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CrysTALLizATIon and the Microscorr.—No. II. 
By Tuomas Daviss, Esq. 
(With Plate VIII.) 
Txoven our knowledge of crystallization is at present far 
from satisfactory, yet it would be impossible to give anything 
hike a complete description of the laws already laid down in 
so short a space as these observations must fill. For this 
reason it will be preferable to examine such salts as may be 
justly termed “representatives ”’ of certain classes. 
In the first part the examination of santonine called out 
certain facts, many of which are equally applicable to all 
crystalline structure—influences of temperature, insoluble 
atoms, impurities, and other accidental disturbances. We 
will, however, now consider what new results we meet with 
when certain salts are mixed together; and by this mode of 
treatment, perhaps, we obtain the most beautiful objects for 
the microscope and polarized light. It is necessary, in most 
instances, that this mixture should be peculiarly intimate, 
and frequently where this is thought to be the case it is 
found, when the microscope’s aid has been invoked, that 
there has been little else save a mingling of particles, which, 
by mutual interference, have prevented anything like unifor- 
mity of design. To obtain this mixture in a perfect state 
salts must be fused or dissolved, and the fusion be allowed to 
cool, or the solvent evaporated after solution; but the treat- 
ment must be varied with the salt we are using. It must 
also be remembered that certain mixtures produce precipi- 
tates, some of which are “curdy,” and little suited for mi- 
croscopic objects. With these we shall not now meddle. 
In the first paper santonine was chosen as a specimen of 
rearrangement of crystalline form after fusion. <A _ little 
further on we will consider a mixture of sulphate of copper 
and magnesia, where the treatment partakes, in some degree, 
of evaporation as well as fusion. But we will now make a few 
observations on an instance where, certain salts being very 
intimately mixed, a “substitution” or “replacement ” of 
part of one by the other takes place, and thus a new and 
frequently permanent salt is produced. This branch is but 
imperfectly understood at the present time. In many in- 
stances the results obtained from exceedingly slight altera- 
tions of proportions are widely different ; but this, I deem, 
is not always caused by a total want of those forms we are 
awaiting, but a certain part of one salt has appropriated the 
necessary quantity of the other, whilst the unchanged mass 
