114 



CARBON AND ITS COMPOUNDS 



pounds (cf. 3). The compounds that contain only car- 

 bon and hydrogen are called hydrocarbons. Among them 



are marsh gas and acetylene (cf. 

 158 and 257), which are a part 

 of illuminating gas (cf. 124). 

 Another important hydrocarbon 

 is benzene, a colorless liquid ob- 

 tained from coal tar. 



FIG. 95. 

 Collecting Marsh Gas. 



Marsh gas gets its name from the 

 fact that it is found in marshes and 

 pools. It is formed by the decay of 

 leaves and twigs under water. If the bottom of a pool is stirred 

 with a stick, bubbles of this gas usually arise, and 

 may be collected over water (Fig. 95). 



Marsh gas can be burned; in mines it is called fire 

 damp. When there is much of it mixed with the air 

 of a mine, and a flame is brought into the mixture a 

 violent explosion takes place. To avoid these explo- 

 sions, Sir Humphry Davy, who was himself a miner 

 when a boy, invented a safety lamp (Fig. 96), which 

 could be taken into the mixture of fire damp and air 

 without setting the mixture on fire. The safety lamp 



is a lamp with its flame entirely surrounded 

 by wire gauze. 



Natural gas is a mixture of hydrocarbons. 

 It is chiefly marsh gas. 



FIG. 96. 



Modern Form 



of the Miner's 



Safety Lamp. 



Drill Hole 



121. Petroleum. Petroleum is a 

 dark liquid obtained from oil wells 

 (Fig. 97). It is a mixture of hydro- 

 carbons, some gaseous, some liquid, 

 and some solid. If we were to distill some of it, we would 

 obtain, first, gases we could not condense, then liquids 



FIG. 97. 



How Petroleum is Found 

 in the Earth. 



