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, &c. 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, 



