AND THE MICROSCOFE. 249 
cube afterwards each spot was surrounded by a liquid flower 
of six petals. At first the leaves were unbroken curves, 
but when the flowers expanded under a long-continued 
action the edges became serrated. ....No matter in what 
direction a solar beam is sent through lake ice, the hquid 
flowers are all formed parallel to the surface of freezing.” 
Thus, not only are the crystals perfect, but all these forms 
lie in successive parallel planes. This fact is clearly shown 
in many examples of microscopic crystals, The surface 
alone is first crystallized, the lower part becoming gradually 
assimilated in form to the higher; though, in certain in- 
stances, a conflicting arrangement becomes visible, and the 
two separate layers of crystals are unlike in some respects, 
It is above stated that crystallization requires “ disturb- 
ance or interference.” It may be asked, what are these dis- 
turbances or interferences? In the present state of our know- 
ledge of this science it would be impossible to give anything 
like a satisfactory answer to the question. A few of these 
causes, however, are known, two of which may be men- 
tioned as the principal : 
Ist. Sudden change im temperature of certain parts of 
the substance, which cause contraction or expansion, and 
so give rise to the formation of crystals. Whereyer the 
substance varies in thickness this action would be materially 
aided. | 
2nd. Insoluble atoms, dust, impurities, &c. 
As instances of the first cause may be mentioned the 
formation of crystals, which is visible when produced on the 
microscopic slide. Those at the edge are almost invariably first 
‘formed, however equally the slide is heated. Of the second, 
the examples are so numerous that they frequently prove a 
great annoyance when a large surface of uniform crystalliza- 
tion is wanted. A small atom of undissolved salt proves a 
nucleus for the accumulation of other portions, and thus 
commences a circular growth of crystals, which materially 
interferes with the particular arrangement which might be 
desired. Dust, which is always floating about in the air, or 
fine impurities in the solution, produce important modifica- 
tions, just as strings are suspended in the syrup-pans to 
serve as nuclei for the formation of sugar-candy. 
It may be here remarked that most fused salts are 
governed by the same laws as those which are dissolved, 
