CHEMICAL HISTORY OP A CANDLE 169 



are very few fuels that act like this. It is, in fact, only that 

 great source of fuel, the carbonaceous series, the coals, 

 charcoals, and woods, that can do it I do not know that 

 there is any other elementary substance besides carbon that 

 burns with these conditions ; and if it had not been so, what 

 would happen to us? Suppose all fuel had been like iron, 

 which, when it burns, burns into a solid substance. We 

 could not then have such a combustion as you have in this 

 fireplace. Here also is another kind of fuel which burns 

 very well as well as, if not better, than carbon so well, 

 indeed, as to take fire of itself when it is in the air, as you 

 see. [Breaking a tube full of lead pyrophorus.] This sub- 

 stance is lead, and you see how wonderfully combustible it 

 is. It is very much divided, and is like a heap of coals in 

 the fireplace : the air can get to its surface and inside, and so 

 it burns. But why does it not burn in that way now when it 

 is lying in a mass? [Emptying the contents of the tube in a 

 heap on to a plate of iron.] Simply because the air can not 

 get to it. Though it can produce a great heat, the great heat 

 which we want in our furnaces and under our boilers, still 

 that which is produced can not get away from the portion 

 which remains unburned underneath, and that portion, 

 therefore, is prevented from coming in contact with the 

 atmosphere, and can not be consumed. How different is 

 that from carbon! Carbon burns just in the same way as 

 this lead does, and so gives an intense fire in the furnace, 

 or wherever you choose to burn it; but then the body pro- 

 duced by its combustion passes away, and the remaining 

 larbon is left clear. I showed you how carbon went on 

 dissolving in the oxygen, leaving no ash, whereas here 

 [pointing to the heap of pyrophorus] we have actually more 

 ash than fuel, for it is heavier by the amount of the oxygen 

 which has united with it. Thus you see the difference 

 between carbon and lead or iron if we choose iron, which 

 gives so wonderful a result in our applications of this fuel, 

 either as light or heat. If, when the carbon burnt, the prod- 

 uct went off as a solid body, you would have had the room 

 filled with an opaque substance, as in the case of the phos- 

 phorus; but when carbon burns, every thing passes up into 

 the atmosphere. It is in a fixed, almost unchangeable con- 



