JOSEPH BLACK was born in 1728, near Bordeaux, in France, where his father, a native of Ireland, but of Scottish extraction, who was engaged in the wine trade, then resided. In 1740 young Black was sent home to receive the rudiments of education at a grammar-school in Belfast. Thence he went, in 1746, to Glasgow, and having chosen the profession of medicine, proceeded in that university with the preliminary studies.

At that period, Dr. Cullen had just entered on the then untrodden paths of philosophical chemistry in his lectures, at which Black was an assiduous attendant. He soon formed an intimacy with his instructor, with whom he associated himself in the toils of the laboratory. It was here that he laid the foundation of his future attainments and discoveries, in an accu

rate and practical knowledge of the science as far as it then reached, and above all in the cultivation of habits of precise and cautious inductive investigation.

In 1750 he removed to Edinburgh to complete his medical course; and it was in connexion with the important inquiries belonging to that department that he made his first discoveries in chemistry.

His first object of research was one which possessed high medical as well as chemical interest-the nature and properties of magnesia. This substance had hitherto been confounded with lime: Dr. Black first showed it to be characterised by peculiar properties which demonstrate its distinct nature as a separate species of earth. The second point of his investigation was the difference between mild and caustic alkalies, between limestone and quick-lime, common and calcined magnesia, &c. The whole of this subject was at that period involved in complete obscurity. Dr. Black showed, by simple and decisive experiments, the real condition of these substances, and indicated the general law by which they are governed, viz. that the difference consists merely in the combination of the simple earth or alkali with a peculiar air, which is driven off by heat, and which was called fixed air by him, and carbonic acid gas by later chemists. He did not, however, prosecute the inquiry into the nature and properties of this gas. This discovery supplied the foundation on which all subsequent researches and theories have been built. He gave an account of these investigations in an inaugural dissertation, composed as an exercise on taking his Doctor's degree, and in a paper entitled Experiments on Magnesia Alba, &c.,' first published in the Edinburgh Physical and Literary Essays' in 1755.

It was almost immediately after the publication of these researches that Dr. Cullen was elected ProVOL. IV.

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fessor of Chemistry at Edinburgh. The reputation which Dr. Black had now acquired pointed him out as the proper person to succeed to the vacant chair at Glasgow, to which he was accordingly appointed in 1756. His department included chemistry and medicine; and he also practised as a physician. His lectures soon became highly popular from the clearness of his style and method, and the beauty and simplicity of his experimental illustrations. He did not, however, prosecute his inquiries, in that particular department of chemistry in which he had already had so much success. But in another branch of science his power of original research was signally displayed.

The relation of bodies to heat, especially in connexion with the changes of state they undergo, was a subject which had hitherto excited hardly any notice; and though some effects were such as might have been supposed obvious, still no one had as yet reasoned on them, or understood their nature.

It is a characteristic of great genius to find important matter of reflection in objects which the vulgar pass by as too common to excite notice, and Dr. Black having remarked some very common facts with regard to heat, was conducted to those great discoveries on which his celebrity rests-that of latent heat, and that of specific heat; which last term is, in fact, only another mode of expressing the same principle. This great truth, the foundation of all our determinate ideas of the causes of those diversities of physical condition which the same mass of matter is capable of assuming, seems to have suggested itself to the mind of the discoverer about the year 1757.

After the invention of the thermometer, it had been among the earliest facts observed that changes in the state of bodies, such as boiling, freezing, melting, &c., take place always at certain fixed temperatures as indicated by the thermometer; and at a different degree of the scale, for each different substance.

And several of these remarkable points came by custom to be marked upon the thermometric scale.

When, however, it was said that water always boiled at 212° of Fahrenheit, or froze at 32o, &c., it was not meant that the mass would boil or freeze the instant the thermometer reached that point. It was supposed that a certain increase or diminution of temperature (as the case might be) was necessary for the production of the effect beyond that precise point; though that point marked, as it were, the commencement of the process. The views generally entertained on this subject were, however, so vague, that it is difficult to make out precisely what was imagined to take place; but it seems to have been supposed that a very slight accession or loss of heat was sufficient completely to accomplish the change.

Such were the notions which prevailed on the subject prior to the commencement of Dr. Black's researches. No one advanced, or seemed to have any desire to advance, a step nearer to the truth: yet the whole was a mere question of fact, and a fact of the most obvious nature. In this we cannot fail to observe one of those instructive instances, which the history of science often brings before us, of the unaccountable blindness, even of inquiring minds, to truths constantly before their eyes, or, if perceived, to the importance of their being thoroughly examined. A very little consideration ought to have shown any observer that the gain or loss of heat in the cases in question is by no means slight or trifling in amount: yet no one thought of this till Dr. Black pointed it out; and no one reasoned upon it, or perceived its bearing, till that philosopher showed the curious inference to be drawn from it. The case was simply this: Two equal vessels, one full of water just at the freezing temperature, the other of actual ice, are brought into a warm room. In a short time the water

acquires the temperature of the room. Exactly the same quantity of heat has been communicated to the vessel of ice; yet, at the end of the same time, it is found to retain precisely the same temperature as at first. A considerable part of it indeed has been melted, but it may take several hours more to melt the whole. Until that change is completed, the temperature does not vary a single degree. As soon as all the ice is liquefied, and not before, the temperature of the mass begins to rise, and proceeds to increase, from this time, as rapidly as that of the water in the other vessel did before, until it acquires the temperature of the room.

What then, Dr. Black inquired, 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 32o 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