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Cullen in the chair of anatomy and chemistry at Glasgow, and he continued to teach there for ten years, when he was appointed to the chemistry professorship at Edinburgh. He then lectured for thirty years to numerous classes, and retiring in 1796 lived till 1799, and died on the 26th of November in that year. His health never was robust; it was indeed precarious at all times from a weakness in the bronchia and chest, but he prolonged life by a system of the strictest abstinence, frequently subsisting

for days together on watergruel and diluted milk. He never was married; but he cherished with unvarying affection his near relatives, who well deserved his care. His favourite niece, Miss Burnet, a person of great sense and amiable temper, was married to his friend and second cousin, Professor Ferguson, the historian and moral philosopher. Dr. Black lived in a select circle of friends, the most illustrious men of the times in science and in letters, Watt, Hutton, Hume, Robertson, Smith, and afterwards with the succeeding generation of Scottish worthies, Robison, Playfair, Stewart. Delighting to commune, to speculate, and to investigate with them, he was careless of the fame which however he could not but be sensible his labours must achieve. He was extremely averse to publication, contemning the impatience with which so many men of science hurry to the press,

often while their speculations are crude, and their inquiries not finished. Nor could the reason often urged in defence of this find much favour with one who seemed never to regard the being anticipated by his fellowlabourers as any very serious evil, so the progress science was secured. Except two papers, one in the

London Philosophical Transactions for 1775 on the freezing of boiled water; the other, in the second volume of the Edinburgh Transactions,' on the Iceland hot springs; he never published any work after that of which we are now to speak, in 1755, and which, but for the accidental occasion that gave rise to it,

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would possibly, like his other original speculations, never have been given by himself to the press.

Upon taking his degree at Edinburgh College he wrote and published a Latin Thesis, after the manner of that as well as the foreign universities. The subject was 'Magnesia, and the Acid produced by Food in the Stomach (De Acido e Cibis orto; et de Magnesia), and it contained the outline of his discoveries already made. Having sent some copies of this Thesis to his father at Bordeaux, one was given to Montesquieu, who at once saw the vast importance of the truths which it unfolded. He called a few days after and said to Mr. Black, “I rejoice with you, my very good friend: your son will be the honour of your name and of your family.” But though the discoveries were sketched distinctly enough in this writing, they were only given at large the following year in his celebrated work Experiments on

• Magnesia, Quicklime, and other Alkaline Substances, incontestably the most beautiful example of strict inductive investigation since the "Optics' of Sir Isaac Newton. His fervent admiration of that masterly work was indicated by his giving it to Professor Robison, then a student, and desiring him to “make it the model of all his studies,” recommending him at the same time a careful study of the mathematics. It appears that this important inquiry concerning the alkaline earths, the results of which were destined to change the face of chemical science, was suggested by the attempts then making to find a solvent for the stone. I distinctly recollect Dr. Black, in his lectures, prefacing the admirable and most interesting account which he gave of his discoveries, with the statement that the hopes of finding a solvent which should not, like the caustic alkalies, destroy the substance of the bladder in melting the stone, first led him to this investigation. Professor Robison has given a note from his memorandum-book indicating that he had at first fallen into the notion of alkalies, when treated with quicklime,

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deriving from it their caustic quality; the common belief (which gave rise to the term caustic) being that lime obtained from the fire the quality of growing extremely hot, even to ignition when united with water. But experiment soon corrected this idea; for, having exposed the caustic or quicklime to the air till it became mild, he says, “Nothing escapes (meaning.no fre or heat); the cup rises considerably by absorbing air.” Another observation on the comparative loss of weight sustained by chalk when calcined (in the fire), and when dissolved in an acid, is followed by the account of a medical case, which the Professor knew to have occurred in 1752. A third note follows, and proves him to have now become possessed of the true theory of causticity, namely, the expulsion of air, and of mildness, namely, its absorption. The discovery was therefore made as early as 1752—it was published generally in 1754—it was given in its fullest details in 1755. At this time M. Lavoisier was a boy at school -nine years old when the discovery was made-eleven when it was published-twelve when it was as fully given to the world as its author ever delivered it. No possibility therefore existed of that great man finding out when he composed his great work that it was a discovery of his own,

