16 BOTANY 



Preparation of Oxygen, — Oxygen may be easily prepared in the school- 

 room or at home in the following manner .1 Heat half a teaspoonful of 



black oxide of manganese with a little more than its bulk 

 of chlorate of potash in a test tube over a bunsen flame 

 or a spirit lamp. Vapors will be seen to arise as the mix- 

 ture becomes heated. After a moment insert a glowing 

 match into the mouth of the test tube; it bursts into a 

 blight flame. In what form does oxygen pass off from 

 the two chemicals in the test tube? How could you 

 determine the presence of oxygen in a substance? Is 

 there oxygen in the air? How do you know? 



Properties of Oxygen. — The physical proper- 

 ties of oxygen are those which we determine with 

 Preparing oxygen, our senses. Oxygen, when carefully prepared, is 



found to be a colorless, odorless, and tasteless gas. 

 It is known to form nearly one half of the earth's surface, to form 

 eight ninths of all water and over three fourths of the weight of the 

 plants and animals inhabiting this world of ours. It has the very 

 important chemical property of causing things placed in it to burn. 

 If, for example, a piece of picture wire is heated red-hot, and then 

 placed in a jar of oxygen, the metal will burn with a bright flame. 



Oxidation. — Light carefully a small piece of magnesium wire and then 

 place it in a test tube in which you have previously made oxygen. Notice 

 the very brilliant flame. A light-colored ash remains. This is magnesium 

 oxide. In the above experiment the oxygen in the test tube unites with 

 the magnesium so rapidly as to form a flame. This process is known as a 

 combustion. 



The chemical union of oxygen with any other substance is called 

 oxidation. Can you distinguish between combustion and oxida- 

 tion? Oxidation takes place wherever oxygen is present. These 

 facts, as we shall see later, have a far-reaching significance in the 

 understanding of some 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 composition of phosphorus, sulphur, and some other 

 materials. Phosphorus is a chemical element distinguished by its extreme 

 inflammability. It unites with oxygen at a comparatively low temperature. 

 Sulphur is another chemical element that combines somewhat easily with 

 oxygen but at a much higher temperature. The rest of the match head 

 is made up of red lead, niter or some other substance that will release oxy- 

 gen, and some glue or gum to bind the materials together. The heat 



* For a conci.se statement of this and following experiments in the scientific form 

 expected from the pupil, see Hunter and Valentine, Laboratory Maniud of Biology, 

 Henry Holt and Company, pages 213 ff. 



