e tissue soft and
flabby; while increases in water content swell the gel and make the tissue
stiff and turgid. No other condition than that of a colloidal gel could
respond in this way to changes in water content.

The formula which is commonly assigned to water is the simplest possible
one; namely, H_{2}O. But if the water were really as simple as this, the
compound would boil at a very low temperature, would have a very low
surface tension, etc.; whereas its actual boiling point, surface tension,
etc., are much higher than those of other compounds having a higher
molecular weight than is indicated by the formula H_{2}O. Actual
measurements of the physical properties of water indicate that at the
temperature at which water is a vapor its formula is at least (H_{2}O)_{2};
while at lower temperatures, at which it exists as a liquid, its formula
may be (H_{2}O)_{3}, or (H_{2}O)_{4}, or even more complex still. The cause
for this association of the compound into multiple molecules undoubtedly
lies in the extra valences of the oxygen. In many organic compounds oxygen
is undoubtedly tetravalent, and it may be easily conceived that in these
complex molecular groupings in the water it exhibits this same property;
the possible molecular arrangements being represented by the formulas

H H
H H \ /
\ / H-O-O-H
O=O and | | etc.
/ \ H-O-O-H
H H / \
H H

Such molecules may be conceived to break down very easily, leaving the
extra valences of the oxygen available to form linkages with other atoms or
molecules. This may constitute one of the ways in which water exerts its
remarkable effects both as a solvent and as an accelerator of all kinds of
chemical reactions. Other organic compounds which contain tetravalent
oxygen are exceedingly active chemically, and there seems to be much to
commend this view of the chemical structure of the water molecule.

Probably the most remarkable property of water is its power of solution. No
other liquid surpasses water as a solvent. This power, as has been pointed
out, is supposed to be due to, or in some way correlated with, the extra
valences of the oxygen atoms, which may perhaps unite with simila