HOTHOUSE FURNACES. 



259 



other half, too much. And here another 

 evil ensues — 



" The temperature of the furnace is 

 lowered below the smoke-burning point. — 

 The water, of which all coal contains 

 more or less, and the bituminous particles, 

 are evaporated, and carry off the heat 

 very rapidly ; the fire thus cooled down, 

 and deprived of a due supply of air, is 

 incapable of raising the bituminous par- 

 ticles to a sufficiently high temperature 

 to cause them to inflame, and a densely 

 black smoke escapes from the chimney- 

 top. It may suggest itself to the reader, 

 that a more regular combustion may be 

 obtained, and a better effect produced, if 

 the fireman were to divide the charge of 

 coal, and throw on 5 lb. of coal every five 

 minutes. This is true in theory; but in 

 practice another evil is induced — the 

 more frequent opening of the furnace 

 door, which admits a volume of cold air, 

 the cooling effect of which on the boiler 

 and fines would more than counteract 

 any saving that could be effected by the 

 greater regularity of the combustion. 



" Stoking is an infallible receipt for mak- 

 ing smoke. When an idle fireman wishes 

 to save his trouble, he fires heavily, and 

 stokes or breaks up his fire when the 

 fresh coal is just warmed, which disen- 

 gages a large body of the gases which 

 escape unconsumed. The secret of econo- 

 mical hand-firing consists in disturbing 

 the fire as little as possible; and regu- 

 larity of combustion is one effectual 

 means of preventing the production of 

 smoke." 



In regard to the effects of smoke on 

 vegetation, we may here observe, that it 

 is perhaps less objectionable as being in- 

 jurious than on account of the unsightly 

 appearance it produces, and, still more, 

 the enormous waste of fuel which passes 

 off in an unconsumed state where no 

 means are used for its combustion. 



Some propositions have been made for 

 economising the fuel in furnaces attached 

 to hothouses ; and, strange enough to say, 

 the old and long ago disused plan of 

 attaching a small lime-kiln to them has 

 been revived of late, and by more than 

 one individual. Speechly published a 

 plan and description of one so long ago 

 as 1796, (vide his " Treatise on the Pine,") 

 and informs us that Mr Thompson at 

 Billing, in Northamptonshire, had them in 



use upon rather an extensive scale ; and, 

 on the authority of Lord John Cavendish, 

 we are also informed that such kilns were 

 in use at Bishop's Court in Ireland prior 

 to 1794. Admitting, however, that some 

 economy in fuel could be attained, the 

 dust and filth attending the operation 

 will, we think, prevent their being used 

 in gardens of high keeping. 



The opinions given in favour of nearly 

 air-tight furnaces, and hence stove com- 

 bustion, are far from being in accordance 

 with those of Sir H. Davy and Dr Ure, 

 both of whom have shown that fuel may 

 be consumed by an obscure species of 

 combustion, but at the same time give 

 out little heat. The former says, in 

 " Researches on Flame," — " The facts 

 detailed on insensible combustion explain 

 why so much more heat is obtained from 

 fuel when it is burned quickly than 

 slowly ; and they show that, in all cases, 

 the temperature of the acting bodies 

 should be kept as high as possible ; — not 

 only because the general increment of 

 heat is greater, but likewise because those 

 combinations are prevented which, at 

 lower temperatures, take place without 

 any considerable production of heat. 

 These facts likewise indicate the source 

 of the great error into which experi- 

 menters have fallen, in estimating the 

 heat given out in the combustion of 

 charcoal ; and they indicate methods by 

 which the temperature may be increased, 

 and the limits to which it may be 

 carried." 



It has been proved by satisfactory 

 experiments, that 1 lb. of charcoal is 

 capable of converting 13 lb. of water 

 at 60° into steam at 212°, if burned to 

 the greatest advantage ; but on account 

 of our hitherto imperfectly constructed 

 furnaces, this heating power is very 

 seldom obtained, even after making 

 allowance for the waste of heat, which, 

 in the best of furnaces, is inevitable. 

 Dr Muller, in his inaugural lecture at 

 King's College, says : " The first and 

 most obvious loss arises from the escape 

 of the heated air from the chimney 

 before it has surrendered to the boiler 

 or flue the full amount of heat which 

 it is capable of relinquishing. It is 

 manifest that the best method of obviat- 

 ing this consists in so arranging the 

 chimney aud passages for the products 



