6 OXYGEN. PAKT i. 



by a coating generally of green silk thread. But in 

 experiments of extreme delicacy where magnetism might 

 vitiate the results, perfectly pure copper wire which is 

 diamagnetic is used for the conducting wires in the 

 thermo-electric pile of the goniometer, and the wires 

 are coated with white silk thread, since it was discovered 

 that the green dye contains some magnetic metal. 



The mass of the metals however constitutes com- 

 paratively but a small part of the terrestrial globe, 

 which is formed of chemical combinations of only thir- 

 teen simple elementary substances, a wonderful mani- 

 festation of creative power that could form a world of 

 such variety and beauty by means of atoms so little 

 diversified ; still more wonderful is it that four simple 

 elements alone constitute the basis of nearly the whole 

 organic fabric. The air we breathe, water, the bodies of 

 men and living creatures, and the vegetation that adorns 

 the earth, are chiefly combinations of three invisible 

 gases, oxygen, hydrogen and nitrogen, with carbon, the 

 ^ <^vruA purest amorphous form of coal.) 



Oxygen gas forms three-fourths of the superficial 

 crust of the terrestrial globe, its productions and its 

 inhabitants. At least a third part of the solid crust of 

 the earth is oxygen in combination ; it constitutes eight 

 parts out of nine in water, and water covers three- 

 fourths of the surface of the globe ; it forms more than 

 twenty parts out of a hundred of atmospheric air, and 

 in the organic kingdom it is an essential constituent. 

 Except in the atmosphere, oxygen is never uncombined, 

 but may be obtained by distilling chlorate of potash, by 

 the decomposition of water by voltaic electricity, and by 

 other means. When pure it is a colourless, tasteless, 

 inodorous, invisible gas ; it is incombustible at ordinary 

 temperatures, yet absolutely essential to combustion ; 

 no animal can live long in it, and none can exist with- 

 out it. In the atmosphere oxygen is highly magnetic ; 



