MOLECULES, ELECTRIC MACHINES. 175 



CHAPTER XX. 



FUNCTIONS OF MOLECULES OF OXYGEN, CARBON, HYDRO- 

 GEN, AND NITROGEN, AS ELECTRICAL MACHINES. 



HPHE molecules of oxygen take precedence of all 

 others, both in comparative quantity and facil- 

 ity of motion by excitation. They constitute nearly 

 half of the matter of our planet, including eight- 

 ninths of the weight of waters, one-fourth of the 

 weight of the atmosphere, and a large part of the 

 rocks, sands, and clays. The molecules of oxygen 

 are kept ever moving in the air and waters, from 

 region to region over the earth, forming unions 

 with other kinds of molecules, and dissevering 

 them ; according to their changing states of exci- 

 tation, as by sunshine and shade, heat and cold. 



Oxygen gas is readily obtained by disuniting it 

 from other kinds of molecules; as by the decom- 

 position of water and of various metallic oxides. 

 For experimental purposes it is commonly pro- 

 cured by heating chloride of potash in a retort. 

 The influence of molecules of oxygen in produc- 

 ing acids, obtained for them their characteristic 

 name ; from the two Greek words, oxus, acid, and 



