206 



Lord Rayleigh. 



[Jan. 16, 



the changes of air pressure ; but we may conclude that with consider- 

 able approximation 



Refractivity of helium _ Q.-j^g 

 Refractivity of air 



The lowest refractivity previously known is that of hydrogen, 

 nearly 0*5 of that of air. 



Viscosity of Argon and Helium. 



The viscosity was investigated by the method of passage through 

 capillary tubes. The approximate formula has been investigated by 

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

 If the driving pressure (p x — p 2 ) is not too great, the volume Y 2 

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



2p 2 8 7 A, 



the volume being measured at the lower pressure p 2 , and n denoting 

 the viscosity of the gas. In the comparison of different gases V 2 , p\, 

 p 2 , R, X. may be the same, and then r/ is proportioual 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 Y 2 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 apparatus was 

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

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

 96; and for argon 121, somewhat higher than for oxygen which 

 at present stands at the head of the list of the principal gases. 



Gas from the Bath Springs. 



In the original memoir upon argonf results were given of weigh- 

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

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

 proportion of argon was only one-half of that contained in the 

 residue, after similar treatment, from the atmosphere. After the 

 discovery of helium by Professor Ramsay, the question presented 



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



f Kayleigh und Kamsay, ' Phil. Trans.,' A, vol. 186, p. 227, 1895. 



