The capacity of receiving heat in the substance is greatest in a state of vapour, and least in that of a solid; so that when ice is converted into water, heat is absorbed, and more still when it is converted into vapour; and on the contrary, when vapour is converted into water, it gives out the heat which it had imbibed, and when it becomes ice it gives out still more.
If equal quantities of ice and water be exposed to heat at the temperature of 32°, the ice will only become water, without receiving any additional sensible heat; but an equal quantity of water in the same situation would be raised to 178°, so that 146 degrees of heat will be imbibed, and remain in latent in the water, in consequence of its passing from a state of ice: and heat communicated by a given weight of vapour will raise an equal weight of a nonevaporable substance, of the same capacity with water, 943 degrees; so that much more heat is latent in steam, than in the water from which it was formed.
This doctrine of latent heat explains a great variety of phænomena in nature; as that of cooling bodies by evaporation, the vapour of water, or any other fluid substance, absorbing and carrying off the heat they had before.
Water, perfectly at rest, will fall considerably below the freezing point, and yet continue fluid: but on the slightest agitation, the congelation of the whole, or part of it, takes place instantly, and if the whole be not solid, it will instantly rise to 32°, the freezing point. From whatever cause, some motion seems necessary to the commencement of congelation, at least in a moderate temperature; but whenever any part of the water becomes solid, it gives out some of the heat it had before, and that heat which was before latent becoming sensible, and being diffused through the whole mass, raises its temperature.
On the same principle, when water heated higher than the boiling point in a digester is suddenly permitted to escape in the form of steam, the remainder is instantly reduced to the common boiling point, the heat above that point being carried off in a latent state by the steam.
Had it not been for this wise provision in nature, the whole of any quantity of water would, in all cases of freezing, have become solid at once; and also the whole of any quantity that was heated to the point of boiling, would have been converted into steam at once; circumstances which would have been extremely inconvenient, and often fatal.