of thermodynamics. 



Suppose a certain volume of carbonic acid is mixed with 

 a fifth of its volume of air at o°C. and one atmosphere. Let 

 us define the composition of a mixture as the volume of one 

 of the constituents, here say carbonic acid, in unit volume 

 of the mixture. The composition x of this particular 

 mixture would then be 5/6. Its properties were investigated 

 by Cailletet and afterwards by the writer of this article. 

 At the temperatures of the experiments, air behaves like a 

 single gas, and the mixture accordingly like a mixture of two 

 substances. When this mixture is compressed at io°C. the 

 volume diminishes regularly until at a certain moment some 

 liquid is formed. This liquid consists of liquid carbonic 

 acid with some air dissolved in it. As compression goes 

 on the quantity of the liquid increases, but the pressure in- 

 stead of remaining constant during the process of condensa- 

 tion, as with sino'le substances, CToes on rising all the time 

 until the whole of the mixture is in the liquid state. As 

 was shown in the first part of this article even a small 

 quantity of a foreign body is sufficient to bring about an 

 increase of the condensation pressure at diminishing volume. 

 For an actual mixture like the one which we are considering 

 this rise of pressure is much more marked, and amounts to 

 several atmospheres. If we look upon the pressure in the 

 usual way as consisting of the sum of the partial pressures 

 for the two constituents we can easily understand why the 

 pressure should be higher the smaller the volume. The 

 partial pressure for carbonic acid is equal to the vapour 



