LEWIS. — THERMAL PRESSURE. 155 



In the meantime it is necessary to consider what advantage in theory 

 may be gained by adopting the pi-oposed assumption. According as we 

 deal with a pure substance or a mixture, the quantity Qnii) n will be a 

 measure of the total thermal pressure, or of the partial thermal pressure 

 of any one molecular species, if we so designate that pressure which may 

 be conceived to be exerted on either side of an infinitesimally thick 

 membrane in the interior of a homogeneous phase by the molecules of 

 that particular molecular species. The principle of thermal pressure 

 offered on page 153 may be put in the form of an equation, as, 



/3 = ^, (10) 



where /? is the partial or total thermal pressure, as the case may be, of 

 some one molecular species ; 7i is the number of gram molecules of this 

 species ; R is the gas constant ; T, the absolute temperature ; and V, the 

 total volume occupied. 



This idea could be otherwise expressed by an extension of the rule 

 of Avogadro, as follows : All substances at the same temperature and 

 the same thermal pressure have the same number of molecules in unit 

 volume. 



Equation (10), which, if correct, represents a universal principle of 

 nature, must be capable of very wide application. In the next section it 

 will be shown that by assuming the correctness of this equation, and with- 

 out any other hypothesis, it is possible to derive all the laws of dilute 

 solutions. It is well to emphasize that while equation (10) was shown 

 to be consistent with the kinetic theory in order that it might appear more 

 probable, still, having once assumed this equation, it is unnecessary 

 henceforth to adopt any kinetic view whatever. 



III. 



A Theory of Solutions. 



Notwithstanding the simplicity- of the phenomena of solutions, they 

 have as yet received no entirely adequate explanation. In fact, in some 

 explanations assumptions have been necessary that are inherently improb- 

 able. In the theory of solutions here presented it will be unnecessary 

 to assume either that the solute does or does not combine chemically 

 with the solvent in any way. It will only be necessary to suppose that 

 when n gram molecules of any substance are dissolved in a solvent, that 



