348 KXPKRIMKNTB OS DIFFUSION IN RELATION TO 



nuJfculf, or, more concisely, the mean path. Its value, according to my cal- 

 culations, is 



1* 



Its value in tenth-metres (1 metre x 10~ w ) is 



TABLE III. 



For Hydrogen . . . 965 Tenth-metres at C. and 760 B. 



For Oxygen . . . 560 



For Carbonic Oxide . . 482 



For Carbonic Acid . . 430 



(The wave-length of the hydrogen ray F is 4,861 tenth-metres, or about ten 

 times the mean path of a molecule of carbonic oxide.) 



We may now proceed for a few steps on more hazardous ground, and 

 inquire into the actual size of the molecules. Prof. Loschmidt himself in his 

 paper "Zur Grosse der Luftmoleciile " (Acad. Vienna, Oct. 12, 1865), was the 

 first to make this attempt. Independently of him and of each other, Mr G. J. 

 Stoney (Phil Mag., Aug. 1868), and Sir W. Thomson (Nature, March 31, 1870), 

 have made similar calculations. We shall follow the track of Prof. Loschmidt. 



TT 



The volume of a spherical molecule is - s 8 , where s is its diameter. Hence 

 if N is the number of molecules in unit of volume, the space actually filled 

 by the molecules is ^Nsf. 



This, then, would be the volume to which a cubic centimetre of the gas 

 would be reduced if it could be so compressed as to leave no room whatever 

 between the molecules. This, of course, is impossible ; but we may, for the 

 sake of clearness, call the quantity 



(8) 



The difference between this value and that given by M. Clausius in his paper of 1858, arises 

 from his assuming that all the molecules have equal velocities, while I suppose the velocities to be 

 distributed according to the " law of errors." 



