66 XON-METALS AND THEIR COMBINATIONS. 



Valence. 



Univalent. Bivalent. ' Trivalent. Quadrivalent. 



Hydrogen. Oxygen. Nitrogen. Carbon. 



Chlorine. Sulphur. Boron. Silicon. 



Bromine. Phosphorus. 



Iodine. 

 Fluorine. 



10. OXYGEN. 



0=16. 



History. Oxygen was discovered in the year 1774 by Priestley, 

 in England, and Scheele, in Sweden, independently of each 

 other ; its true nature was soon afterwards recognized by Lavoi- 

 sier, of France, who gave it the name oxygen, from the two 

 Greek words, #'? (oxus), acid, and yewdu (genao), to produce or 

 generate. Oxygen means, consequently, geperator of acids. 



Occurrence in nature. There is no other element on our earth 

 present in so large a quantity as oxygen. It has been calculated 

 that not less than about one-third of the total weight of our 

 earth is made up of oxygen ; it is found in a free or uncom- 

 bined state in the atmosphere, of which it forms about one- 

 fifth of the weight. Water contains eight-ninths of its weight 

 of oxygen, and most of the rocks and different mineral con- 

 stituents of our earth contain oxygen in quantities varying from 

 30 to 50 per cent. ; finally, it is found as one of the common 

 constituents of most animal and vegetable matter. 



Preparation. The oxides of the so-called noble metals (gold, 

 silver, mercury, platinum) are easily decomposed by heat into 

 the metal and oxygen : 



HgO = Hg + 0; 

 Ag 2 = 2Ag + O. 



A more economical method of obtaining oxygen is the de- 

 composition of potassium chlorate into potassium chloride and 

 oxygen by application of heat. 



KC10 3 = KCl + 3O. 



If the potassium chlorate is mixed with about ten per cent. 

 of manganese dioxide and this mixture heated, the liberation of 



