312 Royal Society : — 



—. — . Professor Landolt's " Refraction-equivalent " is the same 



multiplied by the chemical equivalent, or P — 



The largest generalization arrived at is that the refraction-equiva- 

 lent of a compound is the sum of the refraction-equivalents of its 

 constituents. This has been sufficiently proved in a multitude of 

 instances among the compounds of carbon, hydrogen, and oxygen, 

 and it has been shown, or assumed to be the case, in the combi- 

 nations of many other elementary bodies*. 



My more recent researches have branched off into several lines ; 

 but an especial attempt has been made to answer the following ques- 

 tions. Have any of the elements more than one definite refraction- 

 equivalent ? and what are the refraction-equivalents of the metallic 

 elements ? A large mass of observations bearing on these points 

 has been gathered together, and more or less collated ; but it is yet 

 imperfect, and my present object is rather to indicate the principal 

 method of inquiry than to publish the actual results. 



As the metals are opaque, their refractive indices cannot be deter- 

 mined in a direct manner as those of gaseous hydrogen, liquid phos- 

 phorus, crystallized carbon, and other transparent elements have 

 been. An attempt must therefore be made to determine their 

 effect on the rays of light by examining their compounds ; but their 

 crystalline salts are very frequently doubly refracting, owing to some 

 peculiarities of internal structure, and, where they give only one 

 spectrum, there are practical difficulties about the experiment that 

 are not encountered in dealing with liquid bodies. The solutions 

 of these salts have only one refraction ; and it occurred to me that 

 they might afford an easy means of determining the refraction-equiva- 

 lents, first, of the compounds themselves, and, secondly, of their 

 metallic and other constituents. In practice, many sources of error 

 presented themselves, all of which tell upon the ultimate result, and 

 which necessitated improved apparatus, and great care both in pre- 

 paring the solutions and in taking the observations. 



The method generally adopted was as follows : — An amount of 

 salt representing the chemical equivalent was dissolved in n equiva- 

 lents of water, and the refractive index and density of the solution 

 were taken. From these was reckoned the refraction-equivalent ; and 

 subtracting from this n times the refraction-equivalent of water for 

 the solar line A, there remained the refraction-equivalent of the dis- 

 solved salt for that part of the spectrum. That this fairly repre- 

 sents the action exerted on light by the chemical compound itself is 

 supported by several considerations. 1st. In the few cases (such as 

 chloride of sodium and sugar) where the refraction-equivalent of the 



* See Brit. Association Keport, 1863, Transactions of Sections, p. 12. Ibid. 

 1866, p. 37. Journal of the Chemical Society, ser. 2, vol. iii. p. 108. Landolt, 

 Pogg. Annalen der Physik und Chemie, vols, cxvii., cxxii., and cxxiii. Ketteler, 

 Ueber die Farbenzerstreuung der Gase, 1865. Haagen, Pogg. Annalen, vol. 

 cxxiii. p. 125. Schrauf, Pogg. Annalen, vol. cxxvii. p. 344. Wiillner, Pogg. 

 Annalen, vol. cxxxiii. p. 1. 



