12 APPLIED BIOLOGY 



Such experiments prove that combustible gases may be 

 obtained from animal and plant substances after the water 

 has been removed. Chemists have shown that these gases 

 are not present as such in the animals and plants, but the 

 elements of which they are composed are united in chemical 

 compounds which change to gas when heated. This will be 

 clear when we recall that coal, wood, or oil (all of which are 

 of organic origin) when heated in the retorts at the gas-factory, 

 give off illuminating gas. Gas as such is not present in the 

 solid coal, but the coal has compounds whose elements re- 

 combine when heated and form the gas. So it is in the bodies 

 of animals and plants; the elements to form the gases are 

 present, and the gases are produced by heating, which causes 

 new recombinations of elements. 



14. Carbon is a prominent part of the solid substance of 

 animals and plants. After the gases were given off in the 

 preceding experiments a black substance was left in the 

 pipe. The same thing occurs when we burn an ordinary 

 match; that is, certain gases burn and produce the flame 

 and a charred mass is left. This black material we know as 

 charcoal; at the gas-factory a similar substance from heated 

 coal is called coke. The black color of coke and charcoal 

 is due to the presence of an element known to chemists as 

 carbon. The easiest way to get pure carbon is by charring 

 white sugar with intense heat. Chemists know more than 

 one hundred thousand compounds of carbon with other 

 elements. It is especially abundant in the organic com- 

 pounds of which the bodies of animals and plants are com- 

 posed, and consequently must be also in the food from 

 which the body substances are made. 



15. Mineral substances enter into the composition of 

 animals and plants. If we reheat the charcoal obtained from 

 either the animal or the plant matter in the preceding demon- 

 stration, by holding it with a wire in the hot flame of a gas- 

 or alcohol-burner, the carbon which it contains will soon 



