Page:Handbook of Meteorology.djvu/27

 —of various kinds of matter, comparing them volume for volume, under standard conditions, with the density of a given substance. This ratio of weight is the specific gravity of the substance. Distilled water at its maximum density, 39.1° F (3.94° C) is usually taken for the comparison. Thus, a given volume of mercury weighs 13.6 times as much as an equal volume of water, and an equal volume of alcohol 0.81 times as much as an equal volume of water.

For gases, air and hydrogen are both used as units of comparison. The specific gravity of hydrogen, in terms of air, is 0.069; of air in terms of hydrogen, 14.4; of coal gas, commonly used for inflating balloons, about 0.061; of water vapor, 0.62. When metric units are employed, the weight of a cubic decimeter is the specific gravity of the given substance, a cubic decimeter of water under standard conditions, weighing in theory, but not in fact, 1 kilogram.