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Joseph Black













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JOSEPH BLACK (1728-1799), Scottish chemist and physicist, was born in 1728 at Bordeaux, where his father - a native of Belfast but of Scottish descent - was engaged in the wine trade. At the age of twelve he was sent to a grammar school in Belfast, whence he removed in 1746 to study medicine in Glasgow. There he had William Cullen bor his instructor in chemistry, and the relation between the two soon became that of professor and assistant rather than of master and pupil. The action of lithontriptic medicines,, especially lime-water, was one of the questions of the day, and through his investigations of this subject Black was led to the chemical discoveries associated with his name. The causticity of alkaline bodies was explained at that time as depending on the presence in them of the principle of fire, "phlogiston"; quicklime, for instance, was chalk which had taken up phlogiston, and when mild alkalis such as sodium or potassium carbonate were causticized by its aid, the phlogiston was supposed to pass from it to them. Black showed that on the contrary causticization meant the loss of something, as proved by loss of weight; and this something he found to be an "air," which, because it was fixed in the substance before it was causticized, he spoke of as "fixed air." Taking magnesia alba, which he distinguished from limestone with which it had previously been confused, he showed that on being heated it lost weight owing to the escape of this fixed air (named carbonic acid by Lavoisier in 1781), and that the weight was regained when the calcined product was made to reabsorb the fixed air with which it had parted. These investigations, by which Black not only gave a great impetus to the chemistry of gases by clearly indicating the exi stence of a gas distinct from common air, but also anticipated La voisier and modern chemistry by his appeal to the balance, were described in the thesis De humore acido a cibis orto, et magnesia alba, which he presented for his doctor's degree in°1754; and a fuller account of them was read before the Medical Society of Edinburgh in June 1755, and published in the following year as Experiments upon magnesia, quicklime and some other alkaline substances. It is curious that Black left to others the detailed study of this "fixed air" he had discovered. Probably the explanation is pressure of other work. In 1756 he succeeded Cullen as lecturer in chemistry at Glasgow, and was also appointed professor of anatomy, though that post he was glad to exchange for the chair of medicine. The preparation of lectures thus took up much of his time, and he was also gaining an extensive practice as a physician. Moreover, his attention was engaged on studies which ultimately led to his doctrine of latent heat. He noticed that when ice melts it takes up a quantity of heat without undergoing any change of temperature, and he argued that this heat, which as was usual in his time he looked upon as a subtle fluid, must have combined with the particles of ice and thus become latent in its substance. This hypothesis he verified quantitatively by experiments, performed at the end of 1761. In 1764, with the aid of his assistant, William Irvine (1743-1787), he further measured the latent heat of steam, though not very accurately. This doctrine of latent heat he taught in his lectures from 1761 onwards, and in April 1762 he described his work to a literary society in Glasgow. But he never published any detailed account of it, so that others, such as J. A. Deluc, were able to claim the credit of his results. In the course of his inquiries he also noticed that different bodies in equal masses require different amounts of heat to raise them to the same temperature, and so founded the doctrine of specific heats; he also showed that equal additions or abstractions of heat produced equal variations of bulk in the liquid of his thermometers. In 1766 he succeeded Cullen in the chair of chemistry in Edinburgh, where he devoted practically all his time to the preparation of his lectures. Never very robust, his health gradually became weaker and ultimately he was reduced to the condition of a valetudinarian. In 1795 he received the aid of a coadjutor in his professorship, and two years later he lectured for the last time. He died in Edinburgh on the 6th of December 1799 (not on the 26th of November as stated in Robison's life).

As a scientific investigator, Black was conspicuous for the carefulness of his work and his caution in drawing conclusions. Holding that chemistry had not attained the rank of a science - his lectures dealt with the "effects of heat and mixture" - he had an almost morbid horror of hasty generalization or of anything that had the pretensions of a fully fledged system. This mental attitude, combined with a certain lack of initiative and the weakness of his health, probably prevented him from doing full justice to his splendid powers of experimental research. Apart from the work already mentioned he published only two papers during his life-time - "The supposed effect of boiling on water, in disposing it to freeze more readily" (Phil. Trans., 1775), and "An analysis of the waters of the hot springs in Iceland" (Trans. Roy. Soc. Ed., 1794).

After his death his lectures were written out from his own notes, supplemented by those of some of his pupils, and published with a biographical preface by his friend and colleague, Professor John Robison (1739-1805), in 1803, as Lectures on the Elements of Chemistry, delivered in the University of Edinburgh.



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