292 THE ANIMALS AND MAN 



heat it is called combustion. When a match is rubbed on 

 a surface, the heat produced by friction causes the phos- 

 phorus, and other substances at the match tip to take up 

 oxygen rapidly, causing combustion. Phosphorus is thus 

 oxidized and the combustion that arises from its rapid 

 oxidation sets the match on fire and consumes it. 



Rust is oxidized iron or iron-oxide. 



It is the nature of organic substances (compounds con- 

 taining carbon) to unite easily with oxygen. That is, oxygen 

 has a great affinity for these compounds. It combines easily 

 with other elements. 



To obtain oxygen. — The simplest way to obtain pure 

 oxygen is to heat some compound containing it. The heat 

 breaks up the compound and sets its elements free. The 

 oxygen thus escaping may be collected. 



Experiment i. — Place some oxide of mercury in a test tube and heat 

 it. It gradually disappears from view. The oxygen and the mercury of 

 the compound have separated. The oxygen has become an invisible 

 gas, the mercury has become vaporized. Drops of pure mercury will 

 soon condense on the sides of the glass. 



If, while the experiment is in progress, a live coal on the end of a 

 stick, be inserted into the mouth of the test tube the coal will glow with 

 a. greater brilliancy. This means that oxygen in its pure state unites 

 more freely with the carbon of the wood and makes a more brilliant 

 glow than did the oxygen of the air, mixed as it was with other gases. 



Potassium chlorate gives up oxygen rapidly when heat is applied to 

 it, so rapidly indeed as to cause an explosion. If an equal quantity 

 of black oxide of manganese be mixed with potassium chlorate, the 

 oxygen is given off more slowly and without danger of explosion, and 

 may then be collected in jars as follows: 



Arrange an apparatus as in fig. 155. First fill the jar with water 

 and invert it over the pan of water. Partly fill the test tube with the 

 mixture of potassium chlorate and black oxide of manganese (equal 

 parts). Fit the test tube with a tightly fitting cork and a bent glass 

 delivery rod. Before placing the delivery tube in the water move the 

 alcohol flame along the test tube so as to drive out the air and warm 

 the tube, that no moisture may form on the tube and break the glass. 



Then heat the mixture gradually, beginning at the top and working 



