OE INTERNAL FRICTION OF AIR AND OTHER GASES. 257 



which shows that the viscosity is proportional to the absolute temperature very nearly. 

 The simplicity of the other known laws relating to gases waiTants us in concluding that 

 the viscosity is really proportional to the temperature, measured from the absolute zero 

 of the air-thermometer. 



These relations between the viscosity of air and its pressure and temperature are the 

 more to be depended on, since they agree with the results deduced by Mr. Graham from 

 experiments on the transpiration of gases through tubes of small diameter. The con- 

 stancy of the viscosity for all changes of density when the temperature is constant is a 

 result of the Dynamical Theory of Gases *, whatever hypothesis we adopt as to the 

 mode of action between the molecules when they come near one another. The rela- 

 tion between viscosity and temperature, however, requires us to make a particular 

 assumption with respect to the force acting between the molecules. If the molecules 

 act on one another only at a determinate distance by a kind of impact, the viscosity will 

 be as the square root of the absolute temperature. This, however, is certainly not the 

 actual law. If, as the experiments of Graham and those of this paper show, the visco- 

 sity is as the first power of the absolute temperature, then in the dynamical theory, 

 which is framed to explain the facts, we must assume that the force between two mole- 

 cules is proportional inversely to the fifth power of the distance between them. The 

 present paper, however, does not profess to give any explanation of the cause of the 

 viscosity of air, but only to determine its value in different cases. ' 



Experiments were made on a few other gases besides dry air. 



Damp air, over water at 70° F. and 4 inches pressure, was found by the mean of three 

 experiments to be about one-sixtieth part less viscous than dry air at the same tempe- 

 rature. 



Dry hydrogen was found to be much less viscous than air, the ratio of its viscosity to 

 that of air being "SISG. 



A small proportion of air mixed with hydrogen was found to produce a large increase 

 of viscosity, and a mixture of equal parts of air and hydrogen has a viscosity nearly equal 

 to yf of that of air. 



The ratio of the viscosity of dry carbonic acid to that of air was foimd to be "859. 



It appears from the experiments of Mr. Graham that the ratio of the transpiration 

 time of hydrogen to that of air is '4855, and that of carbonic acid to air -807. These 

 numbers are both smaller than those of this paper. I think that the discrepancy arises 

 from the gases being less pure in my experiments than in those of Graham, owing to 

 the difficulty of preventing air from leaking into the receiver during the preparation, 

 desiccation, and admission of the gas, Avhich always occupied at least an hour and a half 

 before the experiment on the moving disks could be begun. 



It appears to me that for comparative estimates of viscosity, the method of transpira- 

 tion is the best, although the method here described is better adapted to determine the 

 absolute value of the viscosity, and is less liable to the objection that in fine capillary 



* " Illustrations of the Dynamical Theory of Gases," Philosophical Magazine, Jan. 1860. 

 MDCCCLXVI. 2 y 



