FUEL 503 



15 of hydrogen. The fifth column gives the net weights of air required for the 

 oxidation of the fuel. In the sixth column tho theoretical evaporative powers are set 

 down : for rock-oils they are from 227 to 22'5, or, in round numbers, we may say 

 22. In two additional columns are some figures to show how much evaporation is 

 due to carbon and how much to hydrogen. In rock-oils those quantities are 12 due 

 to the carbon, and 10 to tho hydrogen, out of the 22 units of evaporation. 



It is to be observed that the examples given in this table, which are only a few 

 selections out of much more voluminous data, are all taken from good specimens. But 

 it may be taken as the result of practical experience that the evaporative power of 

 bad specimens of fuel from a particular district _ is about two-thirds that of good. 

 This applies particularly to coal. The difference is chiefly due to refuse or ash. But 

 it does not apply to mineral oils, because they can hardly contain anything else but 

 combustible matter. They do not contain ash ; they do not contain earthy matter. 

 The statement, then, about qualities of fuel which have only two-thirds of the total 

 evaporative power of those set down in the table, applies to coal and peat ; but it does 

 not apply to mineral oils. 



We have gone at some length into the- principles of the total evaporative power of 

 different sorts of fuel, because it is of great practical importance to understand them. 

 They show the theoretical limit towards which practice approaches in the course of 

 improvement, but which practice cannot surpass nor even attain : the knowledge of 

 those principles prevents people, on the one hand, from forming too small an estimate 

 of the results which may be got from the economical use of fuel ; and on the other 

 hand from indulging in exaggerated estimates of those results. It shows us, indeed 

 what is the actual waste, when we know the result attained in practice : it enables us 

 to judge how far that result falls short of theoretical perfection, and what room there 

 is for further economy by means of improvements. 



The late Dr. "W. J. Macquorn-Eankine, from whose work on ' The Economy of Fuel ' 

 we are about to quote, writes thus with regard to mineral oil : 



'I may make the following observations on what we may look forward to as the 

 probable result of the introduction of such classes of fuel as mineral oil as substitutes 

 for coal. Coal is a very complex kind of fuel. To ensure the best possible economy 

 in the use of it requires the fulfilment of many different conditions, some of which 

 conflict with each other. "We have to burn fixed carbon, and we have to burn the gas 

 that is disengaged from the coal. We may burn one very efficiently, and not burn the 

 other. It is extremely difficult to manage the introduction of air, so that there shall 

 at once be no risk of a deficiency of air, which causes imperfect combustion, and gives 

 bad economy in one way ; or a surplus of air, which carries heat to waste up the 

 chimney, and causes bad economy in another way. On the one hand, we are exposed 

 to the risk, from any little fault of construction or management of the furnaces, of the 

 hydrogen going off unburned, and of its carrying off a large portion of the carbon un- 

 burned also. On the other hand, we are exposed to the risk of solid carbon being im- 

 perfectly burned, and going off as carbonic oxide. The contrivances for diminishing 

 the causes of waste are somewhat difficult and complex to apply in practice ; and above 

 all, too much depends upon the skill of the fireman or stoker. We may say almost 

 everything depends upon the way the furnace is managed. The very best furnaces, 

 the very best boilers that were ever contrived, may be made extremely wasteful by a 

 careless stoker. On the other hand, in using mineral oils, we are somewhat in the 

 position of the chemist who has got a good burner for burning coal-gas. We have to 

 contrive a suitable apparatus for introducing that oil into the furnace in such a way 

 that it shall be thoroughly mixed with air whether in the state of vapour, or in the 

 state of fine spray, with or without the assistance of some porous substance to act as a 

 wick. A steam-jet seems to be the most efficient apparatus for that purpose. Then 

 if our apparatus is properly contrived and properly constructed, and works in the right 

 manner and produces complete combustion at first, there is no reason to suppose that 

 it will ever act badly if treated with ordinary care. 



' It would seem to be no difficult matter with fuel of that sort to diminish the waste 

 of heat through imperfect combustion to nothing, and the waste of heat through hot 

 gases going up the chimney to something very small indeed. In fact, such an efficiency 

 as 90 per cent, of the total evaporative power being realised, instead of being a very- 

 extraordinary thing, may be looked for as a very ordinary thing ; so that, taking the 

 theoretical evaporative power of some hydrocarbon compounds at 22, we ought not 

 to be surprised that, even with rude apparatus, in a merely experimental state, we get 

 an evaporation of 19 or 20 times the weight of the fuel say 19, at all events. I 

 believe that has been realised in experiments that have been made, and that are now 

 being continued. 



' There is another phenomenon as to which I may say something the mode of 

 operation of steam-jets in cases of this kind. There can be no doubt that one use of 



