contribution to the history of photosynthesis: joseph black

Joseph Black (1728–1799) was a British physician and chemist who is credited for discovering the carbon dioxide, a chemical reactant in photosynthesis, although it was then called “fixed air”. He also takes credit for having discovered specific heat and latent heat.

Black was born in Bordeaux, France of Scottish descent. At the age of 14 he studied Latin and Greek at Belfast. At the age of 16 in 1744, he enrolled at Glasgow University to study arts but shifted to medicine in 1748. He was employed by William Cullen, the professor of medicine, as his laboratory assistant. Cullen started giving lectures in chemistry in the preceding year (1747), although chemisty has not been taught in the university, and Black showed passion for the discipline.

In 1752, Black continued his study at the University of Edinburg and completed his medical degree in 1754. His graduation thesis, On the Acid Humour Arising from Food and Magnesia alba, consisted of chemical experiments which he described in a paper presented to the Philosophical Society of Edinburgh in 1755. This thesis was about the effects of magnesia as antacid.

After more research, he published Experiments Upon Magnesia Alba, Quicklime, and Some Other Alcaline Substances in 1756. This was his most important work. In this paper, he reported his discovery of a component of air which he called fixed air. He identified it by the lime water test (using aqueous calcium hydroxide) in which a white precipitate (calcium carbonate) was formed. He also experimented on birds and small mammals and found that fixed air was unable to support life or the flame of a candle.

He showed that by heating alkaline solids such as magnesia alba (now xMgCO3, yMg(OH)2, zH2O) and lime (CaCo3), “fixed air” was given off. This was the same air which van Helmont previously described as “gas sylvestre”, but it was Black who applied quantitative techniques in the experiment.

Joseph Black spent much of his time teaching chemistry. He was Professor of Chemistry at Glasgow University from 1756 to 1765 and at the University of Edinburgh from 1766 until his death in 1799. His work became the foundation of the chemical revolution (Doyle 2011). His explanation of his experimental results without referring to phlogiston became useful to Antoine Lavoisier in the formulation of his oxygen theory (Cooper 1999; Doyle 2011; Egerton 2008; n.d.).

Note: The lists of contributors and Literature Cited are in the History of Photosynthesis Mainpage.

(Ben G. Bareja, June 2012)

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