OXYGEN AND ITS SALINE r< >.M IM NATIONS 165 



combustion is invigorated and is accompanied by a brighter yellow 



flame. Metallic magnesium, which burns brightly in air, continues to 



burn with still greater vigour in oxygen, forming a white powder, 



which is a compound of magnesium with oxygen (magnesium oxide ; 



magnesia). A strip of iron or steel does not 



burn in air, but an iron wire or steel spring 



may be easily burnt in oxygen. A much 



larger piece of iron might naturally be burnt 



if it only were convenient to heat it to the 



required degree. 27 The combustion of steel 



or iron in oxygen is not accompanied by a 



flame, but sparks of oxide fly in all directions 



from the burning portions of the iron. 28 



In order to demonstrate by experiment 

 the combustion of hydrogen in oxygen, a gas- 

 conducting tube, bent so as to form a con- 



..,.,,,, ,. , . . FIG. 30. Mode of burning a steel 



vemeiit jet, is led from the vessel evolving spring in oxygen. 



hydrogen. The hydrogen is first set light 



to in air, and then the gas-conducting tube is let down into a 'flask 

 containing oxygen. The combustion in oxygen will be similar to 

 that in air ; the flame remains pale, notwithstanding the fact that its 

 temperature rises considerably. It is instructive to remark that oxygen 

 may burn in hydrogen, just as hydrogen in oxygen. In order 

 to show the combustion of oxygen in hydrogen, a tube bent vertically 

 upwards and ending in a fine orifice is attached to the stop-cock of a 

 gas holder full of oxygen. Two wires, placed at such a distance from 



- 7 In order to burn a watch spring, a piece of tinder (or paper soaked in a solution of 

 nitre, and dried) is attached to one end. The tinder is lighted, and the spring is then 

 plunged into the oxygen. The burning tinder heats the end of the spring, the heated 

 part burns, and in so doing heats the further portions of the spring, which thus entirely 

 burns if enough oxygen is present. 



28 The sparks of rust are produced by reason of the volume of the oxide of iron being 

 nearly twice that of the volume of the iron, and as the heat evolved is not sufficient to en- 

 tirely melt the oxide or the iron, the particles must be torn off and fly about. Similar 

 sparks are formed in the combustion of iron, in other cases also. We saw the combustion 

 of iron filings in the Introduction. In the welding of iron small iron splinters fly off in all 

 directions and burn in the air, as is seen from the fact that whilst flying through the air 

 they remain red hot, and also because, on cooling, they are seen to be no longer iron, but 

 a compound of it with oxygen. The same thing takes place when the hammer of a gun 

 strikes against the flint. Small scales of steel are heated by the friction, and glow and 

 burn in the air. The combustion of iron is still better seen by taking it as a very fine 

 powder, such as is obtained by the decomposition of certain of its compounds for 

 instance, by heating Prussian blue, or by the reduction of its compounds with oxygen by 

 hydrogen ; when this fine powder is strewn in air, it burns by itself, even without being 

 previously heated (it forms a pyrophorus). This obviously depends on the fact that the 

 powder of iron presents a larger surface of contact with air than an equal weight in a 

 compact form. 



