218 Formation of Microscopic Crystals. [Monthly Microscopical 
which is an anhydrous salt, requiring 16 parts of water at 60° Fahr. 
to dissolve it, and only y'5th of that proportion at 212°; and equally 
by biborate of soda, which crystallizes with 10 equivalents of water, 
and dissolves in 12 parts of water at 60°, and in »;th of that quan- 
tity at 212°. Again, by sulphate of magnesia, with 6 equivalents 
of water, and in which the respective solubilities at 60° and 212° 
are as 1 to 2; while ferrocyanide of potassium with 3 equivalents 
of water, and in which the solubilities are also approximately as 
1 to 2, exhibits it im a much less degree, the crystals retaining 
their sharpness for several hours, and even after the lapse of some 
days undergoing comparatively little degradation. 
Now, in crystallizing, the vast majority of substances expand, 
chloride of ammonium being the only salt positively known to 
occupy more space in solution than in the solid form. 
Mr. Sorby * shows that in all cases where, “ when they dissolve, 
the total bulk increases, pressure reduces their solubility ; whereas 
if the bulk decreases, pressure makes them more soluble; in other 
words, solution or crystallization is impeded by pressure according 
as mechanical force must be overcome in dissolving or in crystal- 
lizing.” 
oe disposed to seek for an explanation of the phenomena in 
question by reference to this fact, inasmuch as the effect of erystal- 
lization in a closed cell would be, by reason of the increase of bulk 
in assuming the solid form, to produce a corresponding increase of 
pressure on the contents; but on the other hand, the contraction 
of the liquid during cooling must produce an effect in the opposite 
direction more than sufficient to counterbalance the expansion due 
to crystallization. 
Thus, the contraction of water in descending in temperature from 
212° to 32° is about 4°6 per cent., while Mr. Sorby has shown 
that the increase in solubility of ferrocyanide of potassium due to 
the pressure of 66 atmospheres is only 1° 640 per cent., and would 
be under the enormous pressure of 100 atmospheres only 2°485 
per cent.; while in sulphate of copper (in which the increase of 
solubility exceeds that of any other substance which he examined) 
these figures are still only 1:910 and 3°183 respectively. Inas- 
much as I have in some instances obtained a re-solution of the 
crystals first formed amounting probably to 90 per cent. of their 
total quantity, this cause is evidently quite inadequate to explain 
the facts. 
I am indebted to my friend Mr. George Gore for the suggestion 
that we must look to the action of capillary force as a cause perhaps 
adequate to account for this enormous modification of the “ force of 
crystalline polarity.” 
It is indeed evident that the conditions of the experiment are 
* «Proceedings of the Royal Society,’ April 30, 1863. 
