7S PHYSIOGRAPHY. [chap. 



the taper burst again into full flame. The gas is, in fact, 

 what is known to chemists as Oxygen. The red powder is 

 a combination of this oxygen with mercury, and is known 

 therefore as ird oxide of mercury or mercuric oxide. 'When 

 strongly heated, it is completely decomposed or split up 

 into its constituents; every io8 grains of the red oxide 

 jdelding loo grains of metallic mercury and 8 grains of 

 the gas oxygen. 



It was on the ist of August, 1774, that oxygen was 

 originally discovered by Dr. Priestley. He obtained it 

 from the red mercurial powder just as we have obtained it, 

 excepting that he heated the powder by means of a large 

 burning glass. Various other methods were soon discovered 

 for obtaining the gas, and its properties were fully examined, 

 especially by the Swedish chemist, Scheele, and the French 

 chemist, Lavoisier. It was Lavoisier who gave to this 

 curious kind of air or gas the name of Oxygvi^ by which 

 it is now universally known ; and it was he, too, who 

 first showed, by the most conclusive experiments, what was 

 really the composition of atmospheric air. His determina- 

 tion of the constitution of the air was made in the year 1777. 

 It is therefore only within the last century that chemists have 

 become acquainted with the exact nature of so common 

 a body as the air we breathe. 



Lavoisier took a weighed quantity of mercury and exposed 

 it to strong heat in a vessel containing a confined volume of 

 atmospheric air. In the course of twelve days the metal 

 was completely calcined, or converted into the red rust or 

 cxide. During this conversion the air diminished in bulk 

 while the quicksilver increased in weight ; in fact, the 



•' Oxygen, from o|uj, oxus, acid ; y^vvica, gennao, to produce ; a 

 name based on the supposition tliat substances burnt in oxygen always 

 produce acid compounds. 



