FUEL 495 



much of the air never comes into contact "with tho burning body, and where it con- 

 sequently never has tho whole of its oxygen consumed. The heating power stated in 

 the second column is also the maximum effect, and can seldom be realised with ordinary 

 boilers. The draught of air usually carries off at least one-seventh of the heat, and 

 more if its temperature bo very high when it leaves the vessel. In this case it may 

 amount to one half of tho whole heat, or more ; without reckoning the loss by radiation 

 and conduction, which however may be rendered very small by enclosing the fire and 

 flues within proper non-conducting and non-radiating materials. 



It appears that, in practice, the quantity of heat which may be obtained from any 

 combustible in a properly-mounted apparatus, must vary with the nature of tho object 

 to be heated. In heating chambers by stoves, and water boilers by furnaces, the 

 effluent heat in the chimney, which constitutes the principal waste, may be reduced to 

 a very moderate quantity, in comparison with that which escapes from the best con- 

 structed reverberatory hearth. In heating the boilers of steam-engines, one pound of 

 coal is reckoned adequate to convert 7 Ibs. of boiling water into vapour ; or to heat 

 4l Ibs. of water from the freezing to the boiling point. One pound of fir of the usual 

 dryness will evaporate 4 pounds of water, or heat 22 Ibs. to the boiling temperature , 

 which is about two-thirds of the maximum effect of this combustible. According to 

 Watt's experiments upon the great scale, one pound of coal can boil off with the best 

 built boiler, Ibs. of water ; the deficiency from the maximum effect being here 

 |f, or nearly one-sixth. See the Tables at the end of this article. 



In many cases the hot air which passes into the flues or chimneys may be bene- 

 ficially applied to tho heating, drying, or roasting of objects ; but care ought to be 

 taken that the draught of the fire be not thereby impaired, and an imperfect combustion 

 of the fuel produced. For, at a low smothering temperature, both carbonic oxide and 

 carburetted hydrogen may be generated from coal, without the production of much 

 heat in the fireplace. 



Calorimeters. To determine exactly the quantity of heat disengaged by any fuel in 

 the act of burning, three different systems of apparatus have been employed : 1, the 

 calorimeter of Lavoisier and Laplace, in which the substance is burned in the centra 

 of a vessel whose walls are lined with ice, and the amount of ice melted measures the 

 heat evolved ; 2, the calorimeter of Watt and Eumford, in which the degree of heat 

 communicated to a given body of water affords the measure of temperature ; and 3, 

 by the quantity of water evaporated by different lands of fuel in similar circumstances. 



Lavoisier's. The first and most celebrated, though probably not the most accurate 

 apparatus for measuring the quantity of heat transferable from a hotter to a colder 

 body, consisted of three concentric cylinders of tin plate, placed at certain distances 

 asunder ; the two outer interstitial spaces being filled with ice, while the innermost 

 cylinder received the hot body, the subject of experiment. The quantity of water dis- 

 charged from the middle space by the melting of the ice in it, served to measure the 

 quantity of heat given out by the body in the central cylinder. A simpler and better 

 instrument on this principle would be a hollow cylinder of ice of proper thickness, into 

 whose interior tho hot body would be introduced, and which would indicate by the 

 quantity of water found melted within it the quantity of heat absorbed by the ice. In 

 this case the errors occasioned by the retention of water among the fragments of ice 

 packed into the cylindric cell of the tin calorimeter, would be avoided. One pound of 

 water at 172 F., introduced into the hollow cylinder described, will melt exactly one 

 pound of ice ; and one pound of oil heated to 172 will melt half a pound. One con- 

 stant source of error in this instrument is the regelation of the water from the liquified 

 ice in passing through the ice, which still maintains a temperature below 32 F. 



Meyer's Method of Refrigeration, contrived at first by Meyer, was brought to great 

 perfection by Dulong and Petit. It rests on the principle, that two surfaces of like 

 size, and of equal radiating force, lose in like times the same quantity of heat when 

 they are at the same temperature. Suppose, for example, that a vessel of polished 

 silver, of small size, and very thin in the metal, is successively filled with different 

 pulverised substances, and that it is allowed to cool from the same elevation of tem- 

 perature ; the quantities of heat lost in the first instant of cooling will be always equal 

 to each other ; and if for one of the substances, the velocity of cooling is double of 

 that for another, we may conclude that its capacity for heat is one half, when its 

 weight is the same ; since by losing the same quantity of heat, it sinks in temperature 

 double the number of degrees. 



The Method of Mixtures. In this method two bodies are always employed : a hot 

 body, which becomes cool, and a cold body, which becomes hot, in such manner 

 that all the heat disengaged by the former is expended in heating the latter. Sup- 

 pose, for example, that we pour a pound of quicksilver at 212 F., into a pound of 

 water at 32 ; the quicksilver will cool and tho water will heat, till the mixture by 

 Stirring acquires a common temperature If this tomperatnre was 122, the water and. 



