Let us suppose a mass of ice immersed in the mixture of snow and salt which determines the zero point of the thermometer: this mass, if allowed to continue a sufficient length of time submerged in the mixture, will necessarily acquire its temperature, and the thermometer immersed in it will stand at zero. Let the ice be now withdrawn from the mixture, still keeping the thermometer immersed in it, and let it be exposed to the atmosphere at the ordinary temperature, say 60°. At first the thermometer will be observed gradually and continuously to rise until it attain the elevation of 32°; it will then become stationary, and the ice will begin to melt: the thermometer will continue standing at 32° until the ice shall be completely liquefied. The liquid ice and the thermometer being contained in the same vessel, it will be found, when the liquefaction is completed, that the thermometer will again begin to rise, and will continue to rise until it attain the temperature of the atmosphere, viz. 60°. Hitherto the ice or water has received a supply of heat from the surrounding air; but now an equilibrium of temperature having been established, no further supply of heat can be received; and if we would investigate the further effects of increased heat, it will be necessary to expose the liquid to fire, or some other source of heat. But previous to this, let us observe the time which the thermometer remains stationary during the liquefaction of the ice: if noted by a chronometer, it would be found to be a hundred and forty times the time during which the water in the liquid state was elevated one degree; the inference from which is, that in order to convert the solid ice into liquid water, it was necessary to receive from the surrounding atmosphere one hundred and forty times as much heat as would elevate the liquid water one degree in temperature; or, in other words, that to liquefy a given weight of ice requires as much heat as would raise the same weight of water 140° in temperature: or from 32° to 172°.
The latent heat of water acquired in liquefaction is therefore 140°.
(14.) Let us now suppose that, a spirit lamp being applied to the water already raised to 60°, the effects of a further supply of heat be observed: the thermometer will continue to rise until it attain the elevation of 212°, the barometer being supposed to stand at 30 inches. The thermometer having attained this elevation will cease to rise; the water will therefore cease to become hotter, and at the same time bubbles of steam will be observed to be formed at the bottom of the vessel containing the water, near the flame of the spirit lamp. These bubbles will rise through the water, and escape at the surface, exhibiting the phenomena of ebullition, and the water will undergo the process of _boiling_.