308 Free Path Phenomena [on. xvi 



Using the three formulae of 364, this reduces to 



F [l + (*)'(?)'] (743). 



122o V4?- 2 / \c l J 

 Now - is, roughly, the time of describing a free path, and is therefore of 







the order of magnitude of 3 x 10~ 10 seconds. For a sound of moderate pitch 



p is of the order of 3000, so that 4- i s of the order of 10~ 7 . For the sounds 



c 



of highest pitch which are audible, we may take p = 10 5 , so that still 



C 



remains of the order of 3 x 10~ 5 . It is therefore clear that for all sound, 

 the difference between expression (743) and the limiting velocity 7 is in 

 every way quite imperceptible, at any rate upon the supposition that the 



3& 2 

 factor (or the corresponding factor for molecules of different structure), 



is comparable with unity. 



