HYDROGEN. 69 



by a peculiar odor, by being an even stronger oxidizing agent 

 than common oxygen, by liberating iodine from potassic iodide, 

 etc. This latter action may be used for demonstrating the 

 presence of ozone by suspending in the gas paper moistened 

 with a solution of potassium iodide and starch. The iodine, 

 liberated by the ozone, forms, with starch, a dark blue com- 

 pound. Theoretically, we assume that ozone contains three, 

 common oxygen but two, atoms in the molecule. 



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 grains of potassium 

 chlorate ? 



95. State the physical and chemical properties of oxygen. 



96. What is combustion, and how does it differ from slow com- 

 bustion ? 



97. Mention some oxidizing agents. 



98. What is ozone, and how does it differ from common oxygen ? 



99. Under what circumstances is ozone formed ? 



100. State the molecular weight of oxygen and of ozone. 



11. HYDROGEN. 

 H 1 = 1. 



History. Hydrogen was obtained by Paracelsus in the 16th 

 century ; its elementary nature was recognized by Cavendish, 

 in 1781. The name is derived from Mup (hudor), water, and 

 yewdu (genao), to generate, in allusion to the formation of water 

 by the combustion of hydrogen. 



Occurrence in nature. Hydrogen is found chiefly in the form 

 of water, and enters into the composition of most animal and 

 vegetable substances. 



Preparation. Hydrogen may be obtained by passing an elec- 

 tric current through water, by which it is decomposed into its 

 elements : 



HO = 2H O. 



