1881.] On the Viscosity of Gases at High Exhaustions. 447 



before the British Association in 1859, Maxwell* presented the re- 

 markable resnlt that on theoretical grounds the coefficient of friction, 

 or the viscosity, should be independent of the density of the gas, 

 although at the same time he states that the only experiments he had 

 met with on the subject did not seem to confirm his views. 



An elaborate series of experiments were undertaken by Maxwell to 

 test so remarkable a consequence of a mathematical theory ; and in 

 1866, in the Bakerian Lecbure for that year,f he published the results 

 under the title of " The Viscosity or Internal Friction of Air and 

 other Gases." He found the coefficient of friction in air to be practi- 

 cally constant for pressures between 30 inches and - 5 inch ; in fact 

 numbers calculated on the hypothesis that the viscosity was inde- 

 pendent of the density agreed very well with the observed values. 



The apparatus used by Maxwell was not of a character to admit of 

 experiments with much lower pressure than 05 inch. 



Maxwell's theory, that the viscosity of a gas is independent of the 

 density, presupposes that the mean length of path of the molecules be- 

 tween their collisions is very small compared with the dimensions of the 

 apparatus ; but inasmuch as the mean length of path increases directly 

 with the expansion, whilst the distance between the molecules only 

 increases with the cube root of the expansion, it is not difficult with the 

 Sprengel pump to produce an exhaustion in which the mean free path 

 is measured by inches, and even feet, and at exhaustions of this 

 degree it is probable that Maxwell's law would not hold good. 



The experiments recorded in this paper were commenced early in 

 1876, and have been continued to the present time. In November, 

 1876, the author gave a note to the Royal Society on some preliminary 

 results. Several different forms of apparatus have since been used 

 one after the other, with improvements and complexities suggested by 

 experience or rendered possible by the extra skill acquired in manipu- 

 lation. The earlier observations are now of little value, but the time 

 spent in their prosecution was not thrown away, as out of those experi- 

 ments has grown the very complicated apparatus now finally adopted. 



The viscosity torsion apparatus with which all the experiments here 

 given have been performed, is a very complicated instrument, and 

 cannot be well understood without the accompanying drawings. It 

 consists essentially of a glass bulb, blown with a point at the lower- 

 end, and sealed on to a long narrow glass tube. In the bulb is sus- 

 pended a plate of mica, by means of a fine fibre of glass, 26 inches 

 long, which is sealed to the top of the glass tube, and hangs vertically 

 along its axis. The plate of mica is ignited and lamp- blacked over 

 one-half. The tube is pointed at the upper end,, the upper and lower 

 points are 46 inches apart, and are accurately in the prolongation of 



* " Phil. Mag.," 4th ser., vol. six, p. 31. 

 f " Phil. Trans.," 1866, Part I, p. 249. 



