280 



THE REALITIES OF MODERN SCIENCE 



Expressions have been derived for the diameter of 

 a molecule in terms of its mean free path. We shall 

 only indicate the line of reasoning which has been fol- 

 lowed in such derivations. If all the molecules in a 

 c.c. of gas are imagined to be evenly spaced there will 

 be in each row or column of the cube ^N molecules, 

 where N Q is the total number per c. c. The distance 



FIG. 34. 



between the centers of two adjacent molecules will 

 then be D 



Now imagine all the molecules 

 except one to be stationary in this configuration. 

 This molecule will trace out a cylinder having the same 

 diameter as itself, as illustrated in Fig. 34. The 

 average length of all the possible circumscribing cyl- 

 inders which just fall short of inclosing a portion of 

 another molecule, is therefore the mean free path 



