MJourbal, Oct. 1870. Crystals in Closed Cells. 217 
those substances which exert any action on polarized light afford a 
display of colours truly gorgeous, and far exceeding in brilliancy 
those obtained in any other way. 
By a suitable adaptation of the oxy-hydrogen microscope these 
effects may be readily exhibited on a disk of 6 or 8 feet diameter. 
Now it had occurred to me that from the very fact that the 
mother-liquors of any given crystallization are necessarily satu- 
rated solutions of the substance under examination, no medium 
should be so incapable of solvent action upon the crystals deposited 
from them, except, of course, so far as slight changes of temperature 
might cause on the one hand a trifling re-solution, or on the other 
a minute increase of their bulk. 
I found, however, that a very different result actually ensued. 
In the case of nitrate of silver and chlorate of potash, both of 
which afford erystallizations of extreme beauty both as to form and 
as to colour under the polariscope, even before the plates have 
spread to the opposite side of the cell from that at which they have 
commenced, the edges of the portions first formed lose their sharp 
definition ; dark lines appear parallel to their margins; these become 
gradually broader, and finally after the lapse of some hours nothing 
remains but a few shapeless patches. 
This result is not more curious than the further observation that 
the solution can be made with the utmost facility to re-deposit its 
quantum of crystals: in fact by merely re-heating the slide until 
the skeletons of the former crystals have been re-dissolved, or even 
by merely heating for a few seconds to a degree insufficient to pro- 
duce this re-solution, and then allowing it to cool, the crystallization 
is reproduced in all its beauty. 
I have certain preparations of the two salts above named which 
scarcely show a trace of solid matter; yet a very slight elevation of 
temperature suffices to induce so profuse a crystallization, that the 
entire surface of the cells is again covered by their characteristic 
forms, which are in turn as completely re-absorbed after a few hours. 
Not only is the beauty of these forms very great, but their 
variety is well-nigh infinite, no two crystallizations being ever 
precisely alike. 
I have operated in this manner upon a large number of sub- 
stances, organic and inorganic, and although the rate and degree of 
re-solution varies materially, the same general phenomenon holds 
good in all cases. It is therefore a matter of interest to inquire to 
what general law it is due. 
It does not appear to have any definite relation to the solubility 
of the substance, or to the number of equivalents of water of crys- 
tallization which it contains, nor again to its relative solubility at 
different temperatures. 
Thus it is displayed in a marked degree by chlorate of potash, 
