A HISTORY OF SCIENCE 



even more important than his discoveries of chemical 

 properties in general was his discovery of the proper- 

 ties of carbonic-acid gas. 



Black had been educated for the medical profession 

 in the University of Glasgow, being a friend and pupil 

 of the famous Dr. William Cullen. But his liking was 

 for the chemical laboratory rather than for the prac- 

 tice of medicine. Within three years after completing 

 his medical course, and when only twenty-three years 

 of age, he made the discovery of the properties of car- 

 bonic acid, which he called by the name of " fixed air." 

 After discovering this gas, Black made a long series of 

 experiments, by which he was able to show how widely 

 it was distributed throughout nature. Thus, in 1757, 

 he discovered that the bubbles given off in the process 

 of brewing, where there was vegetable fermentation, 

 were composed of it. To prove this, he collected the 

 contents of these bubbles in a bottle containing lime- 

 water. When this bottle was shaken violently, so that 

 the lime-water and the carbonic acid became thorough- 

 ly mixed, an insoluble white powder was precipitated 

 from the solution, the carbonic acid having combined 

 chemically with the lime to form the insoluble calcium 

 carbonate, or chalk. This experiment suggested an- 

 other. Fixing a piece of burning charcoal in the end 

 of a bellows, he arranged a tube so that the gas coming 

 from the charcoal would pass through the lime-water, 

 and, as in the case of the bubbles from the brewer's 

 vat, he found that the white precipitate was thrown 

 down; in short, that carbonic acid was given off in 

 combustion. Shortly after, Black discovered that by 

 blowing through a glass tube inserted into lime-water, 



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