78 ESSAYS BIOGRAPHICAL AND CHEMICAL 



his experiments on magnesia. They are described in a 

 paper entitled ' Experiments upon Magnesia Alba, Quick- 

 lime, and some other Alcaline Substances'; it was the 

 chemical contents of his thesis for the degree of M.D., 

 which he took at Edinburgh in 1754 ; he had been making 

 the experiments since 1752. The actual thesis was in 

 Latin : ' De Humore Acido a Cibis orto, et Magnesia Alba ' ; 

 the pamphlet was published in the following year. 



The medicines in vogue as solvents of the urinary cal- 

 culus were all caustic ; the lapis infernalis, or caustic 

 potash, and the ley of the soap-boilers, or caustic soda. 

 These substances are made from mild alkali, or carbonates, 

 by boiling their solutions with slaked lime, itself produced 

 by slaking quicklime with water. Now quicklime is 

 formed by heating lime-stone in the fire; it thereby 

 acquires its burning properties, or causticity ; and this it 

 was supposed to derive from the fire, of which it absorbed, 

 as it were, the essence. The act of boiling the mild 

 alkalies with lime was supposed to result in a transference 

 of this educt of fire to the alkalies, which themselves 

 became caustic, Lime-water, or a solution of caustic lime 

 was used as a solvent for the calculus; and it was an 

 attempt to produce a less caustic solvent from Epsom salts 

 that induced Black to begin his researches. 



As his notes show, Black began by holding the old view. 

 He attempted to catch the igneous matter as it escaped 

 from lime, as it becomes ' mild ' on exposure to the air : 

 he appears to have made some experiment with this view ; 

 but his comment was, 'Nothing escapes the cup rises 

 considerably by absorbing air/ Two pages further on in 

 his notebook he records an experiment to compare the 

 loss of weight sustained by an ounce of chalk when it is 

 calcined with its loss when dissolved in ' spirit of salt/ or 

 hydrochloric acid ; and he then evidently began to suspect 

 the reason of ' mildness ' and ' causticity/ 



