The reason the degrees were commenced at 32° below the freezing point was, because, when the thermometer was invented, that temperature was supposed to be the lowest degree of cold possible, being that of a certain mixture of [Pg100] snow and salt. This, however, has since been found to be an error, very much lower temperatures being obtained by various physical expedients. The temperature of a body is, then, that elevation to which the thermometer would rise when immersed in that body. Thus, if in plunging the thermometer in water we found the mercury to rise or fall to the division marked 100, we should then say, the temperature of the water was 100°. Let us suppose a spirit lamp, or other regular source of heat, applied to a bath of mercury, so as to maintain the mercury at a fixed temperature of 200°, and let another vessel, containing a quantity of ice at a temperature of 20° be immersed in the mercury. Let a thermometer be placed in the mercury, and another in the ice. The following effects will then ensue. The thermometer immersed in the ice will be observed gradually to rise from 20° upwards, until it indicates the temperature of 32°. It will then become stationary, and the ice which had hitherto remained in a solid state will begin to melt and be converted into water. This process of liquefaction will continue for a considerable time, during which the thermometer immersed in the ice will constantly be maintained at 32°. At the moment, however, when the last portion of ice is liquefied, the thermometer will begin again to rise. The coincidence of this ascent of the thermometer with the completion of the liquefaction of the ice, may be very easily observed, because the ice being lighter, bulk for bulk, than water, will float on the surface, and so long as a particle of it remains unmelted it will be distinctly seen. Now it cannot be doubted that, during the whole of this process, the mercury, supposed to be maintained at 200°, constantly imparts heat to the ice; yet, from the moment the liquefaction begins, until it is completed, no increased temperature is exhibited by the thermometer immersed in the melting ice. If during this part of the process no heat were received by the ice from the mercury, the consequence would be, that the application of the lamp would cause the temperature of the mercury to rise above 200°, which may be easily demonstrated by withdrawing the vessel of ice from the mercurial bath during the process of liquefaction. The moment [Pg101] it is withdrawn, the thermometer immersed in the mercury, instead of remaining fixed at 200°, will begin to rise, although the action of the lamp remains the same as before; from which it is evident that the heat which now causes the mercury to rise above 200° was before received by the melting ice.