Viscous Efect of Strains Mechanically Applied. 155 



Returning to the formula of the first approximation, 



(w-l)m/^=(N-l)U, 

 we get 



(n-l)/c/=(N-l)U/m=(N-l)/D, 



where D is the density of the matter in the atom; so that we 

 see that the quantity (n — l)/d, which is called the specific 

 refractive power of a substance, is equal to the specific refrac- 

 tive power of a single atom of it. Under these circumstances 

 the specific refractive powers of the elements call for study 

 as much as the refraction-equivalents. 



The general results arrived at in this paper are, then, that 

 Gladstone's formula is the best yet advanced, that there is a 

 theoretical foundation for it, and that it can be improved to 

 the form (n — l)/d = A + ^d, where A and B are constants ; 

 that chemists ought to study the refraction of substances 

 in the state of vapour in order to obtain the necessary data 

 for the investigation of refraction-equivalents and the 

 questions of structure connected therewith ; as the practical 

 difficulties in the way of measuring the refractive index and 

 density of vapours are great, the values of A and B in the 

 above formula may be obtained by experiments on liquids at 

 temperatures as far apart as possible, and that where a 

 measurement at only one temperature is available the Lorenz 

 formula may be assumed on empirical grounds to give two 

 thirds of the desired value of A, but there is no guarantee 

 that this will always be the case. 



XVIII. The Viscous Effect of Strains Mechanically Applied, 

 as Interpreted by Maxwell's Theory. By C. Barus^. 



I. The two Species of Molecular Break-up which promote 

 Viscous Deformation. 



1. TT^OLLOWING the argument which underlies Maxwell's 

 J- theory of solid viscosity experimentally, I was obliged 

 to take cognisance of two causes which promote viscous de- 

 formation in solids f. For any structure will give way under 

 impressed conditions of stress, as a whole or in part, either 

 because the cements are insufficiently strong, or because the 

 bricks are insufficiently strong to withstand it. Similarly 

 the underlying cause of viscous motion is either such struc- 

 tural change in which groups of molecules pass without loss 



* Communicated by the Author. 

 t Phil. Mag. November, 1888. 



