COMBUSTION. 323 



bustion, is very different. Hydrogen gas requires a heat equal to that of in- 

 candescence to cause it to begin to burn. Wood, coal, and other combustibles, 

 burn when raised to various temperatures. 



According to the experiments of Sir Humphry Davy, the temperature neces- 

 sary to enable the following substances to combine with oxygen vary in the 

 order in which they stand, the first being that which burns at the lowest tem- 

 perature, and the succeeding ones at temperatures gradually increasing : 



Phosphorus, 



Phosphuretted hydrogen gas, 



Hydrogen and chlorine, 



Sulphur, 



Hydrogen and oxygen, 



Sulphuretted hydrogen, 



Alcohol, 



Wax, 



Carbonic oxide, 



Carburetted hydrogen. 



Olefiant gas, 



The experimental proofs by which combustion is shown to arise from the 

 combination of oxygen with other principles consist of the whole range of one 

 department of chemical science. We may, however, offer an experiment as an 

 example of this species of demonstration. 



Let a short earthenware tube be filled with a coil of iron wire, the weight 

 of which has been previously ascertained. Let one extremity of this tube be 

 connected with a bladder filled with oxygen gas, the weight of which is known ; 

 and let the other extremity be connected with a flaccid bladder, the weight of 

 which, including the air which it contains, is also exactly known. Let the 

 porcelain tube and its contents be raised to incandescence by the application 

 of heat, and let the oxygen contained in the bladder be then forced through the 

 tube in contact with the wire. The wire in this case will burn, and be rapidly 

 oxidised, and the product will be the oxide of iron. When this product is 

 weighed, it will be found to be heavier than the iron ; and when the two blad- 

 ders and their contents are weighed, they will be found to be lighter than be- 

 fore, by exactly the weight which the iron has gained ; the oxygen, therefore, 

 which has been lost by air contained in the bladders, has been combined with 

 the iron during the process of combustion. 



Flame is gas heated to whiteness by the heat produced by the combustion 

 of volatile matter. When a candle burns, the tallow or wax of which it is com- 

 posed is first liquefied, and then drawn up through the interstices of the wick 

 by capillary attraction. As it comes in contact with the source of heat, it is 

 boiled and converted into vapor ; this vapor ascends in a column by reason of 

 its lightness, and is now raised to the temperature which enables it to form a 

 combination with the oxygen of the surrounding air. This combination in- 

 stantly and copiously develops heat, which, being communicated to the sur- 

 rounding current of gas, renders it luminous, and produces the white, bright 

 light of the flame. It will be apparent, from this, that the light from the flame 

 can only exist on its exterior surface, which is in contact with air. The flame 

 of a candle or lamp is, therefore, so far as regards heat, hollow ; or rather it is 

 a column of gas, the exterior surface of which is luminous, while the interior 

 is non-luminous. As the gas in the interior of the flame ascends, it gets into 

 contact with a fresh portion of the atmosphere, from which it receives a supply 

 of oxygen, by combination with which heat is evolved, which produces light. 

 As the gas ascends from the centre of the flame, it comes successively into 

 contact with the air, and in this manner becomes luminous, until at length the 

 column is reduced to a point. Thus the flame of a candle or lamp gradually 

 tapers to a point, until all the gas produced from the boiling matter in the 

 wick receives its due complement of oxygen from the air, and passes off. It 

 speedily loses the temperature necessary to render it luminous, and the flame 

 terminates. 



