64 Henry Riddell on 



had received benefit from a medicine invented by a Mrs. Joanna 

 Stephens, and through their influence she received five thousand 

 pounds for revealing the secret, which was published in the 

 London Gazette of 1739. Any of you anxious to know this 

 medicine in all its details can study it in Ramsay's book ; 

 sufficient to say that it was mostly compounded from such things 

 as egg shells, snails, calcined soap, honey, wild carrot seeds, &c., &c., 

 all burnt to a blackness. Doctor Cullen and his colleagues held 

 opposing views as to the efficacy of such remedies. It must be 

 remembered that the medical treatment of calculus at this time 

 depended upon the use of caustic alkalies, and Black began his 

 experiments on magnesia with the object of obtaining a "milder'' 

 alkali, which would yet be more efficacious than those then in use. 

 Chalk itself had no caustic properties, but when burnt its nature 

 was completely changed. This was a common-place of the know- 

 ledge of the day, but the reason of the change was completely 

 misunderstood. It was the day of the material theory of heat 

 complicated by the extraordinary doctrine of Phlogiston due to 

 Stahl, a doctrine which gave to Phlogiston a set of very puzzling 

 properties. 



Caustic Lime, obtained by burning the so-called calcareous 

 earth, was looked upon as having received its causticity from 

 something added by the fire used to obtain if. Then again, such 

 Caustic Lime, mixed with the fixed or mild alkalies, became itself 

 again mild calcareous earth, while the fixed alkali became caustic 

 in its turn. What more natural explanation than the lime parting 

 with the igneous matter it had received from the fire, and 

 the alkali taking it n})1 It was known also that the Caustic 

 Lime when exposed to the air gradually lost its causticity, 

 becoming again a substance indistinguishable from the calcareous 

 earth from which it had been obtained. What more certain, then, 

 than that the igneous matter, the cause of the causticity, had 

 escaped into the air, and the lime thus become reduced to its 

 original state 1 And yet it had been long known that loss of 

 weight had taken place in the actual calcination, and a curious 



