140 Proceedings of Royal Society of Edinburgh. [mar. 21, 
weight of hydrogen identical, whether it is united to such opposite 
radicles as Cl or OIL Table VL shows further the effect of valence. 
While a given weight of hydrogen produces the same effect in 
solution, whether it he united to the very different but both univalent 
radicles Cl and OK, its effect is reduced by one-half when united to 
the bivalent S0 4 . That valence is not the only factor is shown by 
comparing the effects of hydrogen and potassium when united to 
the common element, chlorine. Hydrochloric acid in solution 
produces a markedly more powerful lowering effect on the melting- 
point of ice than the equivalent amount of chloride of potassium. 
Of all the substances that I have experimented on 5 hydrochloric 
acid is the most energetic in reducing the melting-point of ice, and 
with ordinary strong acid and pounded ice there is no difficulty in 
producing temperatures as low as the freezing-point of mercury. In 
the case of hydrochloric acid, sulphuric acid, chloride of sodium, and 
chloride of calcium, I have carried my experiments tolow temperatures 
and great concentration. But before passing to them it is well to 
consider the more dilute solutions with regard to their density. 
That the mere density of the solution in which the ice is melting 
has no direct connection with the lowering of its melting-point is 
shown by the following table, in which the specific gravities 
(at 15° C.) are given of the solutions of different salts which gave 
the same depressions of melting-point : — 
Temperature of 
ineltirig. 
Specific Gravity of Solutions of 
NaCl 
KCl 
MgCl 2 
CaCl 2 
BaCl 2 
d c. 
- 2-86 
- 1*8 
1-03370 
1-02174 
1-03850 
1-02535 
1-03893 
1-02715 
1-04756 
1-03262 
1-06633 
There are many similarities in the effects produced by greatly in- 
creasing the pressure upon pure water and by dissolving salts in it. 
First, there is an absolute diminution in the volume of the solution 
as compared with the sum of the volumes of its components ; 
second, in virtue of this compression by molecular forces it has be- 
come less compressible by mechanical means ; third, the tempera- 
ture of maximum density and the freezing temperature are lowered ; 
and fourth, the former of these two temperatures is lowered 
more rapidly than the latter. All these effects are produced in kind 