as he did not scruple to describe oxygen, though Dr. Priestley had first communicated it to him in the year 1774; or that Black and he discovered it about the same time, as he was in the habit of stating with respect to other gases, with a convenient degree of ambiguity just sufficient for self-defence, should he be charged with unfair appropriation. Who that reflects on the noble part which this great philosopher acted, both in his life and in his death, can avoid lamenting that he did not rest satisfied with the fame really his due, of applying the discoveries in which he had no kind of share, to the investigation of scientific truths, as entirely the result of his extraordinary faculty of generalization, and genius for philosophical research, as those discoveries, the materials of his induction, were the undivided property of others !

The capital discovery of Black, thus early made, and to any share in which no one has ever pretended, was that the causticity, as it was formerly termed upon a false theory, of the alkalis and alkaline earths, was owing to the loss of a substance with which they had been combined, and that their reunion with this substance again rendered them mild. But the nature of this substance was likewise ascertained by him, and its detection forms by far the most important part of the discovery, for it laid the foundation of chemical science. He found that it was a permanently elastic fluid, like air in some of its mechanical qualities, those of being transparent or invisible, and incondensable, but differing entirely from the air of our atmosphere in its chemical properties. It was separated from alkaline substances by heat, and by the application of acids, which, having a stronger elective affinity for them, caused it to be precipitated, or to escape in the aëriform state; it was heavier than common air, and it gave a slight acidulous flavour to water on being absorbed by it; hence the inference that it was an acid itself. A short time afterwards (in 1757) he discovered that this

peculiar air is the same with that produced by the fermentation of vegetable substances. This he ascertained by the simple experiment of partially emptying in a brewer's vat, where the fermenting process was going on, the contents of a phial filled with lime-water. On shaking the liquid that remained with the air that had entered, he found it become turbid, from the lime having entered into union with the air, and become chalk. The same day he discovered by an experiment, equally simple and equally decisive, that the air which comes from burning charcoal is of the same kind. He fixed a piece of charcoal in the broad end of a bellows nozzle, unscrewed ; and putting that in the fire, he inserted the other end in a vessel filled with lime-water. The air that was driven through the liquid again precipitated the lime in the form of chalk. Finally, he ascertained by breathing through a syphon filled with lime-water, and finding the lime again precipitated, that animals, by breathing, evolve air of this description.

The great step was now made, therefore, that the air of the atmosphere is not the only permanently elastic body, but that others exist, having perfectly different qualities from the atmospheric air, and capable of losing their elasticity by entering into chemical union with solid or with liquid substances, from which being afterwards separated, they regain the elastic or aëriform state. He gave to this body the name of fixed air, to denote only that it was found fixed in bodies, as well as elastic and separate. He used the term “air” only to denote its mechanical resemblance to the atmospheric air, and not at all to imply that it was of the same nature. No one ever could confound the two substances together; and accordingly M. Morveau, in explaining some years afterwards the reluctance of chemists to adopt the new theory of causticity, gives as their excuse, that although this doctrine "admirably tallies with all the phenomena, yet it ascribes to fixed air properties which really make it a new body or existence” (forment réellement un nouvel etre”).*

In order to estimate the importance of this discovery, and at the same time to show how entirely it altered the whole face of chemical science, and how completely the doctrine was original, we must now examine the state of knowledge which philosophers had previously attained upon the subject.

It has often been remarked that no great discovery was ever made at once, except perhaps that of logarithms; all have been preceded by steps which conducted the discoverer's predecessors nearly, though not quite, to the same point. Some may possibly think

Supplement to the · Encyclopédie,' vol. ii., p. 274, published in 1777.

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