Of Chimney Fireplaces. 543 



the whole country, far and wide ; for this dense cloud is 

 certainly composed almost entirely of unconsumed coal, 

 which, having stolen wings from the innumerable fires 

 of this great city, has escaped by the chimneys, and con- 

 tinues to sail about in the air, till, having lost the heat 

 which gave it volatility, it falls in a dry shower of ex- 

 tremely fine black dust to the ground, obscuring the at- 

 mosphere in its descent, and frequently changing the 

 brightest day into more than Egyptian darkness. 



I never view from a distance, as I come into town, 

 this black cloud which hangs over London, without 

 wishing to be able to compute the immense number of 

 caldrons of coals of which it is composed ; for, could 

 this be ascertained, I am persuaded so striking a fact 

 would awaken the curiosity and excite the astonishment 

 of all ranks of the inhabitants, and perhaps turn their 



it has been found by long experience, that the expense attending this preparation is 

 amply repaid by the improvement of the fuel. The coals, thus mixed with clay, not 

 only burn longer, but give much more heat than when they are burned in their crude 

 state. 



It will doubtless appear extraordinary to those who have not considered the subject 

 with some attention, that the quantity of heat produced in the combustion of any 

 given quantity of coals should be increased by mixing the coals with clay, which is 

 certainly an incombustible body ; but the phenomenon may, I think, be explained in 

 a satisfactory manner. 



The heat generated in the combustion of any small particle of coal existing under 

 two distinct forms, namely, in that which is combined with the flame and smoke 

 which rise from the fire, and which, if means are not found to stop it, goes off im- 

 mediately by the chimney and is lost, and the radiant heat which is sent off from the 

 fire, in all directions, in right lines ; I think it reasonable to conclude, that the parti- 

 cles of clay, which are surrounded on all sides by the flame, arrest a part at least of the 

 combined heat, and prevent its escape; and this combined heat so arrested, heating 

 the clay red-hot, is retained in it, and, being changed by this operation to radiant heat, 

 is afterwards emitted, and may be directed and employed to useful purposes. 



In com posing fire-balls, I think it probable that a certain proportion of chaff of 

 straw cut very fine, or even of saw-dust might be employed with great advantage. I 

 wish those who have leisure would turn their thoughts to this subject, for I am per- 

 suaded that very important improvements would result from a thorough investigation 

 of it. 



