204 THEORY OF THE CELLS. 
held in solution. As a specifically lighter fluid poured on one 
specifically heavier so carefully as not to mix with it, yet gra- 
dually penetrates it, so also, every solution, when brought into 
contact with a membrane already infiltrated with water, bears 
the same relations to the membrane, as though it were a solu- 
tion. And crystallization being the transition from the fluid to 
the solid state, we may conceive it possible, or even probable, 
that if bodies, capable of existing in an intermediate state 
between solid and fluid could be made to crystallize, a con- 
siderable difference would be exhibited from the ordinary mode 
of crystallization. In fact, there is nothing, which we call a 
crystal, composed of substance capable of imbibition ; and even 
among organized substances, crystallization takes place only in 
those which are capable of imbibition, as fat, sugar, tartaric 
acid, &e. The bodies capable of imbibition, therefore, either 
do not crystallize at all, or they do so under a form so different 
from the crystal, that they are not recognized as such. 
Let us inquire what would most probably ensue, if material 
capable of imbibition crystallized according to the ordinary 
laws, what varieties from the common crystals would be most 
likely to show themselves, assuming only that the solution has 
permeated through the parts of the crystal already formed, 
and that new molecules can therefore be deposited between 
them. The ordinary crystals increase only by apposition ; but 
there may be an important difference in the mode of this 
apposition. If the molecules were all deposited symmetrically 
one upon another, we might indeed have a body of a certain 
external form like a crystal; but it would not have the struc- 
ture of one, it would not consist of layers. The existence 
of this laminated structure in crystals presupposes a double 
kind of apposition of their molecules; for in each layer the 
newly-deposited molecules coalesce, and become continuous 
with those of the same layer already present ; but those mole- 
cules which form the adjacent surfaces of two layers do not 
coalesce. This is a remarkable peculiarity in the formation of 
crystals, and we are quite ignorant of its cause. We cannot 
yet perceive why the new molecules, which are being deposited 
on the surface of a crystal (already formed up to a certain 
point), do not coalesce and become continuous with those 
already deposited, like the molecules in each separate layer, 
