ix] Doctrines of Respiration. 233 



Glasgow in succession to Cullen. Ten years later he became, 

 again in succession to Cullen, Professor of Chemistry at 

 Edinburgh, where he died in 1799. The Latin dissertation of 

 1754 appeared in the following year as an English essay, 

 entitled, " Experiments on Magnesia Alba, Quick-lime, and 

 other Alkaline Substances." 



Stone in the bladder and gravel in the urine were in those 

 days attracting much attention in the medical profession, and 

 the qualities of various alkaline bodies proposed as remedies for 

 them were being much discussed. 



According to prevalent ideas, governed by the phlogiston 

 theory of Stahl, lime or chalk became quick-lime, became 

 caustic by taking up phlogiston, and when slacked gave out 

 phlogiston; that is to say lime suffered gain in becoming 

 caustic quick-lime, and caustic quick-lime suffered loss in being 

 slacked and becoming mild. 



Black made the notable observation that ordinary or mild 

 lime lost in weight when it was burnt into caustic lime. He 

 further observed that all ' mild ' alkalis, lime, magnesia, and 

 the like, when treated with acids, gave off a particular kind of 

 gas or air. When caustic lime by exposure to the air became 

 mild lime, the change, he argued, consisted in the lime taking 

 up from the atmosphere this particular kind of air. " Quick - 

 " lime therefore does not attract air when in its most ordinary 

 " form, but is capable of being joined to one particular species 

 " only, which is dispersed through the atmosphere either in the 

 "shape of an exceedingly subtle powder, or more probably in 

 " that of an elastic fluid. To this I have given the name of 

 " 'fixed air: " 



Moreover when mild lime was burned and so became caustic 

 lime, this same fixed air was given off. It was the loss of this 

 fixed air which accounted for the loss of weight when mild 

 lime was burned into quick-lime. In fact the mild alkalis 

 were compounds of caustic alkalis with fixed air. 



This discovery of Black really entailed the destruction of 

 the phlogiston theory, but that theory had established itself in 

 the minds of men of the time far too firmly to be driven off at 

 the first assault. 



