60 REPORT— 1871. 



examine his coi'respondence, ■\vliicli has been carefully preserved, and I have been 

 so fortunate as to find in it three original letters from Lavoisier to Dr. Black. 

 They were written in 1789 and 1790, and they appear to comprise the whole of 

 the correspondence on the part of Lavoisier which passed between those distin- 

 guished men. Some extracts from these letters were published soon after Dr. 

 Elack's death by his friends Dr. Adam Ferguson and Dr. Robison ; but the letters 

 themselves, as far as I know, have never appeared in an entire form. I will crave 

 permission to have them printed as an appendix to this address *. Lavoisier, it 

 will be seen, addresses Black as one whom he was accustomed to regard as his 

 master, and whose discoveries had produced important revolutions in science. It 

 may, indeed, be said with truth that Lavoisier completed the foundation on which 

 the grand structure of modern chemistry has since arisen ; but Black, Priestley, 

 Schcele, and Cavendish were before Lavoisier, and their claims to a share in the 

 great work are not inferior to those of the illustrious French chemist. 



Among the questions of general chemistry, few are more interesting, or have of 

 late attracted more attention, than the relations which subsist between the che- 

 mical composition and refractive power of bodies for light. Newton, it will be 

 remembered, pointed out the distinction between the refractive power of a medium 



and its refractive index, and gave for the former the expression f-^ — , where fi is 



the refractive index, and d the density of the refracting medium. Sir J. Herschel, 

 anticipating later observations, remarked, in 1830, tliat Newton's function only ex- 

 presses the intrinsic refractive power on the supposition of matter being infinitely 

 divisible ; but that if material bodies consist of a finite number of atoms, differing 

 in weight for different substances, the intrinsic refractive power of the atoms of 

 any given medium will be the product of the above function by the atomic 

 weight. The same remark has since been made by Berthelot. Later observations 

 have led to an important modification in the form of Newton's function. Beer 

 showed that the experiments of Biot and Arago, as well as those of Dulong, on 

 the refracti^•e power of gases, agree quite as well with a simpler expression as with 

 that given by Newton ; and Gladstone and Dale proposed in 1863 the formula 



^ as expressing more accurately than any other the results of their experiments 



on the refractive power of liquids. The researches of Landolt and AViilluer have 

 fully confirmed the general accuracj' of the new formula. An important observa- 

 tion made, about twenty years ago, by Delfts has been the starting-point for all 

 subsequent investigations on this subject. Delfts remarked that the refractive 

 indices of the compound ethers increase with the atomic weight, and that isomeric 

 ethers have the same refractive indices. The later researches of Gladstone and of 

 Landolt liave, on the whole, confirmed these observations, and have shown that 

 the specific refractive power depends chiefly on the atomic composition of the 

 body, and is little influenced by the mode of grouping of the atoms. These inquiries 

 have gone further, and have led to the discovery of the refraction-equivalents of 

 the elements. By comparing the refractive power of compound bodies diflfering 

 from one another by one or more atoms of the same element, Landolt succeeded in 

 obtaining numbers which express the refraction-equivalents of carbon, hydrogen, 

 and oxygen ; and corresponding numbers have been obtained for other elements 

 by Gladstone and Ilaagen. The whole subject has been recently discussed and 

 enriched with many new observations in an able memoir by Gladstone. As might 

 be expected in so novel and recondite a subject, some anomalies occur which are 

 diflicult to explain. Thus hydrogen appears in different classes of compounds with 

 at least two refraction-equivalents, one three times as gi-eat as the other ; and the 

 refraction-equivalents of the aromatic compounds and their derivatives, as given by 

 observation, are in general higher than the calculated numbers. 



A happy modification of the ice-calorimeter has been made by Bunsen. The 

 principle of the method (to use as a measure of heat the change of volume which 

 ice undergoes in melting) had already occurred to Herschel, and, as it now appears, 

 still earlier to Ilerniann ; but their observations had been entirely overlooked by 

 physicists, and had led to no practical residt. Bunsen has, indeed, clearly pointed 

 * Ordered by the General Committee to be printed among the Eeports, 



