_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. [67] A heat unit, or “calorie,” is the amount of heat required to raise the temperature of a unit-weight (pound, kilogram, or gram) of water one thermometric degree. According to the unit-weight and thermometric scale used, the figures will vary, but in this text the basis is understood to be kilograms and the centigrade scale.