212 MODERN SCIENCE READER 



metals are dissolved in acids, adopted the view that inflam- 

 mable air (hydrogen) was phlogiston, with which metals 

 part on coming in contact with acids. An inconvenient fact 

 in connection with the phlogistic theory was that combus- 

 tion, including the transformation of metals into calces, 

 could only take place in the air. Stahl and his followers 

 referred to this fact as if it were quite natural that if phlo- 

 giston was to be absorbed from metals there must be a 

 medium capable of absorbing it. There were thoughtful 

 men, however, who would not be satisfied with explanations 

 of this kind. Boerhaave, whose Elementa Chemiae (1732) 

 served for many years as the standard text-book of chem- 

 istry, taught distinctly that the conversion of metals into 

 calces involved the absorption of something from the air. 

 This he deduced by combining the fact that the presence of 

 air was necessary with the fact that the conversion involved 

 increase in weight. The latter fact he even freed from an 

 erroneous explanation attached to it by Boyle, who had 

 thought that the increase in weight was due to the absorp- 

 tion of heat during calcination. By the use of the balance 

 Boerhaave showed that metals have precisely the same 

 weight when glowing hot as when cold, and thus proved 

 that heat has no weight. So near the truth were some. 

 Yet none rose to combat the phlogistic theory, and all- 

 even Boerhaave were dominated by it more or less. 



Two things were necessary to make away with phlogiston : 

 First, a clear knowledge of some of the ordinary gases; 

 second, a clear quantitative knowledge of some of the 

 ordinary chemical transformations. The gases in question 

 are carbon dioxide, oxygen and air. As to quantitative 

 chemical knowledge, it can, of course, be acquired ulti- 

 mately only by the use of the balance. Carbon dioxide 

 (called "carbonic acid") was known since the time of Van 

 Helmont; yet chemists were not sure but that it might be 

 impure air, until Joseph Black isolated it and demonstrated 

 its properties in 1755. Bergman completed the study of this 

 gas in 1774. The presence and properties of oxygen were 



