70 



Lord Eayleigh. 



single atoms. From the fact that no appreciable part of the total 

 energy is rotatory, we may infer that the forces called into play 

 during our encounter are of a symmetrical character. It seemed, 

 therefore, more likely that a simple relation between viscosity and 

 temperature would obtain in the case of argon than in the case of the 

 " diatomic " gases. 



The best experimental arrangement for examining this question is 

 probably that of Holman,* in which the same constant stream of gas 

 passes in succession through two capillaries at different temperatures, 

 the pressures being determined before the first and after the second 

 passage, as well as between the two. But to a gas like argon, avail- 

 able in small quantities only, the application of this method is difficult. 

 And it seemed unnecessary to insist upon the use of constant pressures, 

 seeing that it was not proposed to investigate experimentally the 

 dependence of transpiration upon pressure. 



The theoretical formula for the volume of gas transpired, analogous 

 to that first given by Stokes for an incompressible fluid, was developed 

 by O. E. Meyer. t Although not quite rigorous, it probably suffices 

 for the purpose in hand. If pi, Vi denote the pressure and volume of 

 the gas as it enters the capillary, 2^2, ^^2 it leaves the capillary, we 

 have 



i^iVi =P-{V, = ^O^i^-iV) (4). 



In this equation t denotes the time of transpiration, E the radius 

 of the tube, I its length, and /a the viscosity measured in the usual 

 way. 



In order to understand the application of the formula for our pre- 

 sent purpose, it will be simplest to consider first the passage of equal 

 volumes of different gases through the capillary, the initial pressures, 

 and the constant temperature being the same. In an apparatus, such 

 as that about to be described, the pressures change as the gas flows, 

 but if the pressures are definite functions of the amount of gas which 

 at any moment has passed the capillary, this variation does not inter- 

 fere with the proportionality between t and /x. For example, if the 

 viscosity be doubled, the flow takes place precisely as before, except 

 that the scale of time is doubled. It will take twice as long as 

 before to pass the same quantity of gas. 



Although diff'erent gases have been employed in the present experi- 

 ments, there has been no attempt to compare their viscosities, and 

 indeed such a comparison would be difficult to carry out by this 

 method. The question has been, how is the viscosity of a given gas 

 affected l^y a change of temperature 1 In one set of experiments the 



* ' Phil. Mag. ' vol. 3, p. 81, 1877. 

 t ' Pogg. Ann.,' vol. 127, p. 269, 1866. 



