Febmiry 5, 1874. J 



JOURNAL OP HORTICULTUKB AND COTTAGE GARDENER. 



113 



THE VALUE OF FUEL. 



^^ HE remarks of Mr. Abbey iii one of the 

 numbers of the Journal towards the end of 

 last year, and the interesting experiment 

 which he had previously recorded, open-up 

 a subject of great importance to the horti- 

 culturist, as indeed to the public. I will 

 endeavour to reply to Mr. Abbey's queries, 

 but it wUl bo most convenient to do so in 

 the order which a discussion of the ques- 

 tions involved naturally takes. I may, how- 

 ever, here remark that whUe Mr. Abbey's tables of heating 

 power (in which the second column should be from 32", 

 not zero, to 212°) are rouglily correct as regards the 

 comparison of different fuels, yet as they have been con- 

 structed from observation of the gi-eatest amount of heat 

 obtained, they are open to the suggestion that combustion 

 in different forms of apparatus would probably lead to 

 different results. It wiU be better, then, to commence 

 by giving the total quantity of heat which each different 

 combustible is capable of yielding, leaving to after-con- 

 sideration what arrangements in practice are calculated 

 to render the largest possible amount of such heat 

 available. 



The amount of heat given out in combustion is usually 

 estimated in " units of heat" — i.e., the number of pounds 

 of water raised 1" by the combustion of 1 lb. of the fuel. 

 Care must be taken in using the iigures of different ex- 

 perimentahsts not to confuse the degi-ees Centigrade, in 

 which many of the observations are recorded, with de- 

 grees of Fahrenheit. To convert the former into the 

 latter the number must be multiplied by 1.8. I shall, 

 however, use degrees Fahrenheit alone. Further, in 

 order to convert observations of the number of pounds of 

 water vaporised at 212° into " units of heat," it is neces- 

 sary to multiply by 9G2", which is the latent heat of 

 ■ steam at 212". According to the average of the most 

 recent obser^-ers the units of heat evolved, and corre- 

 sponding quantity of water evaporated from 212°, by the 

 principal elements of combustion are in round figures — 



Pounds water 

 Units of heat. evaporated from 212*^. 



14,500 1,5 



61,000 U3.5 



4,000 4.11 



10,000 lU.l 



I give the latter (carbonic oxide) in order that it may 

 be seen that if by imperfect combustion carbon is only 

 converted into this gas, it only produces about one-thu'd 

 of the heat which it gives when converted into carbonic 

 acid by perfect combustion, the remaining two-thirds 

 being evolved by the combustion of the carbonic oxide ; 

 and the reason is that carbon in passing into a gas absorbs 

 and renders latent, just as steam does, a large proportion 

 of the heat evolved, while after it has reached the gaseous 

 stage its heat of combustion becomes wholly sensible 

 heat. 



Partly from these data and partly from dh'ect obser- 

 vation, the heat given out by fuels of compound natm-e 



Ko. 671.— Vol. XXVI., New Series. 



Carbon 



Hydrogen 



Sulphur 



Carbonic oxide . 



Pounds water 

 evap. from 212' 

 ... 20 

 ... 21 

 ... 18i 

 ...* 16 

 ... 15 



ISioOO 13i 



... 13 

 ... 10 

 7J 



has been pretty accurately ascertained. It must, how- 

 ever, be kept in view that coal and coal gas are both 

 of a variable composition, and hence the figures can 

 only be regarded as an average, from which considerable 

 variance may occur. 



Units of heat. 



Coal gas 2.5,200 



Petroleum 20,500 



Oils, wax, &c 18,000 



Welsh Bteam coal 15,400 



Newcastle do 14,400 



Do. household coal . 



Coke 12,500 



Pry peat 0,020 



Dry wood 7,200 



These, however, are the results obtainable only when 

 combustion is absolutely perfect, and when the whole re- 

 sulting heat is obtained for measurement. In the case of 

 compounds containing hydrogen, if the gases formed pass- 

 ofi' before being reduced "below 212°, so as to condense tho 

 steam into water, the latent heat of the steam absorbs 

 about one-fifth of the total heat due to the hydrogen. 

 And it may show how diflioult is the utilisation of the 

 heat if I mention that in the only du-ect experiments 

 made on the heat evolved by coal gas with which I am 

 acquainted, though they were carried out by so eminent 

 a chemist as Dr. Dalton, the heat measured reached only 

 12,000 units, or less than one-half of what theory would 

 assign. So also in regard to coal, the experhuents of 

 Watt, Black, and Eumford assigned it only an average of 

 7000 units, which in practice is still found generally cor- 

 rect, although in the Cornish engines, which are con- 

 structed so as to economise the whole heat, an amount 

 not materially less than that given in the table has been 

 occasionally reached. 



The only method by which, in tho apphcation of heat- 

 ing apparatus to buUdings, it is possible to extract from 

 fuel an amount of heat approaching to the theoretical 

 values is by direct combustion within the building itself, 

 either retaining the products of combustion in the atmo- 

 sphere, or so conducting them through cooling surfaces 

 that they yield the whole of their heat before passing 

 into the open air. Thus a petroleum or oil lamp bm-ned 

 without a chimney in a greenhouse will give heat quite 

 equal to the tabular figures. A gaslight burned under a 

 chimney will give the tabular heat, minus so much as 

 passes off through the chimney. A brick stove, burning 

 coal or coke, will give the heat due to the fuel minus 

 the amount contained in the au- and steani which goes 

 up the chimney. Tliis is reduced to a minimum when 

 the air admitted is just sufiicient for perfect combustion, 

 as in the Arnott stove, or when the chimney is elongated 

 into a flue, which abstracts most of the heat from the 

 gaseous products before they escape : hence the remark- 

 able effect produced by small quantities of fuel in Mr. 

 Elvers' brick Ai-nott stoves, which are wholly within the 

 house to be heated. A flue apparatus with the furnace 

 fed from a stokehole is more wasteful by the amount of 

 heat given off in the stokehole itself, which may be much 

 or Uttle, according to the arrangement. But the most 



No. 1323.— Vol. LI., Old Sekks. 



