The Thermal Unit. 431 



the formula (11) has not been obtained from purely dynamical 

 first principles, but in addition to the usual dynamical prin- 

 ciples of the kinetic theory involves two unproved assumptions 

 — first that formula (10), which applies rigorously to a mixture 

 of two sets of molecules of equal mass but unequal size, can be 

 adapted to the case of two sets of unequal mass and size by 

 replacing the ratios jVg/i^i and 2 v \l2 v 2 by \ v <i l^/i^i i/^i an( i 

 2 v i 2A t i/ / 2 v 2 2^2, where 1 /jr L and ifi 2 are the average momenta 

 exchanged when 1 collides with 1 and with 2 ; and second, 

 that the relative molar velocity of collision of a 1 with a 2, 

 namely, * x + a 2 , is connected with the relative molar velocity 

 of collision of a 1 with a 1 (2aj) by the equation 



_ __ ^(«i+^) 2 =xVJ(2«i) s , 



where 1 V 2 and xVi are the mean relative molecular velocities 

 of a 1 and a 2 and of a 1 and a 1. A dynamical proof of these 

 two assumptions is required before the foregoing dynamical 

 theory of the viscosity of mixed gases can be considered 

 complete. 

 Melbourne, March 1895. 



XXXVIII. The Thermal Unit. By E. H. Griffiths, M.A., 

 F.R.S. y Sidney Sussex College, Cambridge* . 



[Plates VI. & VII.] 



[The following Paper was read at the Ipswich Meeting of the British 

 Association. The consideration of the whole matter has now been 

 referred to the Committee on Electrical Standards. 



The Author would be glad to receive any practical suggestions with a 

 view to their communication to that Committee.] 



Introductory Note. 

 rilHE majority of physicists will, I think, agree with the 

 J- statement that our present system of thermal measure- 

 ments rests on an unsatisfactory basis. Nevertheless the 

 attitude of the scientific world regarding this matter appears 

 to be one of indifference ; and it is open to speculation 

 whether this indifference arises from non-appreciation of the 

 importance of the subject, or from a sense of the difficulties 

 to be encountered if any action is taken. 



I am glad of this opportunity of directing the attention of 

 Section A to such an important question, for I believe the 

 obstacles to progress are not insurmountable, provided they 

 are attacked with the same spirit and energy which charac- 

 terized the efforts of this Association to perfect our system of 

 electrical measurements. The prevailing apathy is possibly 



* Communicated by the Author. 

 2 H 2 



