Page:The fairy tales of science.djvu/192

158 and 100°. The reader will understand that a degree of heat is a mere arbitrary division, and that 212° Fahr. and 100° Cent. indicate the same temperature. We shall adopt the unphilosophical but convenient scale of Fahrenheit throughout this chapter. No indication is afforded by the thermometer of the absolute quantity of heat contained in any substance, but merely of the amount of free or sensible heat capable of producing a certain degree of expansion in a column of mercury. If a quantity of ice, at the temperature of zero, or 0°, be placed in a warm room, it will immediately begin to melt, and a thermometer plunged into it will soon indicate 32°, though at first the column of mercury stood at zero. But, strange to say, the mercury will remain stationary at the freezing-point until the whole of the ice has passed into the liquid form. Thus we see that a large quantity of heat is absorbed by the ice in the act of thawing, so as to be no longer appreciable by the thermometer. Again, if an open vessel containing ice-cold water be placed upon a fire, the temperature of the liquid will rapidly rise to 212°, but at this point it will remain stationary until the whole of the water is converted into steam. The heat thus lost or absorbed during liquefaction and vaporization is called hidden or latent heat, in contradistinction to the heat of temperature. But we must not forget our kettle. The stream