508 Mr. W. Sutherland on the Viscosity 



of gases to be actually constant down to pressures as low as 

 one thousandth of an atmo. Naturally this confirmation gave 

 a great stimulus to the development of the kinetic theory; 

 and as the same equation which asserted that the viscosity of 

 a gas is independent of its pressure also asserted it to be 

 proportional to the square root of the absolute temperature, 

 the experimental examination of the relation between viscosity 

 and temperature was taken up with enthusiasm. When the 

 first experimental difficulties had been overcome, it was proved 

 quite clearly that with the natural gases the variation of 

 viscosity w T ith temperature is more rapid than was asserted 

 by theory ; instead of the relation 97 x T* it was found that 

 7} cc T\ where n ranges from its lowest value of about '7 for 

 hydrogen to about 1*0 for the less perfect gases. 



Maxwell, by some inaccurate experiments, was led to believe 

 that for the perfect natural gases rj cc T, and recast the kinetic 

 theory in a special form to bring it into harmony with this 

 supposed fact of nature. In the original form of the kinetic 

 theory the molecules are supposed to collide with one another 

 as small actual spherical bodies do, only with a coefficient of 

 restitution unity, to which actual bodies approximate but 

 never attain. Maxwell now supposed the molecules to behave 

 as centres of repulsive force, and deduced that if 77 cc T the 

 centres of force must repel one another with a force inversely 

 as the fifth power of the distance between them. But as 

 more accurate experiments proved that 77 does not vary as T, 

 the hypothesis of repulsion inversely as the fifth power had to 

 be abandoned. It is to be remembered that Maxwell probably 

 worked out the details of this hypothesis more for the sake of 

 illustrating the mathematical methods to be applied to centres 

 of force than for the actual results obtained. 



The only other hypothesis which has hitherto been advanced 

 to account for the discrepancy between theory and experi- 

 ment is that of 0. E. Meyer, who pointed out that if the 

 molecules, instead of being regarded as of constant size, were 

 supposed to shrink with increase of temperature, then the 

 experimental results would be explained. But the great 

 objection to this explanation was that it made the size of the 

 molecules vary far too much with temperature : for instance, 

 if cr is the sectional area of the sphere by which the hydrogen 

 molecule may be supposed to be replaced, 97 cc T^/a; that is 

 T 7 cc T 5 /cr, or the sectional area varies inversely as the fifth 

 root of the absolute temperature. No independent confirma- 

 tion of such great variability of molecular size has been given, 

 and has been tacitly regarded as hardly possible. 



