Sept. 8, 1881] 



NATURE 



449 



gratulation inspired by this sviccess is clouded by tlie reflection 

 tliat the steam-engine, even in its best form, remains to this day 

 a most wasteful apparatus for converting the energy of heat into 

 motive power. Our predecessors of that period had not the 

 advantage of the linowledge which we possess of the true nature 

 of heat, and the conditions and Umits affecting its utilisation. 

 In their time heat was almost universally regarded as a fluid 

 which, under the name of caloric, was supposed to lie dormant 

 in the interstices of matter until forced out by chemical or 

 mechanical means. Although Bacon, Ne«ton, Cavendish, and 

 Boyle all maintained that heat was only internal motion, and 

 although Davy and Rumford not only held that view, but proved 

 its accuracy by experiment, yet the old notion of caloric continued 

 to hold its ground, until in more recent times Joule, Meyer, 

 Codling, and others put an end to all doubt on the subject, and 

 established the all-important fact that heat is a mode of moti jn 

 having, lilie any other kind of motion, its exact equivalent in 

 terms of work. By their reasonings and experiments it has been 

 definitely proved that the quantity of heat which raises the tem- 

 perature of a pound of water 1° Fahrenheit, has a mechanical 

 value equal to lifting 772 lbs. one foot high, and that conversely 

 the descent of that weight from that height is capable of e.xactly 

 reproducing the heat expended. 



The meclianical theory of heat is now universally accepted, 

 although a remnant of the old doctrine is displayed in the con- 

 tinued Use of the misleading term " latent heat." According to 

 the new theory, heat is an internal motion of molecules capable 

 of being communicated from the molecules of one body to those 

 of another, the result of the imparted motion being eithi r an 

 increase of temperature, or the performance of work. The work 

 may be either external, as where heat, in expmding a gas, pushes 

 away a resisting body, or it may be internal, as where heat pulls 

 asunder the cohering particles of ice in the process of liquefac- 

 tion, or it may be partly internal and partly external, as it is in 

 the steam engine, where the first effect of the heat is to separate 

 the particles of water into vapour, and the second to give motion 

 to the piston. Internal as well as external work may be recon- 

 verted into heat, but until the reconversion takes place, the heat 

 which did the work does not exist as heat, and it is delusive to 

 call it " latent heat." All heat problems are comprised under 

 the three leading ideas of internal work, external work, and 

 temperature, and no phraseology should be used that conflicts 

 with" those ideas. 



The modern theory of heat has thrown new light upon the 

 theory of the steam-engine. We notv know what is the mecha- 

 nical value in foot-pounds of the heat evolved in the conbustion 

 of one pound of coal. In practice we can determine how much 

 of that heat is transmitted to the water in the boiler, and we are 

 taught how to calculate the quantity which in the pr icess of 

 vajorisation takes the forji of internal work. We can determine 

 how much disappears in the engine in the shape of external work, 

 including friction, and the remainder, with the exception of the 

 trifling quantity saved in the feed-water, we know to be lost. 

 Taking a good condensing engine as an example, we may 

 roughly say that, dividing the whole heat energy into ten equal 

 pirts, two escape by the chimney, one is lost by radiation and 

 friction, six remain unused when the steam is discharged, and 

 only one is realised in useful work. It may be fully admitted 

 that the greater part of the aggregate lo ^s is inevitable ; but are 

 we to suppose that the resources of science, ingenuity, and skill 

 have been exhausted in the attainment of so miserable a result ? 

 Nothing but radical changes can be expected to produce any 

 great mitigation of the present monstrous waste, and without 

 presuming to say what measures are practicable and what are 

 not, I will briefly point out the directions in which amelioration 

 is thesretically pos.sible, and shall afterwards advert to the ques- 

 tion whether we may ho|)e to evade the difficulties of the steam- 

 engine by resorting to electrical methods of obtaining power. 



To begin with the loss which takes place in the application of 

 heat to the boiler ; why is it thit we have to throw away, at the 

 vei-y outset of our operations, twice as much heat as we succeed 

 in utilising in the engine ? The answer is, that in order to force 

 a transmission of heat from the fire to the water in the boiler, a 

 certain excess of temperature over that of the water mUst exist 

 in the furnace and flues, and the whole of the heat below the 

 required excess must pass away unused, except the trifling pir- 

 tio:i of it which disappears in the production of draught. 

