Dr Colquhoun on the Argillaceous Ore of Iron. 87 



These are the two kinds of coal which abound most in the 

 northern division of Great Britain. We shall now take a short 

 glance at their nature and constitution. 



The composition of coals in general may be said to consist of 

 a bituminous principle or principles, and of an earthy ingre- 

 dient. Upon the application of heat to the bitumen, it is re- 

 solved into carbon and certain volatilizable combinations, such 

 as bituminous oil, and carburetted hydrogen gas. The carbon 

 when thus reduced. forms the coke which is employed by the 

 smelter. In regard to the earthy ingredient, although perhaps 

 universally present in all coal, it is nevertheless so extremely 

 variable in its amou\it, and so very changeable in its own na- 

 ture, that it may with propriety be regarded as a substance 

 which is only accidentally present, and which therefore does 

 not form an essential constituent in the composition of coal. 

 But there are various views in which it is important to ascer- 

 tain something of the nature and amount of this earthy ingre- 

 dient, and we shall now consider it a little more particularly. 



This earth may be produced in a separate form by incinerat- 

 ing the coal under the free access of the atmospheric air, until 

 the whole of the carbonaceous matter is consumed. It then 

 always presents itself under the shape of a fine powder. The 

 colour of the powder is sometimes white, but at other times it 

 inclines to red ; and it is material to notice this, because the 

 red indicates the presence of oxide of iron, and affords a pre- 

 sumption that the coal in which it occiirs may be contaminated 

 with iron pyrites. 

 I The general composition of this earthy matter is found to 



consist of silica and alumina, joined with inconsiderable quan- 

 tities of lime, magnesia, oxide of iron, and sulphate of lime. 

 Sometimes the siliceous ingredient preponderates so very much, 

 that all the others above mentioned are of insignificant amount. 

 But in general the alumina is also a preponderating ingredient, 

 a fact which might prove of some consequence in investigating 

 the origin of coals ; for, the well known and almost invariable 

 intermixture of a little bituminous matter with the minerals 

 composing the argillaceous and calcareous strata, on the one 

 hand, and on the other, the distribution of a small amount of 

 clay among the limestones and coals, seem to have been pro- 

 duced by analogous causes. 



