WILLIAM SIEMENS, /.A'.>. 87 



heat is nevertheless produced, and that the amount of heat so 

 produced precisely equals that obtained more rapidly in exposing 

 pulverulent iron to the action of oxygen. Only, in the latter case 

 the heat is developed by slow degrees, and is dispersed as soon as 

 produced, whereas in the former the rate of production exceeds 

 the rate of dispersion, and heat, therefore, accumulates to the 

 extent of raising the mass to redness. It is evident from these 

 experiments that we have to widen our conception, and call fuel 

 " an// substance which is capable of entering into combination with 

 anoihrr substance, and in so doing gives rise to the phenomenon of 

 heat." 



In thus defining fuel, it might appear at first sight that we 

 should find upon our earth a great variety, and an inexhaustible 

 supply of substances that might be ranged under this head ; but a 

 closer investigation will soon reveal the fact, that its supply is, 

 comparatively speaking, extremely limited. 



Constituents of the Earth. In looking at the solid crust of the 

 earth, we find it to be composed for the most part of siliceous, 

 calcareous, and magnesious rock ; the former, silica, consisting of 

 the metal silicon combined with oxygen, is not fuel, but rather a 

 burnt substance which has parted with its heat of combustion ages 

 ago ; the second, limestone, being carbonate of lime, or the com- 

 bination of two substances, viz., calcic oxide and carbonic acid, both 

 of which are essentially products of combustion, the one of the 

 metal calcium, and the other of carbon ; and the third, magnesia, 

 a combination of oxygen with the metal magnesium (which I have 

 just burnt before you,) and which, further combined with lime, 

 constitutes dolomite rock, of which the Alps are mainly composed. 

 All the commoner metals, such as iron, zinc, tin, aluminium, 

 sodium, &c., we find in nature in an oxidized or burnt condition ; 

 and the only metallic substances that have resisted the intense 

 oxidizing action that must have prevailed at one period of the 

 earth's creation are the so-called precious metals, gold, platinum, 

 iridium, and to some extent also silver and copper. Excepting 

 these, coal alone presents itself as carbon and hydrogen in an un- 

 oxidized condition. But what about the oceans of water, which 

 have occasionally been cited as representing a vast store of heat- 

 producing power ready for our use when coal shall be exhausted ? 

 Not many months ago, indeed, on the occasion of a water gas 



