194 



NA TURE 



[December 30, 1897 



this increase of volume for each degree of the F. scale 

 it is inferred that the absolute zero is 460' F. below the 

 ordinary zero. Logically this process is deficient in 

 that it presupposes a known Fahrenheit scale ; and there 

 were just as many perfectly exact Fahrenheit scales as 

 there were previously existing exactly made Fahrenheit 

 thermometers. Our impression is that the law of Charles 

 or Gay-Lussac expressed the fact that the different gases 

 experimented on, all expanded by nearly the same frac- 

 tion of their volume at the temperature of melting ice on 

 being raised under constant pressure from that tempera- 

 ture to the " boiling point " of water. Then this ex- 

 pansion (from Vq to t'loo say) obtained accurately for any 

 one gas enables f C. to be defined for that gas as thermo- 

 metric substance as that temperature for which the 

 volume of the gas under constant pressure is v^^ -t- 

 t (t^xoo - t^oj/ioo, and so also for the Fahrenheit scale. 

 Taking air, we get by this definition the air thermometer 

 scale, which, having its own independent definition, can 

 be used as a standard of comparison for other thermo- 

 metric scales. 



From this we get the absolute zero on this gas 

 thermometer scale as that for which t = — \oov^\{v^(^ - v^j 

 that is for which the volume of the gas is zero. Fulfil- 

 ment of Boyle's law, if it exists, gives agreement of the 

 similarly, but independently, defined constant volume 

 scale for the same gas with the constant pressure scale. 

 Thus we get an independent scale for each gas for 

 which accurate experimental data are available, and the 

 numerical reckoning of the absolute zero of temperature 

 will not be necessarily the same for all. We say this 

 with Mr. Carter's note on p. 107 in view. We believe 

 that a perfect gas is best defined as one which obeys 

 Boyle's law at all pressures and temperatures, and gives 

 a constant pressure scale agreeing with the only properly 

 absolute scale of temperature, that of Lord Kelvin, or, 

 which comes to the same thing, as a gas which, besides 

 fulfilling Boyle's law, has an invariable ratio of specific 

 heats. 



The second law of thermodynamics is hardly that given 

 on p. 106, which is really the so-called "axiom " on which 

 Lord Kelvin based the second law. The law itself is 



most shortly expressed by the equation / dqjt—o, where / 



is absolute temperature, dq a quantity of heat taken in 

 (or given out) at temperature /, and the integral is taken 

 round a reversible cycle. If /q be the lowest available 



temperature, the positive value which -t^idglt has for 



every non-reversible cycle is Lord Kelvin's expression 

 for the heat dissipated in the cycle. 



We are rather disappointed that no treatment, e.g. the 

 beautiful graphical treatment given by Maxwell, is in- 

 cluded of the thermodynamics of change of state. By 

 means of the Protean fundamental parallelogram of the 

 Carnot cycle, the whole of the essential part of thermo- 

 dynamics can be given in two or three pages. A statement 

 from this point of view of the information which thermo- 

 dynamic theory gives as to the density of saturated 

 steam would not have been inappropriate here. 



The account of furnaces and boilers and of steam 

 engines generally seems very full and complete, but a 

 full review of it is a thing to be undertaken only by an 

 NO. 1470. VOL. 57] 



expert in these matters. No one, however, who takes an 

 interest in engineering methods and results, and nothing 

 can be more valuable for the physicist than their study, 

 can fail to be struck with the fulness of the information 

 given by Mr. Carter. 



In Part iii. gas and oil engines are discussed, and we 

 have, after a chapter on gaseous and liquid fuel, the Otto 

 cycle, and engines working on this cycle are fully de- 

 scribed. This, to us, is perhaps the most interesting part 

 of the book. Prime-movers of this description have 

 advanced immensely during the last ten or twelve years, 

 and the lot of those who have to use them is cast in 

 much pleasanter places. We have a lively recollection 

 of having to toil with five or six enthusiastic laboratory 

 students turning the fly-wheel for nearly half an hour at j 

 a time trying to get a large gas engine started. This 

 and other difficulties, and the continual setting to rights 

 which the engine required, would have tried the patience 

 of a saint, and certainly were too much for ordinary 

 mortals. 



In Part iv. water motors and turbin es generally are 

 discussed, in Part v. gearing is dealt with, and in 

 Part vi. we have a most valuable account of types of 

 power stations. , ■ 



The book, it ought to be mentioned, is one of severalp I 

 excellent practical works that have appeared by instal- 

 ments in the Electrician. It is very thoroughly illus-. 

 trated with excellent drawings : engines of all kinds, 

 central power stations, details of machinery, such as 

 valve gear, governors, injectors, &c. Some of these 

 might perhaps have been worked up better, but most 

 are good, and all are thoroughly business-like and 

 intelligible. We regret that we have not space to deal 

 with the work more adequately ; but as it is, in fact, a col- 

 lection of some three or four separate treatises of great 

 value, any attempt to fully review it here is impossible. 



We have nothing but congratulation for the author on 

 his work. Electrical engineers are deeply in his debt. 



A. Gray. 



A PIONEER OF MEDICINE. 

 Masters of Medicine. John Hunter, Man of Science a7id 

 Surgeon. By Stephen Paget, with an introduction by 

 Sir James Paget. 8vo. Pp. 272, with a frontispiece. 

 (London : T. Fisher Unwin, 1897.) 



MR. FISHER UNWIN has undertaken to publish 

 a series of volumes dealing with the life and 

 works of the great scientific men who have brought 

 medicine and the allied sciences to their present state of 

 perfection. The object of the series, which is under the 

 general editorship of Mr, Ernest Hart, editor of the 

 British Medical Journal, is to set before unprofessional 

 readers a plain account of the lives and fortunes of the 

 " Masters of Medicine," with such a survey of their work 

 as may be necessary to show wherein they excelled. 



John Hunter lends himself particularly to this method 

 of treatment. Living only a hundred years ago, the 

 founder of a great school of thought, the hero of a 

 hundred orations by the most eminent surgical minds of 

 the century, there exists plenty of material from which to 

 construct a most readable biography. His life, indeed, 

 has often been written but not always judiciously, for it 



