DAVIES, ON CRYSTALLIZATION AND THE MICROSCOPE. 207 
fern-fronds particularly closely, though this form was an ac- 
cidental one. The sulphate of zinc belongs to the same 
system, and takes the same form as the sulphate of magnesia. 
As an object alone, however, for the microscope, it is more 
valuable than the latter, as when the solution is allowed to 
flow over the slide crystals are formed reminding one of long 
feathers, the colours of which with a selenite plate are very 
beautiful. Sulphate of zinc contains more than 40 per cent. 
of water im its crystals, part of which may be replaced by 
alkaline sulphates, and thus double salts be formed. 
When the double sulphate is wanted, an almost saturated 
solution of the two salts is made with distilled water, in the 
proportion of about three parts of zinc to one of magnesia. 
If this solution is spread evenly upon the plate and allowed 
to evaporate slowly and undisturbed, long, well-defined crys- 
tals are formed, with particularly frequent examples of 
grouping, which adds much to the beauty of the slide. 
Here, two individual specimens are united in the ceutre, 
where the junction is so perfect that one would be almost 
inclined ‘to put it down as a true form, did crystals ever 
exhibit “rentrant” angles; there, a group in which the 
crystals are numerous and intimately joined in the centre, 
resembling a star with many rays; and other irregular and 
grotesque combinations, which greatly increase the interest 
of the slide as an ordinary microscopic object. 
The above is a specimen of “substitution” (before men- 
tioned), where no peculiar means are made use of to obtain 
any uncommon crystalline forms; but we will now consider 
an example of the Class 2, named in No. XVI, where all 
or part of the water of crystallization is driven off and after- 
wards reabsorbed from the atmosphere. Perhaps no salt 
will prove so demonstrative of this as the sulphate of copper 
and magnesia; and, although the salt has been before de- 
scribed as a simple microscopic object, we will here discuss 
the deductions which are forced upon the mind when cause 
and effect are studied. 
SuLPHATE or Copper AND Macnusta.—Sulphate of copper 
is usually placed in System VI of crystallization, 7. e.’ the 
doubly oblique prismatic.* It contains five atoms of water, 
* About this crystal, however, there are different opinions. Dr. Pereira, 
in his ‘ Lectures on Polarized Light,’ referred it to System VI, on the au- 
thority of Gustav Rose and most of the other eminent German crystallo- 
graphers. Dr. Miller, in his ‘Elements of Chemistry,’ has placed it in the 
same class, stating, however, that ‘although the sulphate of copper does 
not crystallize alone with more than 5 Aq., yet, when mixed with sulphate of 
VOL. V.—NEW SER. P 
