132 CHEMISTRY. 



see as we proceed that an acquaintance with the chemical 

 processes which it involves helps us to keep it in our serv- 

 ice, and to prevent it from gaining the mastery. We have 

 had considerable to say incidentally about combustion in 

 the previous chapters, but the subject demands a full and 

 systematic consideration. 



168. Early Ideas of Combustion. Fire was regarded by 

 the ancients as an element; this view prevailed largely 

 during the Middle Ages also, but gave way about the year 

 1700 to the idea that all combustible substances contain a 

 certain principle called Phlogiston, which escapes when they 

 burn. This theory, promulgated by Stahl, a celebrated Ger- 

 man physician, was accepted for nearly a century, but was 

 eventually abandoned when the discoveries of Priestley, of 

 Lavoisier, and others made chemistry a rational science. 



169. Chemistry of Common Combustion. In the combus- 

 tion which we commonly witness there occurs a chemical 

 union between oxygen, on the one hand, and carbon and 

 hydrogen on the other. The oxygen, in uniting with the 

 carbon, forms carbonic anhydride, which is diffused as gas 

 in the air. In uniting with the hydrogen it forms water 

 in the shape of vapor, which passes upward in company 

 with the carbonic anhydride. That this is the chemistry 

 of ordinary combustion you will see as we proceed to con- 

 sider its different modes and circumstances. 



170. Burning Gas. In the burning of hydrogen gas we 

 have only the union of this gas with oxygen, forming water. 

 That this is the product can be proved by holding a glass 

 bell-jar over a burning jet of hydrogen gas, Fig. 50 (p. 133). 

 It will soon become bedewed all over the inside with moist- 

 ure, and if the experiment be continued drops of liquid 

 will at length trickle down, which, if caught in a vessel and 

 examined, can be proved to be water. The metal sodium, 

 as you will hereafter learn, burns on touching water; so 



