20G THEORY OF THE CELLS. 



sponding to the thickness of the layer; that is to say, the 

 molecules which are deposited by intussusception between those 

 already present, must be deposited much more vigorously be- 

 tweeuthose 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 molecules there- 

 fore ceases as regards that layer, and a new one begins. But 

 if the layers grow by intussusception in crystals capable of 

 imbibition, there is nothing to prevent the deposition of more 

 molecules between those which lie side by side upon the sur- 

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

 may continue to grow without the law by which the new mole- 

 cules 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, instead 

 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 sub- 

 stance 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 substance 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 re- 

 sult if these atoms have a certain form, or if in certain axes 

 they attract each other differently. But in bodies capable of 

 imbibition, an atom of one substance is not set upon another 

 atom of the same substance, but atoms of water come between; 

 atoms of water, which are not united with an atom of solid 

 substance, so as to form a compound atom, as in the water of 



