35(5 Dr. L. Bleekrode. On the Deterinin>iti<>n of the 



speculations of Professor Lorenz at Copenhagen,* and of Professor 

 Lorentz of the Leyden University have introduced the term the " con- 

 stant of refraction," expressed by C = (n 2 l)/(n*-|~2)<i, which is inde- 

 pendent of dispersion and temperature, and does not vary whether the 

 substance is solid, liquid, or gaseous ; n and d represent the index of 

 refraction and the specific gravity. Many years ago very extensive 

 and important observations of Gladstone and Dale, Landolt, Wiillner, 

 were applied to the empirically established relation C=( l)/d, and 

 they pointed out, that though wanting any theoretical basis, yet it 

 agrees with the results of experiment and fulfils to some extent the 

 conditions of the former expression. Especially has this relation been 

 used in chemistry for comparing the molecular refractive powers of 

 organic compounds, and has again been applied in the extensive and 

 important researches of BrUhl.f Far less are the attempts made to 

 test both expressions for the different states of substances, and to 

 ascertain which of them agrees the best with the experimental 

 results. The papers of Lorenz and of PrytzJ afford together seven- 

 teen compounds, nearly all belonging to organic chemistry, and 

 examined as fluids at 10 and 20 and compared to the vapours at 

 100. The method used for determining the refraction allowed an 

 exactitude to the fifth and sixth decimal. 



I adduce from the paper of Lorenz some results in the following 

 table : 



* Lorenz deduced this expression from the theory of undulations in a paper 

 published in the Danish language in 1869. A translation appeared in 1881 in the 

 " Wiedemann Annalen," 11, p. 70. Professor Lorentz came to the same relation 

 from the electro-magnetic theory of light, and his investigation was translated in the 

 same periodical in 1880, Bd. 9, S. 641. 



t Dr. T. W. Bruhl, in " Liebig. Annalen d. Chemie," Bd. 200, 1879. 



J Lorenz, loc. cit., and Prytz, "Wiedemann. Annalen," Bd. 11. S. 204. Beside* 

 these papers, I know of no others bearing on this subject. 



