164 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



or pressure. Cases in which these quantities are not thus independently 

 variable will be considered hereafter. 



Inferences in regard to Potentials in Liquids and Solids. 



Such equations as (264), (268), (276), by which the values of 

 potentials in pure or mixed gases may be derived from quantities 

 capable of direct measurement, have an interest which is not confined 

 to the theory of gases. For as the potentials of the independently 

 variable components which are common to coexistent liquid and 

 gaseous masses have the same values in each, these expressions will 

 generally afford the means of determining for liquids, at least approxi- 

 mately, the potential for any independently variable component which 

 is capable of existing in the gaseous state. For although every state 

 of a liquid is not such as can exist in contact with a gaseous mass, it 

 will always be possible, when any of the components of the liquid are 

 volatile, to bring it by a change of pressure alone, its temperature and 

 composition remaining unchanged, to a state for which there is a 

 coexistent phase of vapor, in which the values of the potentials of the 

 volatile components of the liquid may be estimated from the density 

 of these substances in the vapor. The variations of the potentials in 

 the liquid due to the change of pressure will in general be quite 

 trifling as compared with the variations which are connected with 

 changes of temperature or of composition, and may moreover be 

 readily estimated by means of equation (272). The same consider- 

 ations will apply to volatile solids with respect to the determination 

 of the potential for the substance of the solid. 



As an application of this method of determining the potentials 

 in liquids, let us make use of the law of Henry in regard to the 

 absorption of gases by liquids to determine the relation between 

 the quantity of the gas contained in any liquid mass and its potential. 

 Let us consider the liquid as in equilibrium with the gas, and let 

 mS G) denote the quantity of the gas existing as such, m^ the quantity 

 of the same substance contained in the liquid mass, fa the potential 

 for this substance common to the gas and liquid, v (0} and v (L) the 

 volumes of the gas and liquid. When the absorbed gas forms but 

 a very small part of the liquid mass, we have by Henry's law 



m< L) X G) 



(294) 



where A is a function of the temperature ; and by (276) 



m (G) 



(295) 



