The smaller the diameter of the tube, the higher will the water rise in it, and the greater will be the curvature of its upper surface, to which the rise is sensibly proportional. But in the case of liquids which do not “wet” the walls of the tube (as in that of mercury and glass), the curve (meniscus) is convex, instead of concave, and the liquid is depressed instead of rising.
It is in its relations to heat, however, that water is specially distinguished from other substances; and these differences are most vital not only to living organisms, but to the entire economy of Nature.
Density.—As regards the density or specific gravity of water (which is by common consent assumed as the unit of comparison), it will be seen from the “Density” table that whereas all other bodies contract and become more dense as they grow colder, water has its point of (fluid) “maximum density” at 4°C. (49°.2 Fahr.), and expands as it grows colder, until at 0°C. (32° Fahr.) it solidifies into ice. In so doing it departs still farther from the rule obtaining with all other bodies (excepting certain mixtures, such as type metal) and again expands so as to decrease the density from .99988 to .92800; thus causing ice to float on water at the freezing point. Hence water, unlike all other fluids, solidifies first on the surface; and but for this, the thawing of the winter’s ice, which would be formed at the bottom of rivers and lakes, would be deferred until late in summer. The expansion of water in freezing is forcibly illustrated in the bursting of water pipes and pitchers in winter; in the soil, the ice forming in the interstices serves to loosen the compacted land and give it better tilth for the ensuing season.
Specific Heat.—Considering next, the column showing the “specific heat” of water as compared with other substances, we see that it exceeds all other known bodies in the amount of heat required to change its temperature; hence again, its heat capacity is taken as the unit to which all others are compared. The figures given in the table show that even ice and steam require for equal weights only about half as much heat (or burning of fuel) to change their temperature (e. g., 1 degree) as would liquid water. But earthy matters, such as clay or soil and glass, require only one-fifth as much heat for a similar change; charcoal only about one-fourth as much. But vegetable matter as represented by wood on the one hand, and gold and lead on the other, require only about one-thirtieth as much heat as an equal weight of water; zinc about one-tenth as much, steel somewhat more.
It is thus plain that masses of water act powerfully, more than any other substance, as moderators of changes of temperature by their mere presence. The body of an animal or plant is protected against violent changes by the presence of from 60% to 90% of liquid water, the temperature of which can only be raised or lowered slowly; and the presence of the sea tempers the climates of coasts and islands as compared with the heat or cold occurring in the interior of the continents.
Ice.—Again, it is shown in the table that the heat required to melt ice is greater than in the case of any other substance, especially the metals; which when once heated to the fusing point, require only a very little more heat to become liquid. The fusion of salts (including silicate rocks) requires more heat than does that of the pure metals.
Vaporization.—In the amount of heat required for its vaporization water is also especially pre-eminent, and potent in its influence upon organic life. The table shows that the evaporation of water requires six hundred heat units[67] as compared with alcohol, requiring only two hundred; while spirits of turpentine, the representative of a large proportion of vegetable fluids, needs but sixty-seven.
The practical result is that evaporation of water from the surface of animals and the leaves of plants, is exceedingly effective in preventing excessive rise of temperature, the heat of the sun and air being spent in evaporating the perspiration of animals and plants before an injurious rise of temperature, such as would cause sunstroke in animals, and wilting or withering in plants, can occur. But since evaporation is most rapid in dry air, it follows that the cooling effect will be the greater in the arid regions than in the humid. In the latter, therefore, sunstroke is much more frequent than in the fervid regions of the arid west, even though the temperature in the latter may be higher by twenty or twenty-five degrees Fahrenheit. White men who would soon succumb if they attempted to work in the sun in Mississippi or Louisiana when the thermometer stands at 95°F. will experience no inconvenience under the same conditions in the dry atmosphere of the Great Valley of California.
Solvent Power.—To the exceptional properties of water discussed above, should be added another hardly less important one, viz., that of being an almost universal solvent especially of mineral matters, including even those which, like quartz, appear to be most insoluble and refractory ([see chapt. 3]). The water of the soil is thus enabled to convey to the roots of plants, in solution, all kinds of plant food contained in the soil. It should be noted that distilled (hence also rain-) water is a more powerful solvent, e. g., of glass, than ordinary waters containing mineral matter, and even free acids.
Practically, plants take up all their water supply from the soil in the liquid form; and hence the soil-conditions with respect to this supply are of the most vital importance to plant growth. The most abundant supply of mineral plant food may be wholly useless, unless the physical conditions of adequate soil-moisture, access of air, and warmth, are fulfilled at the same time. On the other hand, comparatively few plants are adapted to healthy growth in soils saturated with water, or in water itself; and but few among these are of special interest from the agricultural standpoint.