Water constitutes the largest proportion of the weight of active protoplasm. In living cell contents (except those of such bodies as resting seeds, etc.), water comprises from 70 to 95 per cent of the total weight of the substance; the average proportion being usually between 85 and 90 per cent. The fact that protoplasmic material can exist in turgid form with such high percentages of water as these is due, as has been pointed out, to its existence as a colloidal gel. It is because of this condition that increases in the proportion of water generally increase the turgidity, or turgor, of the protoplasm; instead of, as in all other cases, rendering the mixture less solid and more labile. Losses of water from the protoplasmic gel decrease its "swollen" condition and so render the 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.