OXYGEN. 137 



10. OXYGEN. 1 



O" = 15.88. 



History. Oxygen was discovered in the year 1773 by Scheele, in 

 Sweden, and one year later by Priestley, in England, independently 

 of each other ; its true nature was soon afterward recognized by La- 

 voisier, of France, who gave it the name oxygen, from the two Greek 

 words, oc^c (oxtis), acid, and ysvvda) (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,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 + 3O. 



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

 1 Many instructors prefer to postpone the discussion of the laws of combination, atomic 

 theory, symbols, and chemical equations until after a few elements and compounds have been 

 studied as an introduction and foundation. If such a procedure is followed by those who use 

 this book, the equations in the chapters that may be taken up before the theoretical matters 

 are presented which make the equations intelligible, should, of course, be omitted. They 

 are given in each chapter for the sake of completeness and reference. 



