Page:The fairy tales of science.djvu/196

 two degrees higher in glass than in metal; so that if into water in a glass flask which has ceased to boil, a twisted piece of cold iron wire be dropped, the boiling is instantly resumed.

Solid bodies having different temperatures will, if kept in contact, gradually change until they all acquire the same temperature. But this diffusion does not take place instantaneously, or there would be no such thing as difference of temperature. The rapidity with which heat is conducted varies in different substances; for example, if we place a silver spoon and a wooden one in boiling water, the handle of the former will become too hot to be held before that of the wooden one is sensibly warmed. Silver is, therefore, a good conductor and wood a bad conductor of heat.

Liquids conduct heat very slowly and imperfectly. If mercury be poured into a jar, and boiling water be poured over it, the metallic fluid will receive heat but slowly from the water. A thermometer let down a few feet below the surface of a pond or of the sea, would, on being drawn up, indicate a lower temperature than that of the surface water; for the latter, heated by the rays of the sun, communicates little or no heat to the water below. Indeed, it may be questioned whether water has any conducting power.

It may be reasonably inquired how it happens that water is made to boil so readily by the application of heat. A little consideration will show