186 SCIENTIFIC AMUSEMENTS. 



As a general rule, chemical combination will not take place to 

 any considerable extent between substances in the solid condition, 

 principally because their texture is too stiff and rigid to allow of 

 the two different kinds of matter being brought into sufficient 

 proximity except just at the border line where their outsides 

 touch. If, however, one of the substances be a liquid or gas the 

 action is greatly facilitated; hence, with many kinds of solid 

 chemical substances, one or both of the bodies to be operated upon 

 must be converted into a liquid (either by heating it until it 

 melts, or by dissolving it in a suitable solvent, which amounts to 

 much the same thing) before the chemical action can be brought 

 about. 



In many cases, also, the chemical change will not take place at 

 the ordinary temperature, but ensues readily if the materials are 

 more or less heated ; thus a jet of ordinary coal gas escaping from 

 a burner does not take fire spontaneously, and no chemical action 

 ensues between the coal gas and the air ; but if a light be applied 

 (i.e., if a portion of the mixture of gas and air be considerably 

 heated by a flame) the gas and air undergo a chemical action, the 

 result of which is that the oxygen of the air combines with the 

 constituents of the gas ; in so doing heat is produced, and thus 

 the next portions of gas passing out from the burner are heated 

 sufficiently to set up the chemical change of burning or oxidation ; 

 the heat thus produced propagates the change in the next portions 

 of gas that issue, and so a continuous state of chemical action is 

 kept up, with the result of producing a steady flame. Candles, 

 lamps, torches, &c., burn in just the same kind of way, with this 

 difference that in the case of the coal gas the chemical action takes 

 place directly between the gas and the air ; whereas in an oil lamp 

 the first effect of the heat upon the oil is to decompose it or break 

 it up into gaseous products by a sort of destructive distillation (as 

 in Expts. 200, 201, 202), these products then becoming oxidised 

 by the oxygen of the air as before ; so that an oil lamp or candle 

 flame is actually a kind of gas factory in miniature. 



Expt. 204. To show that Combustible Gases are formed in a 

 Candle or Lamp Flame. Obtain a tallow candle with a thick 

 cotton wick, or an oil lamp (with a round wick rather than a flat 

 one) ; light the lamp and spread out the wick so as to produce as 

 large a flame as possible. Hold a piece of quill glass tubing a few 

 inches long with the lower part of the tube in the centre of the 

 lower part of the flame (fig. 80). If the tube be not too big, and 

 the flame be perfectly steady, it will be seen that a current of 

 smoke and gases passes up the tube, forming a small jet of combus- 

 tible vapours which will burn steadily on applying a light to them. 



