Page:International Library of Technology, Volume 93.djvu/39



25. Although heat is not a material substance, it is a Physical quantity, and may be measured. If a quart of water is placed over a gas or oil flame, it will be found that it takes practically five times as long tQ raise the temperature of the water 5° as it does to raise it 1°, because five times as much heat has been added to the water in the first case as in the second. Now suppose that, instead of 1 quart, 2 quarts of water are placed in a vessel over the same flame. It will be found that it takes twice as long to raise the temperature of the 2 quarts 1° as it did to raise the temperature of 1 quart the same amount. The burning oil or gas below is giving off beat at a uniform rate, and of this heat twice as much has been absorbed by the 2 quarts of water as the 1 quart absorbed in acquiring the same temperature. For any given substance, it may be said that the amount of heat contained in any quantity of that substance is proportional to the product of the weight of the given quantity and its absolute temperature.

The unit commonly employed in the United States of America and in England for measuring the quantity of heat is the British thermal unit, which is the quantity of heat required to raise the temperature of 1 pound of water 1° Fahrenheit. Hence, 10 British thermal units will heat 1 pound of water 10°, or 10 pounds of water 1°. For convenience, the term British thermal unit is commonly abbreviated to B. T. U.

26. Introductory. — The volume of any body — solid, liquid, or gaseous — is always changed if the temperature is changed; nearly all bodies expand when heated, and contract when cooled. In solids, expansion may be considered in three ways: first, expansion in one direction, as the elongation of an iron bar; this is called linear expansion;