402 



Dr. J. H. Gladstone. 



[June 16, 



p /*H flA. i) w here ^ equals the density of the substance and P its- 



Ob 



atomic weight. 



In two communications made to the British Association,* we 

 stated that the dispersion equivalent of any substance is little affected 

 by the manner in which it is combined with other bodies, and we 

 gave as the mean value of CH 2 0*35 in the vinic group, but higher 

 figures in the benzene and pyridine groups ; phosphorus equal to 2"9; 

 chlorine 0'5 ; bromine 1*3 ; and iodine 2"6. In my subsequent paper 

 in the 'Philosophical Transactions,'t in which the refraction equivalents 

 of forty- six elements were worked out, I remarked, " the question of 

 dispersion equivalents is also of interest; the data for the investigation 

 of the matter are given in the Appendix." But there the matter 

 rested. The paramount interest of the refraction equivalents in 

 truth caused both the Continental observers and myself to neglect the 

 question of dispersion ; and with the exception of brief references to 

 it in papers on Refraction, J nothing was published on the subject till 

 last summer, when I applied the measurement of dispersion to the 

 elucidation of the chemical structure of the essential oils ;§ and 

 afterwards in a paper at Geneva || I ventured to give approximate 

 values for fifteen elements. 



Almost simultaneously with these appeared a paper by Bruhl,^[ in 



which he endeavoured to eliminate the influence of dispersion from 



the refraction equivalents of highly refractive bodies. In this he 



seems to establish the fact that for such bodies at least, the theoretical 

 2 y 



formula of Lorenz, ~ — — , gives more uniform results than the 

 empirical formula - ^ ; but he draws as one of his conclusions,- 



CL 



" the dispersion exercised by different bodies stands in no relation 

 which is as yet clearly recognisable and measurable either with the 

 refraction exerted by them, or with the chemical nature of the sub- 

 stances." In this and a following paper** he gives additional proof 

 of the worthlessness of Cauchy's dispersion formula, or any of the 

 suggested modifications of it, to eliminate the influence of dis- 

 persion. 



It will be seen that Briihl's conclusion is inconsistent with the 

 views I have recently expressed, and the determinations I had already 



* 'Brit. Assoc. Eep.,' 1866. (Trans. Sec, pp. 10 and 37.) 



t " On the Refraction Equivalents of the Elements." * Phil. Trans.,' 1869, p. 27. 

 X 'Pbil. Mag,' vol. 11, 1881, p. 59. 'Brit. Assoc. Eep.,' 1881 (Trans. Sec., 

 p. 591). ' Chem. Soc. Journ.,' vol. 46, 1884, p. 258. 

 § « Chem. Soc. Journ.,' vol. 50, 1886, p. 609. 

 || ' Archives Sci. Phys. Nat.,' vol. 16, 1886, p. 192. 

 If 'Liebig's Annalen,' vol. 235, 1886, p. 1. 

 ** ' Liebig's Annalen,' vol. 236, 1886, p. 233. 



