Joseph Black

Joseph Black (1728–1799) was a British chemist and physicist best known for the rediscovery of "fixed air" (carbon dioxide), the concept of latent heat, and the discovery of the bicarbonates (such as bicarbonate of soda).[1] His groundbreaking work in the quantitative study of chemical reactions places him among the founders of modern chemistry.[4] He is considered one of the world's most eminent chemists and one of the founding fathers of the science of chemistry.[6]

Early Life and Education

Joseph Black was born in Bordeaux in France in 1728, one of the 15 children of an Ulster wine merchant of Scots descent based in the town.[2][3] Black was the son of expatriate Ulster merchant John Black and his Aberdeen-born wife, Margaret Gordon.[1] He was educated by his Scottish mother, who taught him English, until the age of twelve when he attended school in Belfast.[3]

At the age of 12, Joseph went to school in Belfast to learn Greek and Latin and at the age of 16 he went to Glasgow University, where he studied arts for four years.[2] Black's father later required him to choose a course of study leading to a profession.[1] In 1748 he started to study medicine and attended the chemistry lectures recently instituted there by the Professor of Medicine, William Cullen.[2]

Key Influences and Mentors

Because he chose medicine, Black came under the influence of an innovative teacher of chemistry, William Cullen, and, unusual for a young student, he sta[1] At Glasgow, Cullen introduced a chemistry that was less subordinate to the pharmaceutical needs of medicine and more an autonomous science posing its own distinct questions—a chemistry, in short, that was more "philosophical." The young Black quickly attracted the attention of the new professor of medicine.[19]

Black went on to become Professor Cullen's laboratory assistant before moving to Edinburgh University in 1752 to continue his medical studies.[2] Chemistry captivated him, his interest being warmly encouraged by Cullen, and for three years he served as Cullen's assistant.[3] He spent three years as Cullen's assistant and a friendship developed that lasted until Cullen's death in 1790.[24]

Professional Development

Black did not graduate in medicine at Glasgow because he was attracted to the University of Edinburgh, where the medical school enjoyed more prestige. In order to graduate, students had to prepare a thesis. Black was particularly assiduous (many students did not take their work seriously), and he conducted a series of experiments on the chemical properties of an alkali—in particular, magnesia alba, now known as magnesium carbonate.[1] His medical degree was awarded in 1754.[1]

In 1756 Cullen was appointed to the chemistry chair in Edinburgh, and Black filled the vacancy created in Glasgow, becoming professor of anatomy and lecturer in chemistry.[1][3] In 1766 Black succeeded Cullen as Professor of Chemistry at Edinburgh; he wrote from Glasgow to his father - 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.[3]

At this point in his career, a change came over Black's approach to chemistry. Instead of continuing to pursue fundamental concepts, Black turned his attention almost entirely to teaching and to advising landowners and entrepreneurs how chemical-based industries in Scotland (and sometimes farther afield) could be developed.[7]

Discovery of Carbon Dioxide

The research on the nature of alkalinity, which Black conducted for his thesis, laid the basis for the most important paper of his career, "Experiments upon Magnesia Alba, Quicklime, and Some Other Alcaline Substances," given to the Philosophical Society of Edinburgh in 1755.[1]

In 1754, Black discovered that when you heat magnesia alba (magnesium carbonate), it loses weight, and he found that the weight loss was due to an "air" that is given off during heating.[8] This "air" differs from ordinary air in that it will not support combustion and is soluble in water. The same air could be extracted from other minerals, such as limestone. In 1756, Black presented a paper to the Philosophical Society of Edinburgh in which he described his new gas, which he called "fixed air", because it could be fixed into a solid substance. We now call it carbon dioxide.[8]

He was thus the first chemist to show that gases could be chemical substances in themselves and not, as had been thought beforehand, atmospheric air in different states of purity.[1] Thus arguably Black's work started the avalanche of research on the respiratory gases carried out by Priestley, Scheele, Lavoisier, and Cavendish.[14]

Latent Heat

Black then turned his attention to heat and he was the first person to describe latent heat, that is the heat added or lost when a liquid changes its state, for example when water changes to ice or steam.[14] Joseph Black was the first to illustrate that heat and temperature are different things.[12]

Black distinguished between the quantity of heat in a body and its intensity, or temperature, realizing that thermometers can be used to determine the quantity of heat if temperature is measured over a period of time while the body is heated or cooled. He took two similar glass flasks, pouring the same quantity of water into both and placing them in a freezing mixture. In one he had added a little alcohol to prevent freezing. They were then removed from the bath, one frozen and the other liquid, though at the same temperature. They were allowed to warm up naturally. The temperature of the water plus alcohol warmed up several degrees, while the ice remained at its freezing point.[1]

As the flasks had to be absorbing heat at the same rate, Black showed that the heat absorbed by the ice in 10 hours would have raised the temperature of the same quantity of water by 78 °C (140 °F). This was the latent heat of fusion of water.[1]

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.[11] Black referred to two different types of heat that he called "sensible heat" and "latent heat." Sensible heat is the heat that raises water temperature. Latent heat is the heat needed to change water (or any other substance) from one phase to another (that is, from solid to liquid or from liquid to vapor).[16]

These he performed late in 1762. From the average of three experiments he calculated that the heat absorbed in vaporization was equal to that the which would have raised the same amount of water to 810, were this actually possible. This gives a figure of 450 calories per gram for the latent heat of vaporization of water, compared to a modern figure of 539.1 calories per gram.[4]

