OXYGEN AND OZONE. 55 



large quantities of the gas are apt to be set free suddenly. 

 This danger is prevented by mixing with it an equal weight 

 of manganese dioxide. What is singular is that the diox- 

 ide does not part with any of its oxygen, and yet it regu- 

 lates and renders more easy the separation of the oxygen 

 from the potassium chlorate. Less heat is required than 

 when the chlorate is used alone, and the gas is driven off 

 gradually and yet very readily. 



54. Other Methods of Preparing Oxygen. There are many 

 other ways of preparing oxygen gas, and by some of them we 

 obtain it indirectly from the atmosphere. One way in which 

 this is done is (1) by passing air over a heated mixture of 

 manganese dioxide and sodium hydrate, and then (2) heat- 

 ing the materials hotter while a current of steam is passing 

 over them. In the first part of the operation sodium man- 

 ganate is formed ; this gives up its oxygen in the second 

 part of the operation, thus reproducing the original materi- 

 als, when the process is repeated. This method was invent- 

 ed by a Frenchman named Tessie du Hotay, and has been 

 tried on a very large scale. 



Oxygen can also be prepared by decomposing sulphuric 

 acid, by heating barium dioxide, and in other ways of less 

 value. Oxygen gas is now a commercial article in great 

 cities, being manufactured for use in the arts. 



55. Properties of Oxygen. Oxygen gas is heavier than 

 that mixture of oxygen and other gases which we call air. 

 If we take 1 as representing air, oxygen would be repre- 

 sented by 1.106. This is said to be the specific gravity of 

 oxygen, air being the standard in reckoning the weight of 

 all the different gases. Oxygen is, like the air, transparent, 

 and without color, odor, or taste. 



56. Oxygen a Supporter of Combustion. It is this ingre- 

 dient of the atmosphere which, to use common language, 

 ordinarily makes things burn. If any thing that is burning 



