32 MOLECULAR MOTION AND ITS ENERGY 17 



But the values of the pressures which the components 

 of the mixture would severally exert if separately occupying 

 the volume of the mixture are 



Pi = f#i> P* = f K v - - J 

 whence 



P=Pi +P*+ -> 



or the pressure exerted by the mixture is equal to the sum of 

 the pressures separately exerted by its several components. 



Thus the law found by Dal ton 1 for the pressure of 

 mixed gases, and confirmed and defended by Henry 2 

 against many attacks, follows as a necessary consequence 

 from the assumptions underlying our theory. Of these one 

 was assumed by Dal ton, viz. that molecules of different 

 gases act on each other neither attractively nor repulsively, 

 and it is the most important for our present case ; for it is 

 this assumption which entitles us to consider that we have 

 to take into account kinetic, and not -also potential, energy. 



That this assumption is not absolutely exact, but only 

 approximate, has already several times been pointed out 

 [ 4, 8]. Just as Boyle's law, this of Dalton can have 

 only a limited validity. 3 



Dal ton's hypothesis, that molecules of different gases 

 neither attract nor repel each other, is often taken to mean 

 that they do not exert pressure on each other. This is 

 quite inadmissible on our theory, for molecules of different 

 kinds moving about in a given volume collide with each 

 other just as much as if they were of the same kind, and, 

 consequently, must exert pressure on each other, as pressure 

 is nothing but the sum of the actions produced by impact. 



And this, reading of Dalton' s hypothesis is also in 

 disagreement with experiment ; for different gases do really 



1 Mem. of the Manch. Lit. and Phil. Soc. v. 1802, p. 535 ; Gilb. Ann. xii. 

 1802, p. 385. 



2 Nicholson's Journal, viii. 1804, p. 297 ; Gilb. Ann. xxi. 1805, p. 393. 



3 Kegnault, Mem. de VAcad. xxvi. 1862, p. 256 ; Andrews, Phil. Trans. 

 clxxviii. 1887, p. 45; Cailletet, Journ. de Pliys. [1] ix. 1880, p. 192; 

 Galitzine, Ueber das Dalian 1 sche Gesetz, Inaug. Diss. Strassburg, 1890 : Wied. 

 Ann. xli. 1890, pp. 588, 770 ; Gott. Nachr. 1890, No. 1 ; U. L al a , Comptes Rendus, 

 cxi. 1890, p. 819, cxii. 1891, p. 426 ; Nalurw. Rundschau, vii. 1892, p. 188. 



