What then, Dr. Black enquired, becomes of the heat which has been all along given to the vessel of ice? Heat has been communicated to it as well as to the other vessel; yet it has not been employed in raising the temperature, but in some way has been expended in converting the ice into water. It is but this simple fact otherwise stated, when we say that the heat so imparted has disappeared as heat of temperature; but may it not have been destroyed or annihilated? To reply to this question we have only to consider that the same vessel of water, cooled nearly to the freezing point, and then exposed to a much greater degree of cold, must, by the same rule, continue parting with its excess of temperature above that of the colder bodies around it. Yet a thermometer immersed in it continues invariably at 32° till the whole has become ice; it then will sink to the lower temperature, but not before. Thus there must be within it a continued supply of heat in order to keep it up to 32° all the time.
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.
Not only indeed is this the case in these changes of state, but it is also the case in the simple instance of mere changes in the temperature of bodies; different bodies require different degrees of heat to be communicated, or thrown into them, in order to produce the same increase of sensible temperature. This was the other great result to which we referred at first as the discovery of Dr. Black: he designated this peculiarity in bodies their capacity for heat; a term sufficiently expressive, but which is now more usually exchanged for the term specific heat. The establishment of the accurate values of this capacity or specific heat, in a number of different bodies, has afforded a wide field of research for subsequent experiments. It has been sometimes said that to Dr. Black’s discovery of latent heat we owe the steam-engine. This is, we think, a mistaken view of the matter. That heat will generate steam, and cold condense it, are facts that were well known, independently of the doctrine of latent heat; though that doctrine undoubtedly gives the explanation of them. The knowledge of these facts might therefore have been practically applied in the construction of the steam-engine, had Dr. Black’s discovery never been made. It is at the same time perfectly true, that this theory supplies us with accurate data dependent on the quantity of heat necessary to be communicated, on which calculation must proceed: and it is on the basis of such exact investigation, that the great improvements in the application of steam have been brought about.
To return however to our narrative: though, as we have said, the leading ideas of these discoveries had occurred to the author probably about the year 1757, yet it was not till a few years afterwards that he had fully made out his theory. The discovery of specific heat was announced in 1760; and that of latent heat, with all the details of its experimental proof, was laid before a literary society in Glasgow, in a paper read April 23, 1762. After this period a full account of both subjects was regularly introduced by the author into his courses of lectures. He did not himself follow out the train of experimental research to which he had opened the way, but his friends and disciples entered largely upon the investigation of those valuable data, the numerical values expressing the quantities of latent heat and specific heat belonging to different substances.
In 1766, Dr. Cullen having been promoted to the chair of medicine, Dr. Black, again treading in the steps of his revered friend and instructor, was called from Glasgow to the professorship of chemistry at Edinburgh. He was thus placed in a more conspicuous position, and the fame of the Edinburgh school was not a little raised by his accession to it. Students flocked from all quarters in increasing numbers, and Dr. Black now devoted himself entirely to perfecting his chemical lectures.
In reference to this period, it has been sometimes remarked as singular, that while chemical science was beginning to make those rapid strides by which its modern advance has been so much accelerated, Dr. Black should have been contented to go on merely as an able expositor and illustrator of what others were doing, without himself taking any share in their labours. Perhaps it might be difficult to assign any better reasons for this conduct than are to be found in the peculiar disposition of the individual, though it has been alleged that he was actuated by a dread of criticism; this, indeed, can only be regarded as itself an indication of a morbid sensitiveness of mind, of which, unhappily, we have other instances in individuals of the highest philosophical genius; and which has probably, in more than one instance, deprived the world of services which would have been invaluable in the cause of science. Be this as it may, Dr. Black, though he continued by constant revisions and additions to make his lectures amply keep pace with the discoveries of the day, yet himself produced during this period only two papers, and those of minor importance: one appeared in the Philosophical Transactions for 1774, in which he assigned the reason why water which has been boiled freezes more easily than that which has not, viz., the expulsion of the air: another was inserted in the second volume of the Edinburgh Transactions, on the analysis of the water from the Geysers of Iceland.