6.2 Early research in Edinburgh
6.2.1 Magnesia alba
After four years with Cullen in Glasgow, Black transferred to Edinburgh to complete his medical studies. He then needed to select a topic for his MD dissertation, one which would involve chemistry, be of topical interest, and also touch upon a medical question. He decided to study the nature of causticity, the corrosive character of alkaline substances, such as quicklime (calcium oxide). He wrote to his father in December 1752 that he had chosen this topic because of a controversy between two Edinburgh medical professors, Robert Whytt (1714–66) and Charles Alston (1683–1760), stemming from their attempts to use limewater (a solution of calcium hydroxide in water) as a chemical means of dissolving excruciating urinary stones (Donovan, 1975, p. 172).
Rather than become directly entangled in a dispute between two professors, Black chose another alkaline substance for his own investigations. This was magnesia alba (magnesium carbonate), which was of medical significance because it was taken (and is still widely used) for acidic indigestion and, to quote Black, ‘it mildly loosens the bowels’ (quoted by Donovan, 1975, p. 193). This was important in a period when overeating of the wrong things and drinking often caused indigestion and constipation. His thesis, De humore acido a cibis orto et magnesia alba (Of the acid humour produced by food and of magnesia alba), was printed in June 1754.
He did not achieve his original aim of producing a substitute for limewater by roasting magnesia and treating the product with water, because magnesium oxide, unlike quicklime, is totally insoluble in water. Nonetheless, Black carried out about thirty chemical experiments on magnesia and calcinated magnesia, which he called magnesia usta. The tentative and disappointing results of Black's thesis were transformed a year later in an essay he read to the Philosophical Society of Edinburgh entitled ‘Experiments on magnesia alba, quicklime, and other alcaline substances’, in which he extended his investigations to quicklime and potash.