274 SCIENCE PROGRESS 



through a medium should be a simple function of the most 

 fundamental properties of the atoms of which it is composed, 

 and that by comparing the retardation when two elements are 

 present but uncombined with that which is caused when com- 

 bination has taken place, we may even obtain some hint of what 

 takes place in chemical combination. 



The subject of physico-chemical research by means of refrac- 

 tive indices may be divided into two main branches : that which 

 deals with solids and liquids, and that which deals with gases 

 and vapours. In this paper it is proposed to give an outline 

 of the more recent developments of the latter division of the 

 subject ; but it is impossible to make clear the present position 

 of the problems involved without a brief summary of the work 

 which has been done on the former. 



The first observation of importance in this subject was made, 

 as usual, by Newton, who remarked that inflammable substances 

 had high indices of refraction, and thence deduced that the dia- 

 mond, which had such an index, would prove to be inflammable. 



But in Newton's day the science of chemistry did not exist, 

 and we have to pass over a century before we come to the first 

 work, which is still of intrinsic value — the research of Biot and 

 Arago on the refractive indices of gases in 1806. This was 

 followed, in 1826, by the much wider research of Dulong; but 

 in both cases the range of substances examined was com- 

 paratively small, and the results were so fragmentary and 

 disconnected that they failed to reveal any orderly connection 

 between the indices of the elements or between the refractivities 

 of compounds and those of their constituents. 



With the rapid growth of the science of chemistry during the 

 first half of the nineteenth century, it soon became evident that 

 the study of the refractive indices of liquids, and in some cases 

 of solids, offered a much wider field for research than that of 

 gases, and, accordingly, the attention of experimenters was 

 chiefly directed to this branch of the subject. Between i860 and 

 1885 an immense mass of work was done in this department, 

 and results of considerable importance were reached. It is only 

 possible to glance at a few of the most important of these efforts. 

 In 1858 and 1863 Gladstone and Dale published researches on 

 the relation between refractive index and temperature, the 

 refractive indices of mixtures, the comparative refractivities of 

 different members of homologous series and of isomerides, and 



