y= ^(1-eWir ^<" 



more kinds of Moving Particles among one anothev. 31 

 therefore in the ratio of 2 to 3, and their sum is 



d V ± /M si dp ... 



If we suppose -£ constant throughout the tube, or, in other 

 words, if we regard the motion as steady for a short time, then 

 -j- will be constant and equal to * l ~^ 1 j or substituting from 

 (48), _ 



whence 



/2k si 



a + b 



By choosing pairs of gases of equal density, and ascertaining 

 the amount of diffusion in a given time, we might determine the 

 value of / in this expression. The diffusion of nitrogen into 

 carbonic oxide or of deutoxide of nitrogen into carbonic acid, 

 would be suitable cases for experiment. The only existing ex- 

 periment which approximately fulfils the conditions is one by 

 Graham, quoted by Herapath from Brande's Quarterly Journal 

 of Science, vol. xviii. p. 76, 



A tube 9 inches long and 0*9 inch diameter, communicated 

 with the atmosphere by a tube 2 inches long and 0*12 inch dia- 

 meter; 152 parts of olefiant gas being placed in the tube, the 

 quantity remaining after four hours was 99 parts. 



In this case there is not much difference of specific gravity 



between the gases, and we have «=9 x (0*9) 2 -r cubic inches, 

 £= qo, c=2 inches, and 5=(0*12) 2 ^- square inches; 



^Vhl^lO.l.H^); . (57) 

 •\ /=000000256 inch = -J- inch. . . . (58) 



Prop. XXI. 2 o find the amount of energy which crosses unit of 

 area in unit of time when the velocity of agitation is greater on 

 one side of the area than on the other. 



The energy of a single particle is composed of two parts, — the 

 vis viva of the centre of gravity, and the vis viva of the various 

 motions of rotation round that centre, or, if the particle be 

 capable of internal motions, the vis viva of these. We shall sup- 

 pose that the whole vis viva bears a constant proportion to that 



