DR. GLADSTONE ON THE REFRACTION-EQUIVALENTS OE THE ELEMENTS. 13 
solutions of salts, bodies which in their solid state are generally doubly refracting, and 
necessarily present difficulties that are not met with in the examination of liquids. 
The method usually adopted was as follows : — An amount of salt representing the 
atomic weight was dissolved in n atoms of water, and the refractive index and density 
of the solution were taken. From these was reckoned the refraction-equivalent, and 
subtracting from this n times the refraction-equivalent of water for the solar line A, 
there remained the refraction-equivalent of the dissolved salt for that part of the spec- 
trum. Thus to take an actual instance: 1 atom or 58-5 parts of chloride of sodium 
were dissolved in 12 atoms or 216 parts of water. The refractive index of the solution 
for A was 1-3683, and the specific gravity at the same temperature was 1-168; 
w.A — 1 
d~ 
therefore was 0-3154, and P ^ — was 0-3154 x (58-5 + 216), that is, 86-57. From this, 
the refraction-equivalent of the whole compound system, 12 times 5-926 {i.e. 71-12) the 
refraction-equivalent of water was subtracted, leaving 86-57 — 71T2, or 15-45, as the 
refraction-equivalent of chloride of sodium. That a number so arrived at fairly repre- 
sents the action exerted by the chemical compound on light, is evident from the follow- 
ing considerations. 
1st. The refraction-equivalent 15-45 closely approximates to that previously deter- 
mined for chloride of sodium from the examination of solid rock-salt, namely 15-02. 
Similarly, cane-sugar dissolved in water gave 119-0, while from Brewster’s observation 
of the crystallized solid it should be 119-3 (see Table III.). Again, crystallized borax, 
after making allowance for the refraction due to the water of crystallization, gave 45-8, 
while from its aqueous solution its equivalent was determined at 45- 9. Chloride of 
ammonium, solid and in solution, gave respectively 22-08 and 22-33. 
2nd. The refraction-equivalents of several solid organic bodies, as determined from 
their aqueous solutions, agree closely with what might be calculated from Landolt's 
values for C, H, and O. Thus, 
Experiment. Calculation. 
Citric Acid 60-89 61-4 
Racemic Acid .... 45-54 45 -8 
Tartaric Acid .... 45-29 45-8 
3rd. The refraction-equivalent as reckoned from a solution is not affected by varying 
the amount of water. This has been proved in the case of the chlorides of sodium, 
potassium, strontium, and copper, iodide of sodium, sulphate of ammonium, and other 
salts, and even in the case of the combinations of water with strong acids, such as sul- 
phuric and nitric acids. The following experiment on chloride of sodium will serve as 
an illustration. 
