THEODOR SCHWANN 263 



assume that it invests the entire crystal. Now, although this layer be 

 formed by the deposition of new molecules between those already pres- 

 ent instead of by apposition, yet this does not involve any change in 

 the law, in obedience to which the deposition of the coalesing molecules 

 goes on more vigorously in two directions, that is, along the surface, 

 than it does in the third direction corresponding to the thickness of the 

 layer; that is to say, the molecules which are deposited by intussus- 

 ception between those already present, must be deposited much more 

 vigorously between those lying together along the surface of the layer 

 than between those which lie over one another in its thickness. This 

 deposition of molecules side by side is limited in common crystals by 

 the size of the crystal, or by that of the surface on which the layer is 

 formed ; the coalescence of HiOlecules therefore ceases as regards that 

 layer, and a new one begins. But if the layers grow by intussuscep- 

 tion in crystals capable of imbibition, there is nothing to prevent the 

 deposition of more molecules between those which lie side by side upon 

 the surface, even after the lamina has invested the whole crystal ; it 

 may continue to grow without the law by which the new molecules 

 coalesce requiring to be altered. But the consequence is, that the 

 layer becomes, in the first instance more condensed, that is, more solid 

 substance is taken into the same space ; and afterwards it will expand 

 and separate from the completed part of the crystal so as to leave a 

 hollow space between itself and the crystal ; this space fills with fluid 

 by imbibition, and the first-formed portion of the crystal adheres to a 

 spot on its inner surface. Thus, in bodies capable of imbibition, in- 

 stead of a new layer attached to the part of the crystal already formed, 

 we obtain a hollow vesicle. At first this must have the shape of the 

 body of the crystal around which it is formed, and must, therefore, 

 be angular, if the crystal is angular. If, however, we imagine this 

 layer to be composed of soft substance capable of imbibition, we may 

 readily comprehend how such a vesicle must very soon become round 

 or oval. But the first-formed part of the crystal also consists of sub- 

 stance capable of imbibition, so that it is very doubtful whether it must 

 have an angular form at all. In common crystals atoms of some one 

 particular substance are deposited together, and we can understand 

 how a certain angular form of the crystal may result if these atoms 

 have a certain form, or if in certain axes they attract each other dif- 

 ferently. But in bodies capable of imbibition, an atom of one sub- 



