138 EQUILIBRIUM OF HETEROGENEOUS SUBSTANCES. 



We shall see hereafter, when we come to consider the properties of 

 gases, that these equations may be verified experimentally in a very 

 large class of cases, so that we have considerable reason for believing 

 that they express a general law in regard to the limiting values of 

 potentials.* 



On Certain Points relating to the Molecular Constitution 



of Bodies. 



It not unfrequently occurs that the number of proximate com- 

 ponents which it is necessary to recognize as independently variable 

 in a body exceeds the number of components which would be 

 sufficient to express its ultimate composition. Such is the case, for 

 example, as has been remarked on page 63, in regard to a mixture 

 at ordinary temperatures of vapor of water and free hydrogen and 

 oxygen. This case is explained by the existence of three sorts of 

 molecules in the gaseous mass, viz., molecules of hydrogen, of 

 oxygen, and of hydrogen and oxygen combined. In other cases, 

 which are essentially the same in principle, we suppose a greater 

 number of different sorts of molecules, which differ in composition, 

 and the relations between these may be more complicated. Other 

 cases are explained by molecules which differ in the quantity of 

 matter which they contain, but not in the kind of matter, nor in 

 the proportion of the different kinds. In still other cases, there 

 appear to be different sorts of molecules, which differ neither in the 

 kind nor in the quantity of matter which they contain, but only 

 in the manner in which they are constituted. What is essential in 

 the cases referred to is that a certain number of some sort or sorts of 

 molecules shall be equivalent to a certain number of some other sort 

 or sorts in respect to the kinds and quantities of matter which they 

 collectively contain, and yet the former shall never be transformed into 

 the latter within the body considered, nor the latter into the former, 

 however the proportion of the numbers of the different sorts of 

 molecules may be varied, or the composition of the body in other 

 respects, or its thermodynamic state as represented by temperature 

 and pressure or any other two suitable variables, provided, it may 

 be, that these variations do not exceed certain limits. Thus, in the 



* The reader will not fail to remark that, if we could assume the universality of this 

 law, the statement of the conditions necessary for equilibrium between different 

 masses in contact would be much simplified. For, as the potential for a substance 

 which is only a possible component (see page 64) would always have the value - oo , 

 the case could not occur that the potential for any substance would have a greater 

 value in a mass in which that substance is only a possible component, than in another 

 mass in which it is an actual component ; and the conditions (22) and (51) might be 

 expressed with the sign of equality without exception for the case of possible 

 components. 



