more kinds of Moving Particles among one another. 29 



even if we admit the original assumption that they are systems 

 of moving elastic particles, we have not very good evidence as 

 yet for the relation among the quantities A, B, C, and D. 



Prop. XX. To find the rate of diffusion between two vessels 

 connected by a tube. 



When diffusion takes place through a large opening, such as 

 a tube connecting two vessels, the question is simplified by the 

 absence of the porous diffusion plug ; and since the pressure is 

 constant throughout the apparatus, the volumes of the two gases 

 passing opposite ways through the tube at the same time must 

 be equal. Now the quantity of gas which passes through the 

 tube is due partly to the motion of agitation as in Prop. XIV., 

 and partly to the mean motion of translation as in Prop. XV. 



Let us suppose the 

 volumes of the two ves- 

 sels to be a and b, and 

 the length of the tube f a 



between them c, and its 

 transverse section s. Let 

 a be filled with the first 

 gas, and b with the second 

 at the commencement of 



the experiment, and let the pressure throughout the apparatus 

 be P. 



Let a volume y of the first gas pass from a to b, and a volume 

 y 1 of the second pass from b to a ; then if p l and p^ represent 

 the pressures in a due to the first and second kinds of gas, and 

 p\ and p'% the same in the vessel b, 



' P. . (48) 



which gives 



2/=2/ f and p l +p 2 =Y=p' 1 +p t < i . . . . (49) 



dv Axi 



The rate of diffusion will be + -^ for the one gas, and — -~ for 



the other, measured in volume of gas at pressure P. 

 Now the rate of diffusion of the first gas will be 



di~ s P = * P 



(50) 



