72 GASEOUS BODIES. 



composition of atmospheric air and water claim the chief 

 consideration. 



The atmosphere, as has been already observed,* is a 

 mixture of three gases, oxygen, nitrogen, and carbonic 

 acid gas. 



Oxygen takes its name from a Greek word, and"literally 

 means a generator of acidity, from the fact of acids being 

 generally a combination of oxygen with a base. Oxy- 

 gen gas is an invisible fluid, without taste or colour, and 

 capable of expansion and compression, similar to at- 

 mospheric air. It is a supporter of life and light ; for it 

 has been found, by experiment, that an animal will die, 

 and light will become extinguished, when the atmosphere 

 is deprived of its oxygen ; even fruit will not ripen in an 

 atmosphere deprived of oxygen. In the process of respi- 

 ration the oxygen, as it passes through the lungs, com- 

 bines with the blood, which it changes from a purple to 

 a red colour, and renders it a stimulus to the action of 

 the heart and arteries, and is the cause of animal heat. 

 Oxygen combines with all combustible bodies, disengag- 

 ing at the same time its caloric, and producing a strong 

 light and flame. It also combines with metals, which 

 act of combination is called oxidizement; the oxide, 

 therefore, or rust of any metal, is the metal combined 

 with oxygen, and it may be observed that the greatest 

 number, if not all such combinations, with the exception 

 of the oxide of iron, are highly poisonous. The specific 

 gravity of oxygen is somewhat greater than that of at- 

 mospheric air. 



Oxygen maybe obtained for experiment in many ways ; 

 the more general way is from the black oxide of manga- 

 nese. The manganese is powdered coarsely, and put 

 into an iron or stone retort, and heated to redness, when 

 the gas will come off, and may be collected over water in 

 a gasometer or other receiver. If the magnanese be 

 mixed with sulphuric acid until it becomes a thin paste, 

 the heat of a lamp will be sufficient to disengage it. It 

 may be also obtained in a very pure state from the 

 chlorate of potash. Vegetables are found to give out 



* See article Pneumatics, 



