Joseph Black

Joseph Black (1728-1799), was a Scottish chemist and physicist, known for the concepts of latent heat and specific heat, and for the discovery of carbon dioxide. He was a founder of thermochemistry, and was the mentor of James Watt. Black was a member of the Poker Club and friend of David Hume, Adam Smith, James Hutton and other leading figures of the Scottish Enlightenment.

Life

"I was born at Bordeaux April 16th, 1728. My father was a merchant settled there, but born in Ireland and the son of a citizen of Belfast of Scottish extraction. My father's residence at Bordeaux was in the suburb called the Chartron, and he had also a farm and country house and vineyard on the other side of the river on a hill called Lormont which commands a fine prospect of the river and city." (from autobiographical memorandum@

Joseph Black was one of fifteen children (8 sons and 5 daughters surviving). His father, John Black, was a wine merchant; his mother, Margaret, was from Aberdeenshire. At the age of twelve, he was sent to a grammar school in Belfast to learn Latin and Greek. In 1744 he enrolled at Glasgow University to study arts, subsequently changing, to study medicine. There, William Cullen was his instructor in chemistry, and the relation between the two soon became that of professor and assistant rather than of master and pupil.

Glasgow years

limestone + heat gives fixed air + quicklime [ CaCO3 → CO2 + CaO ] when quicklime is slaked with water and then boiled with mild alkali (K2CO3), the alkali becomes caustic and the original weight of limestone is restored [ CaO + H2O → Ca(OH)2 Ca(OH)2+ K2CO3 → CaCO3 + 2 KOH ] "If quicklime is mixed with a dissolved alkali it shows an attraction for fixed air superior to that of the alkali. It robs this salt of its air and thereby becomes mild itself..." (from A Short History of Chemistry, J. R. Partington, 3rd edition, (1957)

One of the scientific questions of the day was the action of lithontriptic medicines, especially lime-water. At that time, the caustic nature of alkalines was explained by the supposed 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 carbonate were causticized by its aid, the phlogiston was supposed to pass from it to them. Black showed that, on the contrary, causticization involved 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" (named carbonic acid by Lavoisier in 1781, - fixed air is what we now know of as carbon dioxide). He showed that when magnesium carbonate (magnesia alba) was heated it lost weight because of the escape of this fixed air, but the weight was regained when the product was made to reabsorb the fixed air.

These investigations 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 he published a fuller account of them in 1756 as "Experiments upon magnesia, quicklime and some other alkaline substances". Black left to others the detailed study of this "fixed air" he had discovered.

One of these was his student, Daniel Rutherford (1749-1819), who became famous as the discoverer of nitrogen. In one of his experiments, Rutherford placed a mouse in a confined quantity of air until it died, then burned a candle in what was left until the candle went out, then burned phosphorus in what was left after that, until it would no longer burn. Then, he passed the air through a solution that would absorb carbon dioxide. The air that finally remained would not support combustion, nor could a mouse live in it. Rutherford called this gas (which we now know would be mainly nitrogen) “noxious air” or “phlogisticated air”.

In about 1750, Black developed the analytical balance based on a light-weight beam balanced on a wedge-shaped fulcrum. Each arm carried a pan on which the sample or standard weights was placed. It was much more accurate than any other balance of the time, and became an important instrument in most chemistry laboratories.

Latent Heat

"I, therefore, set seriously about making experiments, conformable to the suspicion that I entertained concerning the boiling of fluids... I imagined that, during the boiling, heat is absorbed by the water, and enters into the composition of the vapour produced from it, in the same manner as it is absorbed by ice in melting, and enters into the composition of the produced water. And, as the ostensible effect of the heat, in this last case, consists, not in warming the surrounding bodies, but in rendering the ice fluid; so, in the case of boiling, the heat absorbed does not warm surrounding bodies, but converts the water into vapour. In both cases, considered as the cause of warmth, we do not perceive its presence: it is concealed, or latent, and I give it the name of LATENT HEAT."

In 1756, Black succeeded Cullen as lecturer in chemistry at Glasgow, and was also appointed Professor of Anatomy and Botany, but in 1757 he exchanged that post for the position of Regius Professor of the Practice of Medicine as he didn't feel competent to teach either anatomy or botany. 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 the studies that led to his doctrine of latent heat. He noticed that when ice melts it takes up heat without undergoing any change of temperature, and he argued that this heat, which, as was usual in his time, he thought of as a 'subtle fluid', must have combined with the ice and thus become latent in its substance. At the end of 1761, he verified this hypothesis experimentally.

In 1764, with the aid of his assistant, William Irvine (1743-1787), he 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. 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.

Edinburgh years

In 1766 he succeeded Cullen in the chair of chemistry in University of Edinburgh, where he devoted most of his time to preparing lectures. Many manuscript versions of his lectures were made by students and by professional copyists, and several of these survive. During his time at Edinburgh he taught chemistry to about 5,000 students and thus was uniquely influential in the dissemination of chemical knowledge during the 18th century.

"I have had the good fortune to teach Chemistry here with applause and last winter was honoured with the regular attendance of four Professors at my lectures. Dr Robertson [Principal of the University of Edinburgh] who misses no opportunity of improving the Medical College at Edinburgh has for several years past been striving to bring me into it and in some hard struggles which he and Dr Cullen had to keep out other Candidates he often did me the honour to declare that he did not act from friendship or attachment to any person but solely to promote the Prosperity of the College".

Black was also pursuing a career as a medical doctor, and his patients included the philosopher David Hume. He was also consulted over the illness of the nurse of Walter Scott, then a child. He diagnosed her with consumption (tuberculosis), which cost the nurse her job, but which may have saved the life of the future Sir Walter Scott.

Black's own health was never robust; he suffered from breathing problems caused by a childhood illness; later in his life he suffered badly from rheumatism and, after becoming a vegetarian, from vitamin deficiencies. In 1795 his health declined sharply, and he negotiated the appointment of a former student, Thomas Charles Hope, as conjoint Professor of Chemistry. The session 1796-7 was the last in which Black lectured, and the University of Edinburgh Library holds a manuscript copy of notes from these lectures. He died in Edinburgh on December 6th, 1799 at the age of 71, and is buried in Greyfriars Kirkyard.

"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." from Encyclopaedia Britannica 11th edition

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.