454 



NA TURE 



{April %, 1875 



tage of the photographic method ; there is no chance of 

 error or for-etfulness. The observations sent home to 

 the Royal Society will enable those on whom the labour 

 and resprnsibility of reducins; them will fall to almost 

 reconstruct the eclippe for themselves. 



" We may remark in conclusion that not only may we 

 hope for many important results in solar physics if the 

 weather be favourable, but that the benefit to science 

 arisint' out of the expedition will b-- by no means limited 

 to the'eclipse results. Already Drs. Vogel and Schuster, the 

 Utter of whom is a distinguished pupil of Owens College, 

 have done some important work on the varying intensities 

 of the different parts of the solar spectrum at different times 

 of the day, and in different climates on the voyage out, but 

 both will' remain some months in India to pursue their 

 inquiries— Dr. Vogel in photographing the solar spectrum, 

 with variously coloured photographic films ; Dr. Schuster 

 in establishing himself at a considerable height for the 

 purpose of photooraphing the various s )lar phenomena 

 and the spectra of some of the most important of the 

 southern stars. The observers, all of whom have made 

 considerable sacrifices in travelling a quarter round the 

 globe and bnck again in the pursuit of science, certainly 

 command our sympathy and deserve success. The 

 Government grant of 1,000/. has been the means of calling 

 forth, and, we hop; sincerely, rendering fruitful, a vast 

 amount of individual effort which would have been power- 

 less without it. We may add that all the instruments have 

 either been purchased by the Royal Society out of its own 

 funds or lent by private individuals." 



ON THE DISSIPATION OF ENERGY* 



''pHE second law of thermodynamics, and the theory of 

 J- dissipation founded upon it, has been for some 

 years a favourite subject with mathematical physicists, but 

 has not hitherto received full recognition from engineers 

 and chemists, nor from the scientific public. And yet the 

 question under what circumstances it is possible to obtain 

 work from heat is of the first importance. Merely to 

 know that when work is done by means of heat, a so- 

 called equivalent of heat disappears, is a very small part 

 of what it concerns us to recognise. 



A heat-engine is an apparatus capable of doing work 

 by means of heat supplied to it at a high temperature and 

 abstract'-d at a lower, and thermodynamics shows that 

 the fraction of the heat supplied capable of conversion 

 into work depends on the limits of temperature between 

 which the machine operates. A non-condensing steam- 

 engine is not, propeily speaking, a heat-engine at all, 

 inasmuch as it requires to be supplied with water as well 

 as heat, but it may be treated correctly as a heat-engine 

 giving up heat at 212° Fahr. This is the lower point of 

 temperature. The higher is that at which the water boils 

 in the boiler, perhaps 360° Fahr. The range of tempera- 

 ture available in a non-condensing steam-engine is there- 

 fore small at bes!', and the importance of working at a 

 high pressure is very apparent. In a condensing engine 

 the heat may be delivered up at 8o°Fahr. 



It is a radical defect in the steam-engine that the range 

 of temperature between the furnace and the boiler is not 

 utilised, and it is impossible to raise the temperature in 

 the boiler to any great extent, in consequence of the 

 tremendous pressure that would then be developed. There 

 seems no escape from this difficulty but in the use of some 

 other fluid, such as a hydrocarbon oil, of much higher 

 boiling point. The engine would then consist of two 

 parts — an oil-engine taking in heat at a high temperature, 

 and doing work by means of the fall of heat down to the 

 point at which a steam-engine becomes available ; and 



* A lecture given at the Royal Institution on Friday, March s, 1875, by 

 Lord Rayleigh, M. A., F.R.S., M.R.I. 



secondly, a steam-engine receiving the heat given out by 

 the oil-engine and working down to the ordinary atmo- 

 spheric temperature. 



Heat-engines may be worked backwards, so as by 

 means of work to raise heat from a colder to a hotter 

 body. This is the principle of the air or ether freezing 

 machines now coming into extensive use. In this appli- 

 cation a small quantity of work goes a long way, as the 

 range of temperature through which the heat has to be 

 raised is but small. 



If the work required for the freezing machine is obtained 

 from a steam-engine, the final result of the operation is 

 that a fall of heat in the prime mover is made to produce 

 a rise of heat in the freezing machine, and the question 

 arises whether this operation may be effected without the 

 intervention of mechanical work. The problem here pro- 

 posed is solved in Carry's freezing app.avatus, described in 

 most of the text-books on heat. There are two communi- 

 cating vessels, A and B, which are used alternately as 

 boiler and condenser. In the first part of the operation 

 aqueous ammonia is heated in A, until the gas is driven 

 off and condensed under considerable pressure in B, , 

 which is kept cool with water. Here we have a fall of 

 heat, the absorption taking place at the high temperature 

 and the omission at tlie lower. In the second part of the 

 operation A is kept cool, and the water in it soon recovers 

 its power of absorbing the ammonia gas, which rapidly 

 distils over. The object to be cooled is placed in contact 

 with B, and heat passes from the colder to the hotter body. 

 Finally, the apparatus is restored to its original condition, 

 and therefore satisfies the definition of a heat-engine. PiT. 

 Carrd has invented a continuously working machine on 

 this principle, which is said to be very efficient. 



Other freezing arrangements depending on solution or 

 chemical action may be brought under the same principle, 

 if the cycle of operations be made complete. 



When heat passes from a hotter to a colder body with- 

 out producing work, or some equivalent effect such as 

 raising other heat from a colder to a hotter body, energy 

 is said to be dissipated, and an opportunity of doing work 

 has been lost never to return. If on the other hand the 

 fall of heat is fully utilised, there is no dissipation, as the 

 original condition of things might be restored at pleasure ; 

 but in practice the full amount of work can never be 

 obtained, in consequence of friction and the other imper- 

 fections of our machines. 



The prevention of unnecessary dissipation is the guide 

 to economy of fuel in industrial operations. Of this a 

 good example is afforded by the regenerating furnaces of 

 Mr. Siemens, in which the burnt gases are passed through 

 a passage stacked with fire-bricks, and are not allowed to 

 escape until their temperature is reduced to a very mode- 

 rate point. After a time the products of combustion are 

 passed into another passage, and the unburnt gaseous 

 fuel and air are introduced through that which has pre- 

 viously been heated. The efficiency of the arrangement 

 depends in great degree on the fact that the cold fuel is 

 brought first into contact with the colder parts of the Hue, 

 and does not take heat from the hotter parts until it has 

 itself become hot. In this way the fall of heat is never 

 great, and there is comparatively little dissipation. 



The principal difficulty in economy of fuel arises from 

 the fact that the whole fall of heat from the temperature 

 of the furnace is seldom available for one purpose. Thus 

 in the iron smelting furnaces heat below the temperature 

 of melting iron is absolutely useless. But when the spent 

 gases are used for raising steam, the same heat is used 

 over again at another part of its fall. There is no reason 

 why this process should not be carried further. All the 

 heat discharged from non-condensing steam-engines, 

 which is more than nine-tenths of the whole, might be 

 used for warming or drying, or other opei'ations in which 

 only low temperature heat is necessary. 



The chemical bearings of the theory of dissipation are 



