8 Lord Bayleigh, [Jan, 17, 



The latter gas, prepared by Professor Eamsay, was brought from 

 London by Mr. W. Randall, who further gave valuable assistance in 

 the manipulations. It appeared at once that the refractivity of 

 helium was remarkably low, 13 inches pressure of the gas being 

 balanced by less than 2 inches pressure of air. The ratios given by 

 single comparisons on July 29 were 0* 14.7, 0*146, 0*145, 0*146, 

 mean 0*146; and on July 30, 0*147, 0* 147, 0*145, 0* 145, mean 

 0*146. The observations were not made under ideal concUtions, on 

 account of the smallness of the changes of air pressure ; but we may 

 conclude that with considerable approximation 



Refractivity of helium 



Refractivity of air 



= 0*146. 



The lowest refractivity previously known is that of hydrogen, 

 nearly * 5 of that of air. 



The viscosity was investigated by the method of passage through 

 capillary tubes. The approximate formula has been investigated by 

 O. Meyer,* on the basis of Stokes' theory for incompressible fluids. 

 If the driving pressure (p^ — ^2) is not too great, the volume Vg 

 delivered in time t through a tube of radius R and length A. is given 



by 



the volume being measured at the lower pressure ^.,5 ^^^ V denoting 

 the viscosity of the gas. In the comparison of different gases Vg, Pi-, 

 ^2) Rj ^ Diay be the same, and then 7/ is proportional to t. 



In the apparatus employed two gas pipettes and manometers, 

 somewhat similar to those shown in Fig. 2, were connected by a 

 capillary tube of very small bore and about 1 metre long. The 

 volume V2 was about 100 c.c. and was caused to pass by a pressure 

 of a few centimetres of mercury, maintained as uniform as possible 

 by means of the pipettes. There was a difficulty, almost inherent in 

 the use of mercury, in securing the right pressures during the first 

 few seconds of an experiment ; but this was not of much importance 

 as the whole time t amounted to several minutes. The ajjparatus was 

 tested upon hydrogen, and was found to give the received numbers 

 with sufficient accuracy. The results, referred to dry air, were for 

 helium 0*96; and for argon 1*21, somewhat higher than for 

 oxygen which at present stands at the head of the list of the principal 



In the original memoir upon argon | results were given of 

 weighings of the residue from the Bath gas after removal of oxygen, 

 carbonic anhydride, and moisture, from which it appeared that the 



* Pogg. Ann. vol. 127, p. 270, 1866. 



t Rayleigh and Ramsay, Phil. Trans. A, vol. 186, p. 227. 1895. 



