An engraving showing Joseph Black

When the pandemic destroyed my faculty’s plans, we thought about how we could offer internships without laboratories and decided to give our students a chemistry kit. Instead of sending them chemicals, we sent them a box of small instruments. We took inspiration from one of the first chemists to take detailed and systematic measurements in the laboratory, Joseph Black.

Born in Bordeaux, France, the son of a wine merchant, Black was sent to boarding school in Belfast, Ireland at the age of 12. He then studied in Glasgow, Scotland. In his senior year of college he was the firm favorite of Robert Dick Sr., then professor of natural philosophy (ie physics). William Cullen, a respected medicin, took him under his wing. Cullen is an avid lecturer and is sometimes remembered for freezing water by evaporating ether with a vacuum pump. Black became his assistant, conducting experiments but also assisting lecture demonstrations that were part of Cullen’s move. Relationships with his mentor became so close that Black later wrote that Cullen “treated him with the same trust and friendship … as if I were one of his children”.

Black aspired to practice medicine and moved to Edinburgh in 1852. In order to look for a thesis topic, he decided to examine kidney stones, which chemical analyzes had associated with lime (calcium and its salts). Black initially thought of examining Kalkwasser, but thought when he became aware of a heated argument between two Edinburgh professors over his action. Instead, he chose a related problem: the behavior of “Magnesia alba” – also known as magnesium carbonate.

When you read your work, you enter another world before the SI

He began by treating the material with acids, observing the “surge” and isolating the products. With sulfuric acid he got Epsom salt, while with “hydrochloric acid” (HCl) he got a salt that disintegrated over time. At the center of Black’s investigations was the use of scales, which he had probably learned from his contemporary Andreas Marggraf, whose textbook he had read and whom he described as an “extremely precise and careful chymist”. Black carefully weighed its raw materials and products. Reading his work, one enters another world in front of the SI: “Two drams of magnesia … were treated with three drams, one scruple and seven grains of the acidic liquid and lost one scruple and sixteen grains through the surge”.

When calcining magnesia alba in his little furnace (his own design, based on previous descriptions by Herman Boerhaave), the substance lost weight, an observation that Black concluded was due to the loss of an “air”, such as when foaming. To his surprise, the calcination residue dissolved in acid to give the same products.

But the “air” was interesting: it put out a candle flame and killed birds in seconds; it was present in his breath and in the ordinary air. Careful experiments showed that lime water became cloudy and reacted with various alkalis. ‘I gave it the name solid air, and perhaps quite inappropriate; but I thought it better to use a word already known in philosophy than to invent a new name. This made carbon dioxide the first atmospheric gas to be isolated and characterized.

It is sometimes said that Black invented the analytical balance, but he never specifically mentioned any particular balance in his essays or lectures. ‘Black’s balance’, an elegant double balance from circa 1760 on display in the National Museum of Scotland, is little different from those used by examiners and pharmacists at the time. What was different was Black’s weighting to study a chemical reaction. His observations are among the first truly quantitative measurements in chemistry that were widely read and had a great influence, not least on Antoine Lavoisier, who later in the century commissioned Nicolas Fortin with an extremely sensitive balance to prove the conservation of mass.

Black’s approach to measurement is illustrated by a letter he wrote in 1790 describing a small improvised scales made of narrow, one-foot-long pine slats marked at intervals in pencil, with a steel pin as a fulcrum. A U-shaped curved rectangle made of brass served as the carrier. Metal loops of known weight could be slid along the bar to weigh an object placed at the other end. With that he said it could weigh 1/1200 of a grain (50 mg).

An image showing the scale by Joseph Black

When Black returned to Glasgow, his interests shifted to heat, perhaps inspired by Cullen’s cooling experiment. Once again he used a familiar instrument, the Fahrenheit thermometer, and showed that temperature can be a direct measure of heat flow. Mixing known amounts of water at different temperatures showed that heat flow and temperature were continuous. He then determined the relative heat capacities of water and several metals, and also observed that the ice melting inside stayed at a constant 32 ° F as the water cooled. He attributed the heat flow to the “latent” heat of the ice. He measured both latent heats of water in relation to its heat capacity. Although he never published these ideas, latent heat was the focus of his lectures, and his discussions with James Watt had a major influence on the development of the steam engine.

Without realizing it, our students used instruments in their kits to replicate some of Black’s experiments and take some of his measurements. And the deepest lesson we can learn from Black is that if you can measure any chemical or reaction, you can measure any one.

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