588 Prof. Kia-Lok Yen on the Coefficients of 



and since w = 2irn = 2tt/T, 



where T is the period of rotation, 



and since in the particular arrangement of the scale as 



described n 1lnT \ 



u = a 'ZD, 



the equation may be written in the following form : 



7rI(V--fl 2 WT 



V ~ 4da 2 b 2 7rtH) 

 o r _ K aP-b 3 ) dT 



V ~ Ua 2 b 2 ' Dt 2 ' 



In this equation the terms I, a, b, and / were determined 

 once for all when the apparatus was constructed — excepting, 

 of course, when the need of a check is felt. Therefore the 

 equation is reduced to the form : 



v = KdT/Dt, 



where K = l(a 2 -b 2 )/Ua 2 b 2 , 



d = the arc in cm. through which the inner cylinder 



is deflected. 

 T = time for one rotation of the outer cylinder, 

 t = period of vibration of the inner cylinder, 

 and D = distance in cm. from the centre of the mirror 



to the transparent scale. 

 The last four quantities d, T, t. and D are the quantities to 

 be measured in each observation. 



The Viscosity of Hydrogen, 



Two different methods were used in generating this gas. 

 First it was generated from the electrolysis of distilled water, 

 and then, when the evolution of gas was found too slow and 

 also for reasons of other inconveniences, the Kipp generator 

 with zinc and hydrochloric acid was resorted to. As it is of 

 great importance that the gas should have the highest possible 

 degree of purity, the gas thus generated was drawn through 

 a series of purifying agents before it was finally introduced 

 into the apparatus. Provisions were made for re-purifying 

 the gas thus once purified, but the repetition of the process 

 was found unnecessary. 



The effect of impurities upon the viscosity will be discussed 

 in a later paper on a The Viscosity of Gaseous Mixtures." It 

 suffices to state here, in order to show the importance of the 

 purifying process, that a slight admixture of a second gas 

 would cause an enormous change in the viscosity. For 

 example, the addition of five per cent., by volume, of nitrogen 



