NATURAL AND PHYSICAL SCIENCE, 1700-1800 305 



to the studies of Black, Bergmann and others on the gas sylvestre 

 (carbonic acid) of Van Helmont. Dr. Joseph Black of Edinburgh, 

 a physician of note and, as we shall see, one of the first to put 

 the science of heat on a sure foundation, seeking to explain the 

 phenomena accompanying the making and the slaking of "quick 

 lime, phenomena now familar to every beginner in chemistry 

 but in 1750 puzzling to all, remembered that Hales had found 

 that "air" could be driven off from certain substances by heat- 

 ing, and suspected that in the burning of limestone to make 

 quicklime, something might be driven off, the loss of which 

 would make it lighter. This something he tried to obtain by 

 causing acid to act upon limestone (in the ordinary laboratory 

 fashion of to-day) and collecting the gas evolved by the aid of 

 Hales' pneumatic trough. He next weighed the gas and the 

 remaining limestone and found that the weight of the former 

 agreed with the loss of weight of the latter. He then reversed the 

 experiment, causing "fixed air" (as he called it) to bubble through 

 a solution of lime, whereupon, as he had anticipated, a white, 

 chalk-like powder appeared and fell to the bottom. This simple 

 experiment proved extremely fruitful, and we can now see that in 

 its use of analysis and synthesis, in its partly quantitative char- 

 acter, and in the chemical reasoning employed, it was also highly 

 instructive. Best of all, it did not require any hypothetical, 

 immaterial or mystical "phlogiston" for satisfactory explanation 

 of all the hitherto puzzling phenomena involved. Black invented 

 for the gas thus driven off by heat or acid the term "fixed air," be- 

 cause it was evidently "fixed" in the limestone or chalky precipi- 

 tate, and because any gas or vapor not obviously something else, 

 was still supposed to be " air," the true nature and chemical com- 

 position of the atmosphere being still (in 1750) quite unknown. 



At this point Bergmann, a Swedish chemist of distinction, by 

 the use of litmus (which Boyle had recommended as a test for acids) 

 and other means, discovered that the "fixed air" of Black is an 

 acid, and accordingly named it "aerial acid." Bergmann also 

 weighed the new gas, finding it heavier than air, and discovered 

 that it is very soluble in water, 

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