50 



CHEMISTRY. 



one third of the earth's solid mass. It is one of the chief 

 components, also, of all vegetable and animal substances. 

 It enters into more combinations with other substances 

 than any other element. There is but one element with 

 which it does not combine. This is not true of any other 

 of the sixty-four elements. Oxygen, therefore, may be said 

 to be the most important substance in nature. This will 

 be still more apparent when we come to consider its agency 

 in the chemical operations every where going on, especially 

 in those of living substances. 



47. One Way of Obtaining Oxygen. Oxygen can be read- 

 ily obtained from many substances which have a great 

 deal of it in them. The red oxide of mercury, formerly 

 called red precipitate, is one of these. This is mercury 

 united with considerable oxygen ; its formula is HgO. By 



heating this oxide the ox- 

 ygen can be made to leave 

 the mercury. One way in 

 which this can be done is 

 shown in Fig. 3. We have 

 here a glass vessel full of 

 mercury, containing the mer- 

 curic acid at the top, stand- 

 ing in the mercury in the 

 dish, B. The heat applied is 

 that of the sun's rays con- 

 Fig, s. cent-rated by a burning-glass, 

 C. The result is the decomposition of the red substance, 

 the oxygen of which accumulates in the upper part of the 

 glass vessel, pushing the mercury down before it. 



Expressed in symbols, the decomposition is very simple : 

 Mercuric Oxide. Mercury. Oxygen. 



HgO Hg O 



48. Discovery of Oxygen. The above was the original ex- 



