DR BLACK.
Joseph Black was born in 1728, near Bourdeaux 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 accurate 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 characterized 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 alkalis, between limestone and quick-lime, common and calcined magnesia, &c. The whole of this subject was at that periodinvolved 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 calledfixed airby him, andcarbonic acid gasby 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 Professor 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 relations 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 oflatent heat, and that ofspecific 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 theearliest 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 32°, &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 thistime, 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 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 hasdisappearedas 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 ofheatin 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 theamountin 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 becomeslatentin 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 merelyconcealed, or dormant for the time; and no term can be so proper to describe its condition aslatent 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 theircapacity for heat; a term sufficiently expressive, but which is now more usually exchanged for the termspecific 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 probablyabout 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.
It appears from an anecdote related on good authority, (see Edinburgh Encyclopædia, article, Dr. Black) that so early as 1766, when the low specific gravity of hydrogen as discovered by Mr. Cavendish had been announced, the idea of employing it for balloons occurred to Dr. Black; and that he actually exhibited a small one, to the extreme astonishment of a party of friends. It was not till 1782 Montgolfier claimed the merit of originating this idea.
Dr. Black never enjoyed very robust health, but by great care and attention he managed to the best advantage a constitution naturally delicate, pursuing, especially towards the latter part of his life, an extremely regular and abstemious mode of living. About 1793 his strength began to fail. In 1796 he became unequal to the sole discharge of his duties as a lecturer, and employed an assistant. In the following year he was compelled to relinquish lecturing altogether. Though in great weakness, he was able by unremitting precautions to preserve a considerable share of general health. He had always expressed a hope that he might be spared the distress of a long illness; and, in accordance with this wish, while sitting at table partaking of his usual simple fare, he expired November 26, 1799, in so tranquil a manner, that a cup of milk which he had placed on his knee remained unspilt; and it was some time before his servant perceived that life was extinct.
The cast and character of Dr. Black’s mind is illustrated by the whole nature and course of his labours and investigations. Methodical precision and originality of thought were the qualities which pre-eminently distinguished him. In framing general conclusions he was peculiarly cautious and exact. It is clear that he possessed abilities which might have placed him much higher in the rank of original discoverers, had not an unfortunate backwardness, perhaps the result of natural timidity or indolence, perhaps of weak health and incessant employment, withheld him from pushing his researches to a greater extent, and even from asserting his just claims to what he had done, which was in some instances wrongfully appropriated by others. Some charges of this nature have been brought against Lavoisier, in reference to the discovery of the nature of alkalies; but in his writings Lavoisier certainly does ample justice to Black.
In all the best and most substantial qualifications of a teacher and lecturer, he has seldom been surpassed. His method was luminous and natural; his style unadorned, but beautifully perspicuous; his experimental illustrations completely satisfactory and convincing, yet always of the simplest possible kind. He manifested a great disliketo any unnecessary parade of apparatus, and the exhibition of showy and striking, but useless phenomena. He aimed not at display and popular fame, but to arrive at the best means of interesting, instructing, and enlightening his pupils. He led them by his own example pre-eminently to value accuracy in the establishment of facts, caution in deducing general conclusions, and a resolute adherence to the results derived from experiment and induction.
Dr. Black’s moral and social character was exactly such as harmonized with his mental endowments. He was moderate in his desires, temperate in his enjoyments, benevolent and warm in his affections. He manifested a strong love of order, propriety, and decorum, and a total absence of jealousy against scientific rivals, or envy of their fame. His disposition was at once serious and cheerful; and he was distinguished by a happy equanimity of temper. He was sometimes accused of penuriousness: but the charge is wholly denied by his relative, Dr. Ferguson; and his intimate friend, Professor Robison, has related many instances of his conduct totally incompatible with such a disposition. In person he was rather above the middle height; of a slender figure, with a mild and engaging countenance.
After Dr. Black’s death his manuscript lectures were revised and published by Professor Robison, in two quarto volumes, in 1803. The first and most important portion of the work is devoted to the subject of heat; and contains the development of the author’s original researches to which we have referred. The simplicity of style, the admirable taste and propriety of language, and the perspicuous and luminous method of illustration, cannot be too highly praised. With respect to the other portion, embracing the details of chemistry properly so called, though the same commendation as to the manner must be bestowed, the matter, which was not less excellent for the time at which the lectures were delivered, was yet, at the period of the publication, necessarily much behind the advance of discovery.
Engraved by J. Posselwhite.LORD BACON.From a Print by J. Houbraken 1738.Under the Superintendence of the Society for the Diffusion of Useful Knowledge.London, Published by Charles Knight & Co. Ludgate Street.
Engraved by J. Posselwhite.LORD BACON.From a Print by J. Houbraken 1738.Under the Superintendence of the Society for the Diffusion of Useful Knowledge.London, Published by Charles Knight & Co. Ludgate Street.
Engraved by J. Posselwhite.LORD BACON.From a Print by J. Houbraken 1738.Under the Superintendence of the Society for the Diffusion of Useful Knowledge.London, Published by Charles Knight & Co. Ludgate Street.