ENVIRONMENT OF LIVING THINGS 



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compounds; that is, chemical elements which are united by certain 

 chemical laws to iorm substances called compounds. Compounds 

 are quite different in their 

 properties from the elements 

 which compose them. Such 

 is the chlorate of potash, which 

 contains the elements oxygen, 

 chlorine, and potassium. Heat- 

 ing this with oxide of manga- 

 nese causes the oxygen, to be 

 released in the form of a gas. 

 This is an example of a chem- 

 ical change. 



Properties of Oxygen. — ^ Oxy- 

 gen, when carefully prepared, 

 is found to be colorless, odor- 

 less, and tasteless. It makes 

 up about one fifth of the air. 

 Combined with other sub- 

 stances, it forms a very large 

 part by weight of water, rocks, 

 minerals, and the bodies of plants and animals. Oxygen has 

 the very important property of uniting with many other sub- 

 stances. 



The chemical union of oxygen with any other substance is called 

 oxidation. Oxidation of some sort may take place wherever oxy- 

 gen is present. This facb has a far-reaching significance in the 

 understanding of the most important problems of biology. 



Oxidation in a Match. — The simple process of striking a sulphur match 

 gives us another illustration of this process of oxidation. The head of the 

 match is formed of a combination of phosphorus, sulphur, and some other 

 materials. Phosphorus is a chemical element distinguished by its extreme 

 inflammability ; that is, it unites with oxygen at a comparatively low tem- 

 perature, producing a flame. Sulphur is another chemical element that 

 combines somewhat easily with oxygen but at a mueli higher ti'mperature. 

 The rest of the match head is made up of red lead, niter, or sonui other sub- 

 stance that will release oxygen, and some glue or gum to bind the materials 

 together. The heat caused by the friction of the match head against the 

 striking surface is enough to cause the phosphorus to ignite ; this in turn 



Preparation of Oxygen. 



