76 THE REALITIES OF MODERN SCIENCE 



of course, easily to be met, if one's science was a matter 

 of qualities and principles, by describing phlogiston 

 as a " principle of levity" such that it decreased the 

 weight of the body into which it entered. In the 

 choice of such additional principles the advocates of 

 this theory were not always consistent. 



The problem of combustion was not, however, to be 

 solved until it was shown that air was a mixture of 

 dissimilar gases. That gases did differ in kind and in 

 density, that is, mass per unit volume, was definitely 

 established by Black's experiments in 1752 on carbonic 

 acid (carbon dioxide and water). Black, who was a 

 Scot and a student of medicine, was interested early 

 in his life in the medicinal properties of mineral waters. 

 In his study of limewater he found that limestone 

 (now known as a compound of calcium, carbon, and 

 oxygen, CaC0 3 ) when heated lost a gas (carbon dioxide, 

 C02). The weight of the gas thus produced he found 

 by the balance to be equal to the loss in weight of the 

 stone. He also brought about the reverse reaction, 

 obtaining calcium carbonate from lime (calcium oxide) 

 and carbonic acid, and demonstrated for this reaction 

 the conservation of mass. 



In 1773 Scheele, a Swedish chemist, discovered 

 oxygen as a constituent of air. The succeeding year 

 Priestley also discovered oxygen. The name, however, 

 was suggested later by Lavoisier. Priestley obtained 

 oxygen by heating a tube of red oxide of mercury by 

 focusing the sun's rays upon it with a burning-glass. 

 The gas thus liberated was found to support com- 

 bustion better than did ordinary air. For example, 

 a glowing ember thrust into a vessel containing oxygen 



