It is upon the property which liquids possess, of being convertible into the gaseous state, that the processes of evaporation and distillation depend. The first of these is performed, when our object is to separate the more vola tile parts of a compound, for the purpose of procuring those that are less so ; and the second, on the contrary, is done ill order to procure the volatile part itself. A minute account of these operations belongs more to chemistry, and to the various branches of chemical manufactures, than to the general view which we are taking of die effects of heat. (See DISTILLATION and EVAPORATION.) In both cases, the separation is effected by the combination of heat with the liquid ; in the one, it is sometimes produced by the heat which naturally exists in the atmosphere, aided by a current of air, continually presenting a new stratum to be acted upon ; and at other times a quantity of heat is thrown into the substance by the combustion of some kind of fuel. It is, however, necessary, in either case, that a large sur face should be exposed to the atmosphere, for the purpose of carrying off the vapour as rapidly as it is produced ; for it appears that it is principally at the surface that it is generated. In the process of distillation, artificial heat is always employed, and commonly in a greater degree than where the object is merely to produce evaporation. Here the liquid is confined in close vessels ; and the operation depends upon a portion of it, which is nearest the source of heat, becoming converted into vapour, and passing up through the other parts of the fluid in the form of bubbles, constituting what is styled ebullition.
We have already remarked, that when a solid is con verted into a liquid, or a liquid into an elastic fluid, the con version is brought about suddenly. The substance in ques tion, before it changes its state, continues to receive heat, is expanded in a certain degree, and has its temperature raised ; but if an additional quantity of heat be still given to it, the expansion no longer goes on in the same manner, and the temperature is no longer elevated, but it assumes a new form, becoming, according to circumstances, either a liquid or a vapour. It was formerly supposed, that this change did not depend upon any peculiar or specific ac tion, but that the mere addition of a certain small portion of heat was adequate to effect it. Dr Black, however, per ceived the insufficiency of the opinion usually entertained upon the subject, and was induced to investigate it with great assiduity ; the result of which was, the establishment of his celebrated theory of latent heat. The fundamental position of this theory is, that when a solid is converted into a liquid, or a liquid into a gas, a much greater quantity of heat is absorbed by it than is perceptible by the sensa tions, or the thermometer, the effect of which is to unite with the particles of the body, and thus to alter its form. When, on the contrary, the vapour is reduced to the state of a liquid, or a liquid to that of a solid, heat is disengaged froni it, without the substance in question indicating any diminution of temperature, either to the sensations, or to the thermometer. This phenomenon is considered to be
the reverse of the former ; here the heat that escapes is not supposed to have affected the body in the way that free caloric acts, but simply to have maintained it in the state of elastic fluidity, or of liquidity.
We are hence led to regard heat as existing in two states that are essentially different from each other : the one pro ducing temperature and expansion, and of course capable of being measured by the thermometer ; the other not manifesting these properties, but uniting itself to bodies, and changing their form. The former we call free or un combined heat ; the latter latent or combined heat. As we are in the habit of conceiving temperature and ex pansion* to be the most certain indications of the presence of heat, it required a train of minute experiments to esta blish a position, which appeared, at first view, so much at variance with our ordinary conceptions, as that heat could be communicated to bodies without producing these effects. The following facts may be considered as the ground-work of Black's doctrine. Let a mass of pounded ice, cooled several degrees below the freezing point, be exposed to the heat of a furnace, so that it may receive equal quantities of caloric in intervals of time. The ice, as it receives the heat, becomes warmer at each suc cessive interval, until the whole acquires the temperature of .32° ; but now, although the heat still remains applied as before, the ice acquires no additional warmth ; it still indicates 32° to the thermometer, but it is observed gra dually to dissolve. No increase of temperature takes place until the whole is rendered fluid, and then it again begins to grow warmer, and, as before, acquires successive incre ments of heat, until it arrives at the 212th degree. Here, again, the process ceases ; and whatever quantity of heat be sent into the fluid, its temperature remains unchanged ; but now the fluid is observed gradually to assume the state of vapour, until the whole of it is converted into the aeri form state. Now there is every reason to conclude, that in this case the ice and water continue to receive equal quan tities of heat for the whole period ; yet during some part of the process, the thermometer remains stationary. At this very time, however, the change of form occurs ; and therefore it is natural to conclude, that it is connected with the absorption of a portion of heat, which, in consequence of its not producing either temperature or expansion, has obtained the name of latent. The legitimate inference is, that a certain quantity of caloric, which would otherwise have been employed in producing temperature and expan sion, is necessary in the one case for melting the ice, and in the other for v-aporizing the water.