Is not this a sufficient answer to the question just proposed? Adopt any theory you please respecting the nature of heat: suppose a material substance, or conceive an effect, or quality, or a series of vibrations; in any case, what is apparently lost in the former case is regained in the latter. Without sacrifice of accuracy we may affirm, in any sense, that the heat which had disappeared in the process of thawing has re-appeared in the process of congealing. Moreover, the most exact thermometric observations showed the _amount_ in the two cases to be the same. Thus, without reference to any particular theory of the nature of heat, Dr. Black was justified in asserting that a certain portion of heat becomes _latent_ in the water; and that it owes its fluid state to this latent heat. We have here referred only to one class of these phenomena; to one particular application of the general law. Similar results take place when water boils: the boiler receives as much heat from the fire during the time requisite to raise it to 212° as it does during the next equal portion of time; but its temperature (in an open vessel) will not rise beyond that point. Here then again a quantity of heat has disappeared; but the water is converted into vapour. Collect the vapour in a cold receiver; it produces a high degree of heat, and is re-condensed into the form of water. The heat then, whatever it be, Dr. Black inferred, is latent in the steam. It is not destroyed; it disappears as temperature, but under other circumstances it can be made to re-appear: it is therefore merely concealed, or dormant for the time; and no term can be so proper to describe its condition as _latent heat_. Analogous facts are presented by all other bodies which have been subjected to examination. Whenever a change of state from the aëriform to the liquid, or from the solid to the liquid takes place, a corresponding evolution or absorption of heat accompanies it. Every research of experimenters on this subject, since Dr. Black, has contributed fresh instances confirming the universality of this great law of nature. A solid body then requires a certain portion of heat to be thrown into it, in order to melt, or convert it into a liquid: and the liquid again requires a similar supply to evaporate it into steam, or convert it into an elastic fluid state; and this portion of heat produces no influence on the temperature of the body. The reverse is true of the reverse processes. The quantity of heat so absorbed or given out is different in different bodies.