OF AIR AND OTHER GASES. H 



The ratio of the viscosity at the upper temperature to that at the lower was 



1-2624, 



which shews that the viscosity is proportional to the absolute temperature very 

 nearly. The simplicity of the other known laws relating to gases warrants 

 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 constancy 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 relation 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 

 shew, the viscosity 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 molecules 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 temperature. 



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

 its viscosity to that of air being '5156. 



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 fj- of that of air. 



The ratio of the viscosity of dry carbonic acid to that of air was found 

 to be '859. 



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



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