Mr. Maxwell 07i the Viscosity of Air, ^c. 



[Feb. 8, 



air. I have never succeeded in filling my apparatus with perfectly pure 

 hydrogen, for air leaks into the vacuum during the admission of so large 

 a quantity of hydrogen as is required to fill it. The ratio of the viscosity 

 of my hydrogen to that of air was -5156. That obtained by Graham was 

 •4855. 



4. The ratio for carbonic acid was found to be '859. Graham makes 

 it "807. It is probable that the comparative results of Graham are more 

 exact than those of this paper, owing to the difficulty of introducing so 

 large a volume of gas without letting in any air during the time of filling 

 the receiver. I find also that a very small proportion of air causes a con- 

 siderable increase in the viscosity of hydrogen. This result also agrees 

 with those of Mr. Graham. 



5. Forty experiments on dry air were investigated to determine whether 

 any slipping takes place between the glass and the air in immediate con- 

 tact with it. 



The result was, that if there were any slipping, it is of exceedingly small 

 amount ; and that the evidence in favour of the indicated amount being 

 real is very precarious. 



The results of the hypothesis, that there is no slipping, agree decidedly 

 better with the experiments. 



6. The actual value of the coefficient of viscosity of dry air was deter- 

 mined, from forty experiments of five different kinds, to be 



/7 = -0000149 (461° + 0), 

 where the inch, the grain, and the second are the units, and the tempera- 

 ture is on Fahrenheit's scale. 

 At 62° this gives '007802. 



Professor Stokes, from the experiments of Baily on pendulums, has 

 found 



which, with the average temperature and density of air, would give 



;z=-00417, 



a much smaller value than that here found. 



If the value of is expressed in feet instead of inches, so as to be uni- 

 form with the Britiv.h measures of magnetic and electric phenomena, as 

 recorded at the observatories, 



/u=-OOO179(461 + 0) 

 = •08826 at 32°. 

 In metre-gramme-second measure and Centigrade temperature, 



yu = -0l878 (1 + -00366 6)). 

 M. O. E. Meyer (Pogg. Ann. cxiii. (1861) p. 383) makes fi at 18° C. 

 = •000360 in centimetres, cubic centimetres of water, and seconds as units, 

 or in metrical units, /[« = ^0360. 



According to the experiments here described, fx at 18°C, = ^02. 