Specific Heat

The second, and closely related, discovery made by Black concerning heat was that different substances have different heat capacities.[4] He also discovered that different liquids have different capacities to take up heat, and he introduced the concept of "specific heat." This term refers to the amount of heat required to increase the temperature of a particular substance by a known amount.[14]

For example Black described an experiment in which a known weight of mercury at 150°F was mixed with an equal weight of water at a temperature of 100°F. He found that the temperature of the mixture at equilibrium was not 125° as might be expected, but 120°. In other words the mercury was cooled by 30° while the water was warmed by only 20° despite the fact that the quantity of heat gained by the water was the same as that lost by the mercury.[14]

But very soon after I began to think on this subject, (anno 1760) I perceived that this opinion was a mistake, and that the quantities of heat which different kinds of matter must receive, to reduce them to an equilibrium with one another, or to raise their temperature by an equal number of degrees, are not in proportion to the quantity of matter in each, but in proportions widely different from this, and for which no general principle or reason can yet be assigned.[11]

Relationship with James Watt

During his time at Glasgow, Black was in contact with the Scottish inventor James Watt, who was employed as instrument maker to the university. Watt worked on developing improvements to the steam engine, and his double-cylinder version essentially recognized the phenomena of latent heat. The two men, who became firm friends, were at pains to declare that their researches were conducted independently, however.[1]

Black was a friend of the young James Watt (1736-1819) who was responsible for the development of early steam engines. Watt was puzzled why so much cooling was necessary to condense steam into water, and Black realized that the answer was the latent heat. The resulting improvements in steam engines ushered in the Industrial Revolution.[15]

Teaching and Influence

Joseph Black was a highly effective teacher, famous for his practical demonstrations during lectures. He moved back to Edinburgh University to focus on chemistry in 1766, and students from across Europe and as far afield as North America came to hear him teach, his course of lectures taking place five times each week between November and March.[2]

In the 1790s the average number of students attending Black's lectures was over 200 and during his tenure of the Chair he must have taught chemistry to about 5,000 students and thus was uniquely influential, in Britain, in the dissemination of chemical knowledge during the latter half of the eighteenth century. Former students of his held Chairs of chemistry in Edinburgh, Glasgow, Cambridge, Oxford, and not a few made significant contributions to chemistry, pure and applied.[3]

Black certainly seems to have had an aversion to publishing his research; for example, he wrote nothing on his theories of heat for publication. His work became well known because of the large number of students (sometimes in excess of 300) who registered for his annual course of lectures and diffused his concepts throughout Great Britain, Europe, and the United States.[7]

Personal Life

Although Black never married, he had a very active social life and became an eminent member of the literary and scientific circles in what became known as the Scottish Enlightenment: he was also renowned for his flute playing. Amongst those he knew well were Adam Smith, David Hume, Alexander Carlyle and James Hutton.[2]

Black was one of the luminaries of the Scottish Enlightenment the spirit of which is nicely illustrated in that Adam Smith, the political economist, loved nothing so much as to get Black, the chemist, and Hutton, the geologist, together at an Oyster Club and listen to their talk.[3] Dearest to Black was the Oyster Club, weekly dinners with his closest friends William Cullen, the geologist James Hutton, and Adam Smith, the author of The Wealth of Nations (1776).[19]

Meanwhile, Black was also pursuing a career as an eminent medical doctor. Amongst his patients were the philosopher David Hume. He was also consulted over the illness of the nurse of Walter Scott, then still a child. He diagnosed her with consumption, or tuberculosis, which lost the nurse her job, but possibly saved the life of the future Sir Walter Scott.[2]

Joseph Black himself never enjoyed the best of health, suffering from breathing problems caused by a childhood illness; and later in his life he suffered badly from rheumatism and, after becoming a vegetarian, from vitamin deficiencies.[2]

Industrial Consulting

Black's skill at judging the viability of new proposals for industrial processes became renowned. After he had masterfully judged the financial benefits for establishing a tar works, Sir John Dalrymple, solicitor to the Board of Excise, described Black as "the best judge, perhaps in Europe, of the merit of such inventions." Black was certainly in demand for his opinions, being consulted by a considerable number of industrialists on an extraordinarily broad range of topics.[7]

In the surviving correspondence these include sugar refining, alkali production, bleaching, ceramics glazing, dyeing, brewing, metal corrosion, salt extraction, glass making, mineral composition, water analysis, vinegar manufacture, and furnace construction.[7]

Legacy and Impact

The work was a foundation stone of the impending revolution in chemistry as acknowledged by the great chemist Lavoisier who in 1789 wrote to Black as "One of the most zealous admirers of the depth of your genius and of the important revolutions which your discoveries have caused in the Sciences".[3]

In the scientific arena, Black's discoveries formed one of the first foundation blocks of what was to become the science of thermodynamics. This field of study, elevated to an independent discipline by William Thomson, Lord Kelvin (1824-1907), has proven enormously fruitful over the intervening two centuries.[16]

Black died on 6 December 1799 aged seventy-one. He left the substantial sum of £20,000 and his quantitative exactitude was expressed in his will for he divided the bequest into 10,000 shares so that he could allot to each of the legatees the exact amount he considered appropriate.[3] Black died a celebrated death, being found by his servant with a cup of milk balanced between his knees, not a drop having been spilled. It was commented upon that this was entirely in line with the ordered nature of his life and, further, that it reflected the perfection of his experimental procedures.[5]

He was an eminent Scottish physicist and chemist, a renowned teacher, and a practicing medical doctor: and the chemistry buildings at both Edinburgh and Glasgow Universities are named after him.[2]