206 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 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 molecules therefore 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 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, 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 m 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
