Shrauf on the Optical Constants of Crystallised Substances. 481 



§ 10. The consequence from § 9 is, that the density of the ether 

 may be set in proportion to the density of the substances* . 



§11. Not the elasticity, but rather the density is subject to varia- 

 tion (Fresnel's theory). 



§ 12. The consequence of § 9 is, that the propagation of light 

 may be equally conceived as being independent of the luminous ether, 

 and only in dependence on the substantial molecules. 



§ 13. If Fresnel's formula is derived from the principle of conser- 



A 2 — 1 

 vation of vis viva, and — ^ — is substituted, a formula similar to 



Cauchy's in structure is thus obtained. 



§ 14. In consequence of § 9, it appears possible to calculate the 

 density in the three dimensions of any crystal, and to bring this new 

 moment in connexion with the rest of the physical properties. 



§ 15. The densities being proportional to the 7nasses, and these to 

 the distance r of the molecules, the coefficient of dispersion must be 

 subject to the general law of gravitation, and it would be admissible 

 to write g 



It must be remarked that, in consequence of the more or less relative 

 mobility of the molecules, N must have a different value for different 

 substances. 



§ 16. The elementary substances, according to Boedecker's theory, 

 not entering into mutual combination with the same density and 

 correlation of atoms as they possess in the state of liberty, it may be 

 admitted that they are probably compelled to form double molecules, 

 or semi-molecules. In the second case of combination the specific 

 power of refraction would be the double, in the first only the half 

 of what it was in the elementary substance, when still chemically free. 



§ 17. The indices of refraction of chemical combinations and 

 mixtures may be calculated on the supposition enounced in § 16. 

 Absolute exactitude may be obtained for any extensive groups, of 

 which some members, and consequently the law of their chemical 

 combination, are known f. 



The general law here is — Substances with nearly equal powers of 

 refraction may combine without alteration in them. If these powers 

 are very different, and far distant from each other, they tend to 

 become homogeneous, so that the greater power of refraction is 

 halved, and the lesser is doubled (see § 1G). 



§ 18. The coefficient of dispersion of combined substances % seems 



* Calcareous spar, Arragonite ; diamond, graphite, coal; water, ice ; different 

 varieties of topaz, beryl, apatite, &c. ; and all the substances examined by Dale and 

 Gladstone may serve as evidences and exemplifications of the above propositions. 



t Evidence is afforded by Dulong's, Dale's, and Gladstone's determinations, also 

 by all other, especially organic, combinations, on account of the simplicity of their 

 formulae ; also by Deville's, Grailich's, Ilandl's, and Weiss's determinations con- 

 cerning mixed substances. 



\ Our knowledge of the coefficients of dispersion proper to chemical elements 

 being still very defective, it is impossible to bring this proposition to full evidence. 

 Sulphuret of carbon may serve for demonstration, and similar inferences may be 

 deduced from investigations concerning mixtures. 



Phil.Mag. S. 4. Vol. 23. No. 156. June 1862. 2 K 



