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oj Edinburgh , Session 1875 - 76 . 
in optical contact, while in the other many different forms may be 
observed, in which the component crystals are more or less of a 
laminate structure, often presenting most beautiful appearances. 
I have made this distinction, having observed that in some cases 
at least the true crystals are permanent, while the laminate are not 
so under like conditions, but change into the true form by time, or 
may become altogether disintegrated in a damp atmosphere into a 
confused mass, having few optical properties. The crystals also 
possess distinctive optical properties. 
The production of either of these classes of crystals appears to 
depend on two conditions, namely, on the thickness of the film, and 
on the amount of moisture applied in their production, — a thin 
film and a moderate supply of watery vapour inducing the true 
form, while a thick film and an increased quantity so alter the 
structure that at last, although the crystallisation may proceed 
from a centre, the circular character .is entirely lost. 
Spherical crystallisation, to which the circular is to be referred, is 
veryfrequent in mineral substances. It may be seen also in the well- 
known experiment of the rapid crystallisation of a supersaturated 
solution of acetate of soda, by the introduction of a centre round 
which the salt forms a spherical mass, and the surface, when the 
action has ended, presents all the appearance of a circular crystal. 
However difficult it may be to account for the origin of crystals, 
their growth in a circular form, when once the centre is determined 
in a film, is very obvious in those cases where they are produced. 
In the preparation of the film, not only is the superfluous solvent 
rapidly evaporated by heat, but a considerable part of the water of 
crystallisation is also driven off, and there is left on the glass plate 
a film of an amorphous substance, which, either by attracting 
moisture spontaneously from the atmosphere, or by having it added, 
allows the salt, whatever it may be, to resume its crystalline form. 
That this is the case may be seen from the fact that the crystallisa- 
tion will take place, and that in a circular form, if the drying of a 
plate is stopped just at that point when there is sufficient water 
left to enable the crystals to form when the plate is cooled. In 
this case their formation is a repetition of the acetate of soda ex- 
periment already alluded to. A plate may sometimes also be dried 
and crystallised, and on being again exposed to heat the crystals 