 Further, that since we cannot avoi 1 admitting the nitrogen of 

 the air along with the oxygen, we have to heat a large volume of 

 neutral gas which has no other effect than to rob the fire. Con- 



sidering what efforts have been made to facilitate the transmission 

 of the heat by augmenting the evaporative surface, and using thin 

 tubes as flues, it is vain to expect any great result from further 

 perseverance in that direction, and urdess a method can be devised 

 of burning the fuel inside instead of outside the apparatus, so as 

 to use the heated gases conjointly with the steam as a working 

 medium in the engine, a remedy appears to be hopeless. We 

 already practise internal combustion in the gas-engine, and it is 

 clear that with gaseous fuel, at all events, we could associate 

 such a mode of combustion with the vaporisation of water. We 

 may even regard a gun as an engine with internally-burnt fuel, 

 and here I may remark that the action of heat in a gun is strictly 

 analogous to that of heat in a steam-engine. In both cases the 

 heat is evolved from chemical combination, and the resulting 

 pressures differ only in degree. The gun is the equivalent of the 

 cylinder, and the shot of the piston, and the diagrams repre- 

 senting the pressure exerted in the two cases bear a close resem- 

 blance to each other. While the powder is burning in the gun 

 we have a nearly uniform pressure, just as we have in the cylinder 

 while the steam is e itering, and in both cases the uniform pressure 

 is followed by a diminishing pressure, represented by the usual 

 curve of expansion. If in the steam-engine we all jwed the 

 piston to be blown out it woi\ld act as a projectile, and if in the 

 gun we opposed mechanical resistance to the shot, we might 

 utilise the effect in a quieter form of motive power. But it is a 

 remarkable fact that such is the richness of coal as a store of 

 mechanic il energy that a pound of coal, even as used in the 

 steam-engiue, produces a dynamic effect about five times greater 

 than a pound of gimpowder burnt in a gun. I cannot, however, 

 on thi^ account encourage the idea that steam may be advan- 

 tageously substitute for gunpowder in the practice of gunnery. 



And now to turn from the fire which is the birthplace of 

 the motive energy, let us follow it in the steam, to the con- 

 denser, where mDst of it finds a premature tomb. From the 

 point at which expansion commences in the cylinder the tem- 

 perature and pressure of the steam begin to run down, and if 

 we could continue to expand indefinitely, the eritire heat would 

 he exhausted, and the energy previously expended in separating 

 the water into steam would be wholly given up in external 

 effect ; but this exhaustion would not be ci>mplete until the 

 absolute zero of temperature was reached (viz. 461° below the 

 zero of Fahrenheit). I do not mean to say that an ideally 

 perfect engine neces-arily involves unlimited expansion, seeing 

 that if instead of discharging the steam at the end of a given 

 expansion, we made the engine itself do work in compressing it, 

 we might, under the conditions of Carnot's reversible cycle, so 

 jutly celebrated as the foundation of the theory of the steam- 

 engine, recommence the action with all the unutilised he\t in an 

 available form. But an engine upon this principle could only 

 give an amount of useful effect corresponding to the difference 

 between the whole work done by the engine, and that very large 

 portion of it expended in the operation of compression, and this 

 difference viewed in relation to the necessary size of the engine, 

 would be quite insignificant, and would in fact be wholly swal- 

 lowed up in friction. Carnot did not intend to suggest a real 

 engine, and his hypothesis therefore takes no cognisance of 

 lo-^es incident to the application of an actual fire to an actual 

 boiler. His ideal engine is also supposed to be frictionless, and 

 impervious to heat except at the point where heat has to be 

 transmitted to the water, and there the condition of perfect 

 conduction is assumed. In short an engine which would even 

 approximately conform to the conditions of Carnot's cycle is an 

 impossibility, and a perfect steam-engine is alike a phantom 

 whether it be sought for in the cyclical process of Carnot, or 

 under the condition of indefinite expansion. Practically we 

 have to deal with a machine which, like all other machines, is 

 subject to friction, and in expanding the steam we quickly arrive 

 at a point at which the reduced pressure on the piston is 

 so little in excess of the friction of the machine as to render the 

 steam not worth retaining, and at this point we reject it. In 

 figurative language we take the cream off the bowl and throw 

 away the milk. We do save a little by heating the feed water, 

 hut this gain is very small in comparison with the whole loss. 

 What ha°ipens in the condenser is, that all the remaining energy 

 which has taken the form of internal work is reconverted into 

 heat, but it is heat of so bw a grade that we cannot apply it to 

 the vaporisation of water. But although the heat is too low to 

 vaporise water it is not too bw to vaporise Ether. If instead of 

 condensing by the external application of water we did so by 

 the similar application of ether, as proposed and practised by 



