50 FORMATION OF SHELLS OF ANIMALS, ETC., 
hypothesis, are exactly lke those of the smallest crystals 
on the heated slides, before mentioned. And also it may 
be observed, that in examining good specimens of these 
crystals the larger can be seen to be made up of crystals 
of a variety of sizes, and therefore to be divisible into 
smaller ones, and these again into crystals still more 
minute, though of the same form, and so on, until the size 
becomes so diminutive that the true figure is inappreciable 
by the highest magnifying powers. (I have slides contaiming 
distinct rhomboidal crystals of 4¢'55th of an inch im length, 
and containing other crystals not more than 39$5oth or 
zodooth of an inch long.) Hence it may fairly be inferred 
that this same form, being the simplest and most natural, 
and of all rectilmear figures most easily produced, would 
continue diminishing in size until a crystal would be 
arrived at whose further division would separate it into its 
constituent molecules, that is, into those portions of matter 
which, their inertia being overcome, are the first to be put 
in motion under the combined influence of impulsion and 
gravity. Now, applying the reasoning employed at pages 
47 and 48 to the formation of just such a crystal, it will 
be seen that one of the molecules, thus impelled, which is 
to form a part of this crystal, would instantly encounter 
another molecule put im motion at the same time and 
under the same mechanical conditions, and the two being 
as instantly arrested by the predominant action of gra- 
vity, would become fixed in their position. And by the 
same process of reasoning, the other molecules could be 
shown to be added, until the formation of the crystal 
is completed, all the molecules which composed it 
bemg put in motion and brought into the condition of 
rest by the simultaneous operation of the same forces, 
namely electricity, the agent generating the impulsive 
