566 JAMES CLERK MAXWELL. 



description of the experiments and the statement of the 

 result arrived at form the subject of the Bakerian lecture of 

 1866. There plates of glass were suspended by a steel piano 

 wire, and caused to oscillate in their own planes between four 

 fixed plates, which were placed at equal distances apart, and 

 so that the moving plates were suspended half way between 

 each pair of fixed plates (see Part I. p. 341). The distance 

 between the plates was varied in different experiments, and 

 the whole system was enclosed in a receiver which could be 

 filled with different gases and the pressure regulated at will. 

 The oscillations of the discs were observed by means of a 

 mirror and scale, and the experiment consisted in determin- 

 ing the rate at which the oscillations died away, from which 

 the viscosity of the gas was calculated. For experiments 

 at high temperatures the receiver was surrounded by a steam 

 jacket. The results arrived at showed that dry air is more 

 viscous than damp air, and that the viscosity of air is 

 greater than that of hydrogen or carbonic acid, nearly in the 

 same ratio as was determined by Graham in his experiments 

 on the transpiration of gases through capillary tubes. But 

 the most remarkable results were (1), that the viscosity of 

 any particular gas is independent of the pressure; and (2) 

 that it is directly proportional to the temperature measured 

 from the absolute zero of the air thermometer. 1 From the 

 last result, Maxwell deduced " that the force between two 

 molecules is proportional inversely to the fifth power of the 

 distance between them." If the molecules only acted by 

 impact, it would follow that the viscosity must be pro- 

 portional to the square root of the absolute temperature. 



In the paper " On the Dynamical Theory of Gases," read 

 before the Eoyal Society on May 31, 1866, the molecules 

 are dealt with in the most general manner, no hypothesis 

 being made respecting their constitution. The paper dis- 

 cusses (1) the phenomena of diffusion, depending on the 

 average value of the velocities ; (2) the phenomena depend- 

 ing on the average square of the velocities, which deter- 



1 More recent experiments indicate that this statement should be 

 modified. 



