OXYGEN. 



243 



Preparation. Oxygen gas is generally disengaged from 

 some compound containing it, by the action of heat. 



1. It was first procured by Priestley, by heating red preci- 

 pitate (peroxide of mercury,) which is thereby resolved into 

 fluid mercury and oxygen gas. To illustrate the formation of 

 oxygen in this way, 200 grains of red precipitate may be in- 

 troduced into the body of a small retort a of hard or difficultly 



FIG. 29. 



fusible glass, and the retort united in an air- tight manner with 

 a small globular flask b, having two openings, both closed by 

 perforated corks, one of which admits the beak of the retort, 

 and the other an exit tube c, of glass, bent as in the figure. 

 The extremity of the exit tube is introduced into a graduated 

 jar d } capable of holding 50 or 60 cubic inches, and placed in 

 an inverted position, full of water upon the shelf of a pneumatic 

 water-trough. Heat is then applied to the retort, by means 

 of an Argand spirit lamp, powerful enough to raise it to a red 

 heat, and maintain it at that temperature for a considerable 

 time. The first effect of the heat is to expand the air in the 

 retort, bubbles of which issue from the tube c, and rise to the 

 top of the jar d, displacing water ; but more gas follows, which 

 is oxygen, and at the same time metallic mercury condenses 

 in the neck of the retort and runs down into the intermediate 

 flask b. When the red precipitate in the retort has entirely 

 disappeared, the lamp may be extinguished, and the retort 

 allowed to cool completely. The end of the exit tube c being 

 now above the level of the water in the jar, which is nearly full 

 of gas, a portion of the latter, equal in bulk to the air which first 

 left the retort, will return to it, from the contraction of the 

 gas within the retort. The jar will be found in the end to 

 contain 48 cubic inches of gas, which is therefore the measure 



R 2 



