OXYGEN AND OZONE. 53 



mercury will not evaporate under 40. The space above 

 the mercury in a barometer or a thermometer is spoken of 

 as a vacuum ; but it is not strictly so, for there is some 

 little of the vapor of mercury diffused through that space. 



While it is proper, then, to speak of substances appearing, 

 under certain circumstances, in the form of vapor as being 

 gaseous or aeriform, they can not properly be called gases. 

 This name belongs only to those substances which main- 

 tain this state under all circumstances ; or perhaps we 

 should say under all ordinary circumstances, for some of 

 the gases under extraordinary circumstances have been 

 made to take on another form, either solid or liquid. 



51. Obtaining Oxygen from Oxide of Manganese. The 

 chemist does not now get his oxygen from mercuric oxide, 

 for there are other compound substances that contain more 

 of it, and furnish it more readily and abundantly. One of 

 these is an oxide of a metal called manganese ; there are 

 several oxides of this metal containing different propor- 

 tions of oxygen ; the one used for the preparation of oxy- 

 gen is called the dioxide, MnO 2 . Manganese dioxide oc- 

 curs native as a mineral, and when ground fine it is a con- 

 venient and cheap source of oxygen ; it is not now so much 

 used as formerly, for 

 still better methods 

 have been invented. In 

 obtaining oxygen from 

 it great heat is em- 

 ployed ; it must there- 

 fore be heated in an 

 iron retort, such as you 

 see in Fig. 5, placed in 

 a furnace. Only one Fig. 5. 



third of the oxygen in the dioxide is driven off, leaving 

 behind the red oxide of manganese. The dioxide is of a 

 dark color, and is commonly called the black oxide. 



