AND MODERN PHYSICS. 127 



collision takes place and its motion is changed, the 

 viscosity will be considerable ; if, on the other hand, 

 the mean free path is small, the reverse will be tnie. 

 Thus it is possible to obtain a relation between 

 the mean free path and the coefficient of viscosity, 

 and from this, if the coefficient of viscosity be known, 

 a value for the mean free path can be found. 



Maxwell, in the paper under discussion, was the 

 first to do this, and, using a value found by Professor 

 Stokes for the coefficient of viscosity, obtained as the 

 length of the mean free path of molecules of air 

 4 4 w f an inch, while the number of collisions per 

 second experienced by each molecule is found to be 

 about 8,077,200,000. 



Moreover, it appeared from his theory that the co- 

 efficient of viscosity should be independent of the 

 number of molecules of gas present, so that it is not 

 altered by varying the density. This result Maxwell 

 characterises as startling, and he instituted an elaborate 

 series of experiments a few years later with a view of 

 testing it. The reason for this result will appear if 

 we remember that, when the density is decreased, the 

 moan free path is increased ; relatively, then, to the 

 total number of molecules present, the number which 

 cross the surface in a given time is increased And it 

 appears from Maxwell's result that this relative in- 

 crease is such that the total number crossing remains 

 unchanged. Hence the momentum conveyed across 

 each unit area per second remains the same, in spite 

 of the decrease in density. 



Another consequence of the same investigation is 

 that the coefficient of viscosity is proportional to the 



