—~ 
TRANSACTIONS OF THE SECTIONS. 13 
and solid condition vary directly as their densities. This observation is now ex- 
tended to other bodies and to the gaseous condition. 
Condition. Water.} Phosphorus. |Sulphur. | Bisulph. Carb. | Ether. 
_ Sa 336 58 50 ie sa 
ICRP so 'hie\.s). sata 2184 « 333 “59 “49 “49 “49 
NREECOUAY 6 oie: oh oy 5 mia is) 321 “50? *A9? "44, “46 
Calculation from constituents | 339 Pots at “48 53 
In the above table the density of water at 0° C. is taken as unity throughout. It 
will be observed that the specific refractive energies of the solid and liquid con- 
dition are almost identical, and that those of a gas are somewhat less, with one 
doubtful exception, sulphur. As these are taken for the most luminous part of the 
spectrum, the disturbance caused by dispersion, at least, will make itself felt; and 
it is worthy of remark that (setting aside sulphur) the more dispersive a substance 
is, the greater is the difference between its specific refractive energy in the liquid 
and gaseous condition. 
There is no resemblance between the absolute refractive powers of a substance in 
these two states, as was observed long since by Arago. 
4. Specific refractive energy and solution—Ilt was laid down in a former paper 
as approximately true that the specific refractive energy of a mixture of two 
liquids is the mean of the specific refractive energies of its constituents. The 
following observations on solutions of two gases, two liquids, and two solids in 
water will serve to test whether this law can be extended to solution in general, 
even where there is a change of aggregate condition in one of the bodies, or where 
a feeble chemical affinity exists between the two. The specific refractive energy 
of water is taken at ‘3285 for Fraunhofer’s line A, and °333 for the line D. 
Specific refractive Specific refractive energy of solution in water. 
Substance. 
energy: Observed. Calculated. 
Ammonia.... 506 375 378 
Hydrochloric acid} 277 "344 “316 
Alcohol «./3 s+ “456 *396 395 
Nitric acid. ... *289 “310 “311 
BULACT wisvays «e *340 340 3365 
Common salt . . "260 “318 315 
Tn all these cases the observed and the calculated numbers are nearly coincident, 
with the exception of hydrochloric acid, where the combination of the gas with 
water seems to have materially altered its optical property. It is so likewise with 
sulphuric acid. 
The above-mentioned solution of sugar in water, when mixed with an equal 
weight of water, gave the specific refractive energy 337, instead of ‘3865, which 
may be taken as the same thing. 
The doubly refracting crystals of tartaric acid gave as their specific refractive 
power, deduced from the mean of the two spectra, ‘319 for the line A, and a solution 
in water gave ‘316—a lower, instead of 324, a higher number, as the theory 
requires. 
tt may be a matter for consideration how far the molecular forces that cause 
crystallization, or maintain a body in a solid or liquid state, influence the velocity 
with which light is propagated through the medium. 
5. Specific refractive energy and chemical combination.—Dulong showed long ago 
that the absolute refractive power of a compound gas is nearly, but not exactly, the 
mean of the specific refractive powers of its gaseous constituents. This is equally 
true of the specific refractive energy. But the observation need not be confined 
to gases. In the following table the actual specific refractive energies of three 
liquids is compared with the mean of the specific refractive energies of their con- 
stituents, whether gaseous, liquid, or solid, 
