450 [March 5, 



constant at different temperatures. Now every refractive index 

 contains at least two coefficients : the one of refraction, which is repre- 

 sented by the theoretical limit of the spectrum ; the other of 

 dispersion, for which the difference between the refractive indices 

 of H and A may be taken as the exponent. The refractive index, 

 minus unity (/* 1), is termed by the authors the " refractive energy" 

 of the substance, and this multiplied by the volume (p. 1), or 

 divided by the density, is termed the " specific refractive energy." 

 It was not found as a rule that the theoretical limit of the spectrum 

 gave more truly a constant than the line A ; but the difference is 

 within experimental errors. The empirical law was therefore ex- 

 pressed as follows : The refractive energy of a liquid varies directly 

 with its density under the influence of change of temperature, or, in 

 other words, the specific refractive energy of a liquid is a constant 

 not affected by temperature. Yet the influence of dispersion renders 

 this not absolutely accurate in the observed numbers, for the change 

 of dispersion does not follow the same law, the spectrum contracting 

 in some cases much more, and in other cases much less rapidly than 

 the volume increases ; indeed no relation is as yet discoverable be- 

 tween the change of dispersion and that of density. 



II. The refraction and dispersion of mixtures of liquids. This 

 question has engaged the attention of several experimenters, only one 

 of whom, however, M. Hoek, has offered a solution. His formula 

 depends on p. 2 1. Yet most of the results recorded were equally 

 well explained on the supposition that the specific refractive energy 

 of a mixture is the mean of the specific refractive energies of its 

 components. It was clearly desirable to test this in some cases 

 where the refractive indices of the liquids mixed were very wide 

 apart. Fortunately, bisulphide of carbon and ether, substances 

 almost at the opposite limits of the scale, were found to mix without 

 condensation ; and another good experiment was obtained with aniline 

 and alcohol, on mixing which, however, some diminution of volume 

 occurs. In both these cases the experimental numbers were slightly 

 below those deduced from the mean of the specific refractive energies, 

 the discrepancy being beyond the limits of probable error ; yet no 

 other formula could be devised which would give a nearer approxi- 

 mation to the indices actually observed. 



III. The refraction, dispersion, and sensitiveness of different 



