OXYGEN. 33 



quantity of oxygen, — the gas coming both from the acid and base, and 

 the salt becoming converted into the chloride of potassium. This is 

 shown by the following diagram : 



r c Chlorine '/- Chloride of 



Chloric Acid \ ^ ^^ ' Potassium. 



Chlorate of J ' ^ Oxygen ^ ^ - 



Potassa \ ^"\^^ 



T» . 5 Potassium ^\v^ 

 L ^o^ass^ J Oxygen J::_ 6 eq. oxygen. 



If a little powdered peroxide of manganese be previously mixed 

 with the chlorate of potassa, a less amount of heat will be required, 

 although none of the oxygen comes from the manganese. Oxygen 

 should be collected over water, in a p7ieumatic trough^ — a large 

 vessel holding water, and fitted with a shelf for holding the receivers, 

 which must always be below the level of the liquid. 



Properties. — Colourless, tasteless, inodorous; sp. gr. 1*1 026; — has 

 never been condensed into a liquid ; it is the most perfect negative 

 electric ; is very sparingly soluble in water ; a powerful supporter of 

 combustion and of life; has a strong attraction for most simple 

 bodies. Its power of supporting combustion shown by immersing in 

 it a candle with a red hot wick, which is instantly relighted; or by 

 the burning of fine iron wire, or of phosphorus and sulphur, in it ; the 

 compounds which are always thus formed are termed either oxides 

 or acids, according as they do, or do not, possess acid properties. 

 Ordinary combustion is only the rapid union of a body with oxygen ; 

 and the body, when burnt, is always increased in weight ; which in- 

 crease is exactly equal to the amount of oxygen which has disap- 

 peared. Oxygen is equally essential to respiration : no animal can 

 live in an atmosphere deprived of oxygen ; pure oxygen is injurious 

 to life, on account of its highly stimulating properties, all the func- 

 tions becoming extremely excited. Eq. = 8 ; — combining vol. =50 ;— 

 Symbol, O. 



Tiieory of combustion. — The term combustion, in its widest sense, 

 signifies "a chemical combination, attended with the evolution of 

 light and heat." In its restricted sense, it means " the rapid union 

 of a combustible with oxygen." Before the discovery of oxygen 

 gas, the phenomena of combustion were explained on the Stahlian, 

 or phlogistic theory, that all combustibles contained a principle named 

 phlogiston, to which they owed their combustibility ; and that when 

 they burned, they gave out their phlogiston, and then ceased to be 

 combustible. A metallic oxide was consequently regarded as a 

 simple substance, and the metal itself as a compound of the oxide 

 with phlogiston. 



Lavoisier proved the incorrectness of this doctrine, by showing 

 that a body on undergoing combustion, so far from giving out any 

 thing, always acquires oxygen ; thus on burning a piece of phospho- 



