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William Cullen

In chemistry, William Cullen (1710-1790) was a Scottish physician and chemist who in 1757 pioneered affinity reaction diagrams (chemical reaction diagrams) during his lectures on the use of French chemist and physician Étienne Geoffroy’s affinity table. [1]

Cullen was a charismatic lecturer. He always came before a class with a manuscript or lengthy notes that he revised each year, but he rarely used them and spoke extemporarily. He gave the students the necessary background of information, but this was never presented as a list of facts to be memorized, rather as a journey of exploration into the mysteries of diseases and the processes that caused them. The professor and his students were companions on the journey. Phrases like ‘as far as I know’ and ‘I am persuaded’ abound in his lectures and textbooks. [2] One of his most notable pupils was Scottish physicist and chemist Joseph Black.

Chemical reaction diagrams
In 1757, Cullen began to use Geoffroy’s affinity table in lectures at the University of Aberdeen. [1] To help in explaining to his students the idea of ‘single elective attractions’ (single displacement reactions) and ‘double elective attractions’ (double displacement reactions), Cullen pioneered the use of reaction arrows ‘→’ or ‘darts’, as he called them, drawn diagonally to represent the affinity tendencies of the individual species in the reaction. In addition, he introduced the conception of using brackets ‘{’ to represent a bonded association.

In Cullen’s lecture notes, a core example used is the list of species in the third column of Geoffroy’s affinity table. At the head of the third column, i.e. row-one, column-three (R1:C3), is nitric acid HNO3 which Cullen labels as N : A. The relative affinities of nitric acid, according to Geoffroy, are in the order: iron ♂, copper ♀, lead ђ, mercury (R5:C3), and silver (R6:C3). Therefore, iron would displace copper from a solution of copper in nitrous acid and would similarly displace lead, mercury, and silver from their solutions of nitrous salt. Using the conception of affinity arrows and bonding brackets, Cullen used diagrams in his lectures to represent these individual reactions steps of single elective affinities or single displacement reactions, such as shown below, in the middle diagram, with the modern day chemical reaction equivalent adjacent, to the right:

William Cullen's single elective affinity diagrams

References
1. (a) Crosland, M. P. (1959). “The use of diagrams as chemical ‘equations’ in the lecture notes of William Cullen and Joseph Black.” Annals of Science, Vol 15, Num 2, June.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview), (pgs. 385-388). Morrisville, NC: LuLu.
2. William Cullen – James Lind Library.

Sadi-Carnot	Latest page update: made by Sadi-Carnot , Apr 28 2010, 8:08 AM EDT (about this update About This Update Moved from: Biographies (connected) - Sadi-Carnot No content added or deleted. - complete history)
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