Page:Elementary Text-book of Physics (Anthony, 1897).djvu/200

 CHAPTER I.

MEASUREMENT OF HEAT.

157. General Effects of Heat.—Bodies are warmed, or their temperature is raised, by heat. The sense of touch is often sufficient to show difference in temperature; but the true criterion is the transfer of heat from the hotter to the colder body when the two bodies are brought in contact, and no work is done by one upon the other. This transfer is known by some of the effects described below.

Bodies, in general, expand when heated. Experiments show that different substances expand differently tor the same rise of temperature. Gases, in general, expand more than liquids, and liquids more than solids. Expansion, however, does not universally accompany rise of temperature. A few substances contract when heated.

Heat changes the state of aggregation of bodies, always in such a way as to admit of greater freedom of motion among the molecules. The melting of ice and the conversion of water into steam are familiar examples.

Heat breaks up chemical compounds. The compounds of sodium, potassium, lithium, and other metals, give to the flame of a Bunsen lamp the characteristic colors of the vapors of the metals