82 



Mr. S. W. Holman on the Relation betiveen 



where M is the mass of a molecule, u the " velocity of mean 

 square" of the molecules, and s the distance between the 

 centres of two molecules at impact. This formula, if true, 

 shows that the viscosity of any gas is independent of the pres- 

 sure upon it at a constant temperature, and increases propor- 

 tionally to the value of u, i. e. to the \ power of the absolute 

 temperature. Maxwell, however, has shown*' that the ^ as- 

 sumption with regard to the nature of the impact determines 

 the value of the exponent in this expression. From the expe- 

 rimental investigators of the subject, this exponent, x, has re- 

 ceived the various values x=l, f, f, J; but no results which 

 I have yet seen furnish any accurate determination of the law. 



The fo lowing method, with which I have as yet made some 

 experiments on air only, is designed for the study of the va- 

 riation of the viscosity with the temperature of gases, and, 

 although capable of being used to furnish measurements in 

 absolute units of the coefficient, has been arranged for differen- 

 tial effects only, for which it possesses special advantages. 



Two glass capillary tubes, A B and C D, were placed side by 

 side, each in a tin trough to contain a bath to regulate the 

 temperature of the gas. Air-tight glass and rubber connectors 

 extended from G to the gauge F, and to the end D of one ca- 



ra 



pillary. The ends B and of the capillaries were connected 

 with the gauge E by means of a "]" joint of glass. The end A 

 of the second tube communicated with the external air through 

 the chloride-of-calcium tubes H and I. The size of the con- 

 nectors at the ends of A B and C D was sufficient to allow the 

 gas to assume the temperature of the bath. The tube at G 

 was connected with a large flask, from which the air was con- 

 tinuously exhausted by means of a Kichards's jet-aspirator. 

 The size of this flask rendered the pressure constant in spite of 



* Phil. Mag. [IV.] vol. xxxv. p. 211. 



