208 DAVIES, ON CRYSTALLIZATION AND THE MICROSCOPE. 
one of which may he replaced by alkaline sulphates, thus 
producing double salts of great beauty. By a great degree 
of heat it is rendered anhydrous, becoming a white powder ; 
but it is again converted blue by the addition of water. The 
sulphate of magnesia has been already described. About 
three parts of pure sulphate of copper and one of sulphate 
of magnesia must be taken and dissolved in distilled water, 
in such proportion that the solution may be almost saturated. 
A small quantity of this is spread evenly upon the central 
portion of the slide, which must then be held over a spirit or 
gas lamp until fusion in its water of crystallization has pro- 
ceeded to such a degree that, upon touching the film with a 
needle, the matter may be drawn into threads. The amor- 
phous substance upon the glass will now, upon cooling, take 
flower-like forms here and there, or these may be started at 
pleasure by breaking the surface with a fine needle-point, and 
especially quickly where breathed upon. A slight warmth at 
the fire will stop the growth in any desired state, or by again 
allowing to become cool a fresh band of petal-like branches 
may be added. If covered with pure balsam at any moment 
the increase is stayed, and may then be mounted as usual ; 
but the preparation is less liable to change when there is no 
chance of any continued action going on from the uncovered 
portions of the film. 
This preparation I have before described, but what I am 
about to say would not be complete without the above relation. 
I think that my numerous experiments and results justify 
me in stating that it is by the absorption of water alone from 
the atmosphere or other vehicle that crystallization takes 
place. In proof of this one or two facts are almost sufficient. 
When the slide is finished and ready for covering with balsam, 
wherever the surface of the film is broken and allows moisture 
to reach the inner portion a fresh circular crystal arises, and 
grows in asimilar manner to those formed at first. Again, 
in a dry hard frost I have worked with the same solutions 
and apparatus that have given me great success, and during 
three days of industrious work I have not obtained a single 
iron, it, like this salt, assumes 7 Aq., and then is isomorphous with the ferrous 
salt” (7.e. belongs to System V). Mr. Brooke, in the ‘Encyclopedia Me- 
tropolitana,’ and others, term it an oblique rhombic prism, thus referring it 
to System V, the same as sulphate of magnesia, before mentioned. 
It would be vain in me to attempt to decide any question: betwixt such 
men; but I must say that, from my own experience, I am forced to sub- 
scribe to the last opinion. I believe that few salts combine in this way 
eo are not of the same class, but Iam now seeking for some law in this 
matter. 
