322 REPORT-—-1890. 
from the observed freezing point we can calculate the composition of the 
hydrates which must exist in the solution (Col. VII.), and these are 
found to agree so fully with those indicated by the examination of the 
curved figures representing various properties of the solution (Col. VIII.) 
that the maximum difference between the two is only 0-48 in the per- 
centage of acid present. 
When we can by simple calculations, based on one series of deter- 
minations, prove that the hydrates in solution must be the same as those 
which totally independent experiments have led us to suppose, we have, 
I think, arrived at proof as nearly absolute as it is possible to conceive ; 
and, if I have succeeded in showing that this proof may be accepted with- 
out in any way rejecting the facts on which the advocates of the osmotic 
pressure theory rely—approximate constancy, approximate regularity, 
and approximate similarity between dissolved and gaseous matter—I 
shall feel that I have done far better work than the mere establishment 
of the hydrate theory, by pointing out a possible modus vivendi for both 
theories almost in their entirety, and by helping to break down that wall 
of separation between physicists and chemists which is fast crumbling 
into dust. 
Dr. GLADSTONE made a communication on ‘The Molecular Refraction 
of Substances in Solution,’ in which he reconsidered the five reasons 
given in 1865 and 1869 for believing that ‘ the specific refractive energy of 
a solution is the mean of the specific refractive energies of the solvent 
and the substance dissolved.’ In describing the present state of our 
knowledge, he brought forward some facts which have a bearing on the 
views under discussion. 
In the first place, although it may be accepted as a rule that a solid 
when dissolved retains its former refractive power, it is a rule not without 
exceptions. Thus the experiments, both of the speaker and of Dr. Bedson, 
on rock salt agree in giving 14°6 as the molecular refraction of chloride 
of sodium for the solar line A or R; in which R represents the value Se 
multiplied by the molecular weight. But the molecular refraction for the 
same ray as calculated from aqueous solution is 15:3, showing that the 
water has perceptibly increased the refractive power. And this is not an 
isolated instance, for the observations of Topsoe and Christiansen on crys- 
tals of potassic bromide and iodide show a molecular refraction for the 
line D, or k,, of 24°85 and 36:29 respectively, while the solntions indicate 
25°7 and 36°9 respectively. In fact, the chlorides, bromides, and iodides 
in general, when dissolved in water, are known to exhibit a higher 
refraction and dispersion than would be calculated by adding together tlre 
generally received values for the metal and the halogen, and this increase 
is uniform for each series of salts. 
It is also known that there is a slight change in the molecular reftac- 
tion of certain liquid substances, such as acetic acid, when they are mixed 
with water. 
In the second place the molecular refraction of a substance in solution 
is not varied by varying the amount of the solvent. In the case of water, 
however, there are some marked exceptions. With the hydracids the — 
values increase with the dilution up to acertain extent, when they become 
stationary. Nitric and sulphuric acids are also exceptional. It is evident 
» 
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