THE STEAM-ENGINE. 131 



to consume the whole of the fuel and convert it into carbonic 

 acid, and that none for want of air shall escape out of the 

 fire in the condition of carbonic oxide, but, on the other 

 hand, every particle of air which enters the fire-grate beyond 

 the quantity required is a source of loss by increasing the 

 weight of heated material which escapes unutilized up the 

 chimney. It is a common thing for newspaper writers to 

 point out that smoke is not only a nuisance, but is a waste 

 of fuel which might be burnt. So it is, but it is far less 

 waste than would go on if you had furnaces so managed 

 as to pour up the chimney carbonic oxide. This being in- 

 visible does not excite attention, and thus two men might be 

 employing fuel ; one sending up no visible smoke (although 

 plenty of carbonic oxide), would receive commendation as 

 an economical man, while the other, who did make smoke 

 (which he ought not to do), but made no carbonic oxide, 

 would be improperly stigmatised as the extravagant man. 

 I believe what we have to do is to see if we can obtain 

 mechanical stoking, and on this point I should like to 

 be allowed to say a few words on combustion generally. It 

 appears to me that you may use fuel either in the form 

 of gas, as is practised by Dr. Siemens ; in the form of 

 liquid, as in the use of hydrocarbons ; in the form of almost 

 impalpable powder as used by Mr. Crampton, or in the 

 ordinary form in which we get it as coal, and as we employ 

 it in steam-engines. If you employ it as coal, then clearly 

 the only portions which are available for combustion at 

 one time are the surfaces. In order to develop sufficient 

 heat for your purpose, you are compelled to have within the 

 fire-box, or on the fire-grate, as much coal or coke as will 

 when you take its surface into account (if you could measure 

 it), unite with enough oxygen to produce the required heat in 

 a given time ; and for this to be done you must either have 

 a Jarge extent of grate, or must keep the fire open, or must 

 have a powerful draft ; with a large extent of grate, there 

 is the difficulty that it should at all times be covered, 

 otherwise air runs freely through the spaces uncovered by 

 fuel, causing much loss. On the otljer hand, with a great 

 depth of fire you have the very difficulty I have been point- 

 ing out that of making carbonic oxide. Carbonic acid is 

 produced in the lower part of the fire, and this carbonic 

 acid rising through the hot fuel above, takes up another 



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