n 90 to 95 per cent of water can be
cut into chunks with a knife and no water will ooze from them. The water is
not in chemical union with the solid matter in the form of definite
chemical hydration, however, as the same gel is formed with all possible
variations in the water content.

Gels may be either rigid, as in the case of those of silicic acid, etc., or
elastic, as are those of gelatin, egg-albumin, agar-agar, etc. The latter
are the common type of gels among organic colloids. They can be easily
changed in shape, or form, without any change in total volume.

In gel-formation, the two phases of the system take a different
relationship to each other. The disperse, or solid, phase becomes
associated into a membrane-like, or film, structure, surrounding the liquid
phase in a cell-like arrangement. That is, the whole mass takes on a
structure similar to a honeycomb except that the cells are roughly
dodecahedral in shape, instead of the hexagonal cylinders in which the bees
arrange their comb cells, in which the original disperse phase constitutes
the cell-walls and the original liquid, or continuous phase, represents the
cell-contents. The cells of an elastic gel resemble closely the cells of a
plant tissue in many of their physical properties. They are roughly
twelve-sided in shape, as this is the form into which elastic spherical
bodies are shaped when they are compressed into the least possible space.

=Imbibition and Swelling of Gels.=--When substances which are natural gels,
such as gelatin, agar-agar, various gums, etc., are submerged in water,
they imbibe considerable quantities of the liquid and the cells become
distended so that the mass of the material swells up very considerably.
This swelling will take place even against enormous pressures. For example,
it has been found that the dry gel from sea-weeds will swell to 330 per
cent of its dry volume, if immersed in water under ordinary atmospheric
pressure; but that it will increase by 16 per cent of its own volume when
moistened, if under a pressure of 42 atmospheres.

During the swelling of gels by imbibition of water, the total volume of the
system (i.e., that of the original dry gel plus that of the water absorbed)
becomes less. For example, a mixture of gelatin and water will, after the
gelatin has swelled to its utmost limit, occupy 2 per cent less space than
the total volume of the original gelatin and water. It has been computed
that a pressure equi