288 SCIENCE PROGRESS 



Ketteler and Lorenz, who were mainly interested in dis- 

 persion, re-determined the indices of some of the substances 

 dealt with by Dulong; but the next important landmark in 

 the history of the subject does not occur till 1874-8, when 

 Mascart 1 published his researches on a large number of com- 

 pounds. 



These papers form the largest store of facts extant. But 

 for the purpose of an inquiry into the relation between the 

 refractivities of the elements and their constituents, the value of 

 his work is discounted by three circumstances. The range 

 of compounds which he could attack was limited by the form of 

 his apparatus, which did not permit experiments at a higher 

 temperature than if C. His results were in some cases cal- 

 culated to express what the refractive index of the compound 

 would be at o° and 760, irrespective of the density ; in others 

 they were simply compared with that of air at 12 C, and they 

 cannot easily be reduced to a form strictly comparable with 

 that of the indices of the permanent gases. Further, in a large 

 number of cases, the compounds which he examined contained 

 carbon, and, as the gaseous refractivity of this element has not 

 been measured, all these figures are unavailable for the purposes 

 of the present inquiry. Indeed, at the end of his research he 

 was in possession of only six compounds, of which the re- 

 fractivities of all the components were known to him. These 

 were NH 3 , NO, N 2 0, HC1, HBr, H 2 0. 



It was very unfortunate that, of these six, three were nitrogen 

 compounds, which subsequent comparison proves to be anoma- 

 lous. This circumstance misled Mascart to the conclusion 

 that the refraction of a compound is in general more than that 

 of its components. He added that it was not possible to 

 calculate the refractivity of a gas from its components. 



This view seems to be that which is generally held at the 

 present time. Ostwald, in an able review of the subject, goes 

 so far as to state that there is no relation to be traced between 

 the indices of the elementary gases and their compounds, and 

 Briihl, in a well-known paper, definitely concludes that in 

 gaseous compounds refractivity is certainly not an additive 

 quality, but one which is constitutive. Since Bruhl's paper was 

 published not much work appears to have been done on this 



1 Mascart, Ann. de PEcole Normale, sup. (2), t. vi. p. 9, 1877 '> C.R., t. lxxxvi. 

 pp. 321, 1 182 (1878). 



