OXYGEN. 73 



10. OXYGEN. 

 OH = 16 (15.96). 



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 afterward recognized by Lavoisier, of France, 

 who gave it the name oxygen, from the two Greek words, bf-vs (oxus), 

 acid, and yew&u (gennao), to produce or generate. Oxygen means, 

 consequently, generator 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, possibly as much as 45 per cent., of the 

 total weight of our earth is made up of oxygen ; it is found in a free 

 or uncombined 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 constituents 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 matters. 



If the unknown interior of our earth should be similar in composition to the 

 solid crust of mineral constituents which have been analyzed, then the sub- 

 joined table will give approximately the proportions of those elements present 

 in the largest quantity. 



Oxygen . . .45 parts. Calcium . . .4 parts. 



Silicon . . . 28 " Magnesium . . 2 " 



Aluminum . 8 " Sodium . . . 2 " 



Iron . . . 6 " Potassium . . 2 " 



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

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

 metal and oxygen : 



HgO=Hg + O; 



Ag 2 0=2Ag + O. 



A more economical method of obtaining oxygen is the decomposi- 

 tion of potassium chlorate, KC1O 3 , into potassium chloride, KC1, 

 and oxygen by application of heat : 



KC1O 3 = KC1 + 30. 



While the above formula represents the final result of the decomposition, it 

 takes place actually in two stages. At first potassium chlorate gives up but 

 one-third of its total oxygen, forming potassium chloride and perchlorate, 



KC10 4 , thus : 



2KC1O 3 b= KC10, + KC1 + 2O. 



