166 Dr. J. H. Gladstone on the Specific Refraction 



rimental errors are accumulated on these residual numbers. 

 The optical determinations of Soret are so uniform and exact 

 as to inspire the greatest confidence ; but he is not satisfied 

 with the specific gravities. Errors may also arise from im- 

 purity of the salt, or want of homogeneity in the crystal. An 

 aqueous solution is probably a more uniform substance than a 

 hydrated crystal, and better fitted for the purpose of deter- 

 mining optical equivalents. 



On this account any determination of the refraction of 

 indium and gallium made from these alums must be open to 

 considerable question : the rubidium compounds of these 

 metals, however, are believed by Soret to be fairly pure salts, 

 and we can compare them with three other alums of rubidium . 

 It would appear from the first table that indium has the 

 value of aluminium + 8*0, or of chromium + 1*5, or of iron 

 — 1*8 ; and, similarly, that gallium has the value of aluminium 

 + 5*4, or of chromium — 1*1, or of iron —4*4. Now alumi- 

 nium sulphate has already been estimated at 70*5, chromium 

 sulphate at 82*5, and ferric sulphate at 89'1, while the value 

 of S0 4 is held both by Kanonnikoff and myself to be 17*0. We 

 may therefore deduce the following refraction-equivalents : — 

 Aluminium . . . . 9*7 

 Chromium .... 15'7 

 an d :_ Iron . 19-0 







From Aluminium 



Salt. 



From Chromium 

 Salt. 



From Iron Salt. 



Indium 



Gallium 



177 



151 



17-2 

 14-6 



17-2 

 146 



From the mean of the above the following constants may be 

 derived : — 





Atomic 

 weight. 



Specific 

 refraction. 



Refraction- 

 equivalent. 



Indium 



1136 

 698 



•1532 

 •2120 



17*4 

 14-8 



Gallium 





These numbers can only be looked upon as approximate. 



Turning to the matter of dispersion. — It has already been 

 shown that the refraction-equivalent of an alum is the sum of 

 the refraction-equivalents of its constituents for the line A. 

 If this law holds good equally for the more-refrangible part 



