164 



PEINCIPLES OF CHEMISTRY 



degree. Examples of this are familiar to all from every-day experience. 

 Combustion proceeds in oxygen with greater rapidity, and is accom- 

 panied by a more powerful incandescence, than in ordinary air. This 

 may be demonstrated by a number of very convincing experiments. If 

 a piece of charcoal, attached to a wire and previously brought to red- 

 heat, be plunged into a flask full of oxygen, it rapidly burns at a white 

 heat i.e., it combines with the oxygen, forming a gaseous product of 

 combustion called carbonic anhydride, or carbonic acid gas. This is the 

 same gas that is evolved in the act of respiration, for charcoal is one of 

 the substances which is obtained by the decomposition of all organic 

 substances which contain it, and in the process of respiration part of the 



constituents of the body, so to speak, slowly 

 burn. If a piece of burning sulphur be laid on 

 a small cup attached to a wire and be placed 

 in a flask full of oxygen, then the sulphur, 

 which burns in air with a very feeble flame, 

 burns in the oxygen with a violet flame, 

 which, although pale, is much larger than 

 in air. If the sulphur be exchanged for a 

 piece of phosphorus, 24 then, unless the phos- 

 phorus be heated, it combines very slowly 

 with the oxygen ; but, if heated, although 

 on only one spot, it burns with a very bril- 

 liant white flame, which is unbearable to 

 the sight. In order to heat the phosphorus 



inside the flask, the most simple way is to bring a red-hot wire into con- 

 tact with it. Before the charcoal can burn, it must be brought to a state 

 of incandescence. Sulphur also will not burn under 100, whilst phos- 

 phorus inflames at 40. Phosphorus which has been already lighted in air 

 cannot so well be introduced into the flask, because it burns very rapidly 

 and with a large flame in air. If a small lump of metallic sodium be put 

 in a small cup made of lime, 25 melted, and inflamed, 26 then it burns very 

 feebly in air. But if burning sodium be immersed in oxygen, the 



FIG. 29. Mode of burning sul 

 phur, phosphorus, sodium, &c. 

 in oxygen 



24 The phosphorus must be dry ; it is usually kept in water, as it oxidises in air. It 

 should be cut under water, as otherwise the freshly-cut surface oxidises. It must be dried 

 carefully and quickly by wrapping it in blotting-paper. If damp, it splutters in burning. 

 A small piece should be taken, as otherwise the iron spoon will melt. In this and the 

 other experiments on combustion, water should be poured over the bottom of the vessel 

 containing the oxygen, to prevent it from cracking. The cork closing the vessel should not 

 fit tightly, otherwise it may fly off with the spoon and burning substance, owing to the 

 expansion due to the heat of the combustion. 



25 An iron cup will melt with sodium in oxygen. 



26 In order to rapidly heat the lime crucible with the sodium, they are heated in the 

 flame of a blow-pipe described in Chap. VIII. 



