OXYGEN. 77 



perature, and it is for this reason that a substance continues to burn until it is 

 consumed, provided the supply of oxygen be not cut off, and the temperature 

 be not through some cause lowered below the kindling temperature. 



The total amount of heat evolved during the combustion of a substance is 

 the same as that generated by the same substance when undergoing slow com- 

 bustion, but the intensity depends upon the time required for the oxidation. 

 A piece of iron may require years to combine with oxygen, and it may be 

 burned up in a few minutes ; yet the total heat generated in both cases is the 

 same, though we can notice and measure it in the first instance by most deli- 

 cate instruments only, while in the second it is very intense. 



Ozone is an allotropic modification of oxygen, which is formed 

 when non-luminous electric discharges pass through atmospheric air 

 or through oxygen ; when phosphorus, partially covered with water, 

 is exposed to air, and also during a number of chemical decomposi- 

 tions. Ozone differs from ordinary oxygen by possessing a peculiar 

 odor, by being an even stronger oxidizing agent than common oxygen, 

 by liberating iodine from potassium iodide, etc. This latter action 

 may be used for demonstrating the presence of ozone by suspending 

 in the gas a paper moistened with a solution of potassium iodide and 

 starch. The iodine, liberated by the ozone, forms with starch a dark- 

 blue compound. Theoretically, we assume that ozone contains three, 

 common oxygen but two, atoms in the molecule, which is substan- 

 tiated by the fact that three volumes suffer a condensation to two 

 volumes when converted into ozone, which would indicate that three 

 molecules of oxygen furnish two molecules of ozone, thus : 



30 2 = 20 3 

 or 



0=0 



O O 



O=O = 



O 



O=O /\ 



O O 



Ozone occurs in small quantities in country air, but is rarely noticed 

 in cities, where it is decomposed too quickly by the impurities of the 

 atmospheric air. It has been assumed that ozone acts advantageously, 

 as it has a tendency to destroy matters which are unwholesome. Too 

 little, however, is known of the subject to justify a positive opinion 

 in regard to it. 



QUESTIONS. 91. By whom and at what time was oxygen discovered? 92. 

 How is oxygen found in nature ? 93. Mention three processes by which oxygen 

 may be obtained. 94. How much oxygen may be obtained from 490 grammes 

 of potassium chlorate? 95. State the physical and chemical properties of 



