which is kept constantly cold by being surrounded with the cold water in which it is immersed; and the vapour, being thus immediately reduced below the temperature of 212°, is reconverted into water. At first it collects in a dew on the surface of the vessel D; but as this accumulates, it drops into the bottom of the vessel, and forms a more considerable quantity. As the quantity of water is observed to be gradually diminished in the vessel B, the quantity will be found to be gradually increased in the vessel D; and if the operation be suspended at any stage of the process, and the water in the two vessels weighed, it will be found that the weight of the water in D is exactly equal to the weight which the water in B has lost. (55.) The demonstration is, therefore, perfect, that the gradual diminution of the boiling water in the vessel B is produced by the conversion of that water into steam by the heat. In the process first described, when the top of the vessel B was supposed to be open, this steam made its escape into the air, where it was first dispersed, and subsequently cooled in separate particles, and was deposited in minute globules of moisture on the ground and on surrounding objects. (56.) In reviewing this process, we are struck by the fact, that the continued application of heat to the vessel B is incapable of raising the temperature of the water contained in it above 212°. This presents an obvious analogy to the process of liquefaction, and leads to inquiries of a similar nature, which are attended with a like result. We must either infer, that the water, having arrived at 212°, received no more heat from the mercury; or that such heat, if received, is incapable of affecting the thermometer; or, finally, that the steam which passes off carries this heat with it. That the water receive heat from the mercury, will be proved by the fact, that, if the vessel B be removed from the mercury, other things remaining as before, the temperature of the mercury will rapidly rise, and if the fire be continued, it will even boil; but so long as the [Pg107] vessel B remains immersed, it prevents the mercury from increasing in temperature. It therefore receives that heat which would otherwise raise the temperature of the quicksilver. [Illustration: _Fig._ 16.]