Page:Elementary Text-book of Physics (Anthony, 1897).djvu/226

212 The elementary gases obey a similar law with considerable exactness; the constant given by the product of their specific heats at constant pressure and their atomic weights is about 3.4.

The following table will illustrate the law of Dulong and Petit. The atomic weights are those given by Clarke.

194. Fusion and Solidification.—When ice at a temperature below zero is heated, its temperature rises to zero, and then the ice begins to melt; and, however high the temperature of the medium that surrounds it may be, its temperature remains constant at zero so long as it remains in the solid state. This temperature is the melting-point of ice, and because of its fixity it is used as one of the standard temperatures in graduating thermometric scales. Other bodies melt at very different but at fixed and definite temperatures. Many substances cannot be melted, as they decompose by heat.

Alloys often melt at a lower temperature than any of their constituents. An alloy of one part lead, one part tin, four parts bismuth, melts at 94°; while the lowest melting-point of its constituents is that of tin, 328°. An alloy of lead, tin, bismuth, and cadmium melts at 62°.

If a liquid be placed in a medium the temperature of which is below its melting-point, it will in general begin to solidify when its temperature reaches its melting-point, and it will remain at that temperature until it is all solidified. Under certain conditions, however, the temperature of a liquid maybe lowered several degrees below its melting-point without solidification, as will be seen below.