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Dr. Gladstone on Refraction-Equivalents. 



[June 18, 



energy of a body, notwithstanding change of temperature, aggregate con- 

 dition, solution, or even chemical combination, has been confirmed, and 

 upon this has been built the doctrine of Refraction-equivalents. 



Our " specific refractive energy " is the refractive index of any substance 



minus unity, divided by the density ; in symboUc language Pro- 

 fessor Landolt's " Refraction-equivalent is the same multiplied by the 



chemical equivalent, or P ^ ^ 



The largest generalization arrived at is that the refraction-equivalent of 

 a compound is the sum of the refraction-equivalents of its constituents. 

 This has been suflSciently 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 combinations of many other elementary 

 bodies*. 



My more recent researches have branched oif into ses^eral lines, but an 

 especial attempt has been made to answer the following questions. 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 determined 

 in a direct manner as those of gaseous hydrogen, liquid phosphorus, crys- 

 tallized carbon, and other transparent elements have been. iVn 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 ultim.ate result, and which necessi- 

 tated improved apparatus, and great care both in preparing the solutions 

 and in taking the observations. 



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

 presenting the chemical equivalent was dissolved in n equivalents of water. 



See Brit. Association Report, 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. 

 Schraui, Pogg. Annalen, vol. cxxvii. p. 344. Wiillner, Pogg. Annalen, vol. cxxxiii. p. 1. 



