414 



NATURE 



[March 3, 1S92 



and with superheating, 



/A-B 14^ _ 

 Va + b a 



•7+|^)(A-B, + l--A.|-;-^ 



1437 + 



B 



Numerical example. Say that A = 800°, B =: 600°, S = 1000°. 

 Substituting these values, we get — 



E = •2301 without superheating, 

 Ej = '2389 with superheating. 

 That is, less than 4 per cent, is gained by superheating 200°. 



So far, I supiDort Lord Rayleigh's view, or, rather, he says 

 what I have been impressing upon engineers for the last twenty 

 years. If this had been all I had to say, I would not have 

 written now ; but Lord Rayleigh adds to his statement what is 

 to me an astounding announcement, that, "by the addition of 

 saline matters, such as chloride of calcium or acetate of soda, 

 ... the possible efficiency, according to Carnot, may be in- 

 creased." I hasten to call this assertion into question, because 

 there are so many people ready to bring engines on new prin- 

 ciples into the field of joint-stock bubbles ; and I am afraid we 

 may be having, quite apart from Lord Rayleigh, a new field 

 engine syndicated and floated on the strength of this communica- 

 tion and the signature thereto, before its meaning is understood. 

 As I understand thermodynamics there would be no gain from 

 superheating by a saline solution, over the usual method of 

 superheating steam raised from pure water. The saline mixture 

 is not the working substance. Carnot's law refers to the work- 

 ing substance only, and not to anything left in the boiler. The 

 first step in evaporation from the saline mixture is to separate a 

 particle of water from the salt. In the act of separation, the 

 temperature of the water particle falls to the temperature due 

 to the pressure, and at that temperature it is evaporated into 

 steam particles, which immediately become of the same tem- 

 perature as the saline mixture. These steps are followed by 

 every particle of water, each independently of every other par- 

 ticle. Of course, we cannot practically test those temperatures, 

 as the complete series is run through for each particle in a frac- 

 tion of the twinkling of an eye, and immersed in a liquid of 

 greatly higher temperature. A thetaphi diagram for this would 

 give, at B A, and extending upwards to temperature S, a very 

 narrow figure 8, whose loops are equal, and drawn, as in a 

 figure 8, one right-hand and the other left-hand. The line for 

 the reception ol latent heat would be identically the same line, 

 the horizontal through A, as when the evaporation was from 

 pure water. It is evident, therefore, that, according to my 

 lights, the efficiency will be precisely the same as without the 

 salt in solution. 



Some ten years ago this plan was submitted to me for my 

 opinion by an eminent mechanical engineer, Mr. S. Geoghegan, 

 who, I understood, had then patented it. The above is the 

 substance of the opinion I then expressed, and nothing I have 

 learned since induces me to change my view of it now. j 



The "complete elaboration of this method," hinted at in the 

 last paragraph of Lord Rayleigh's communication, is not clear [ 

 to my mind ; and it is just possible that a few sentences of 

 explanation would show me that I have been hitting away at 

 something that was not intended by the writer. If so, my 

 excuse must be that I have read the statement, as every prac- 

 tical engineer would, to m.ean that the latent heat is imparted } 

 along the isothermal of the superheat. When I get to under- 

 stand the first sentence of the last paragraph of the communica- I 

 tion, I may be able to confirm the anticipation of higher 

 economy. J. Macfarlane Gray. 



The passage quoted by Lord Rayleigh from my book on the 

 steam-engine, in some remarks on this subject in your number 

 of the l8th inst. (p. 375), is taken from one of the earlier 

 chapters, which is devoted to engines which receive and reject 

 heat at constant temperature. When such an engine is used as 

 a standard of perfection, by comparison with which some other 

 engine is tried, it appears to me that the maximum and mini- 

 mum temperatures of the working fluid must in the first instance be 

 adopted as the temperatures of reception and rejection of heat ; 

 and in fact, without entering on questions reserved for discussion 

 in a later chapter, no lower value than the maximum could well 

 have been adopted. There is no doubt that the practice of com- 

 NO. I 166, VOL. 45] 



paring together engines with diffeient cycles has been a source 

 of considerable misapprehension, and very probably the language 

 used in the passage in question may be insufficiently guarded. 

 The use of superheated steam on this method of comparison is- 

 not a gain, but a considerable loss, for the heat might ideally all 

 have been used at the maximum temperature, and is so used in 

 the standard of comparison. 



The practical case in which the boiler pressure is given is, of 

 course, quite different. There is a gain by superheating, but, 

 putting aside cylinder condensation, the gain is small, because 

 such a small percentage of the heat is employed at temperatures 

 above that of the boiler. 



The process was originally introduced with the object of 



drying the steam and diminishing cylinder condensation ; and 



{ now that the practical difficulties attending its use have been in 



I great measure removed (as I am informed), by the employment 



; of mineral oil for lubricating purposes, it may be hoped that it 



maybe revived, and be the means of a considerable economy. 



The action of ."superheated steam in a cylinder was explained 

 and its economy experimentally demonstrated by Hirn some 

 fifteen or twenty years ago. I have given the explanation briefly 

 on p. 352 of my book, but I purposely avoided discussing ques- 

 tions relating to it, being of opinion that, in the present state of 

 our knowledge, theoretical investigations are of doubtful value. 

 I am certainly, however, under the impression that the true 

 nature of the economy obtained by its use has for a long period 

 been very generally recognized, though some writers in dealing 

 with the theory of heat engines may have expressed themselves 

 incautiously. It would, I think, be very desirable, in teaching 

 the subject, to introduce as early as possible the idea of a mean 

 temperature of supply. I have dwelt on the importance of this 

 conception in the latter part of my book, and I am sure its 

 introduction would remove many difficulties. 



Greenwich, February 24. James H. Cotterill. 



Lord Rayleigh's interesting communication on superheated 

 steam in your last issue (p. 375) leads me to ask whether it is 

 generally known that solutions can be heated up to temperatures 

 higher than 100° by passing into them steam at 100°. 

 The late Peter Spence at the Exeter meeting of the 

 British Association in 1869 called attention to the fact 

 that by simply passing steam at 100° directly into a strong 

 solution of nitrate of soda (other salts will of course answer) 

 it was possible to raise the liquor to its boiling-point, about 121°. 



Superheated steam is frequently used for heating up liquors in 

 chemical processes on the large scale, but where a slight dilu- 

 tion is no disadvantage, the simpler operation of heating with 

 ordinary low pressure steam might be adopted more generally 

 than it is. Sj)ence used steam in this way for the purpose of 

 extracting sulphate of alumina from alum shales. 



G. H. Bailey. 



The Owens College, Manchester, February 22. 



Poincare's "Thermodynamics." 



Permeitez-moi de repondre en quelques mots a I'article que 

 M, Tait a consacre a ma thermodynamique, non que je veuille 

 prendre la defense de mon imprimeur, ou refuter des reproches 

 generaux, contre lesquels ma preface proteste suffisamment. 



J'abuserais ainsi de votre hospitalite et de la patience de vos 

 lecteurs ; je me bornerai done a discuter une seule des critiques 

 de M. Tait, et je choisirai celle que ce savant parait regarder 

 comme la plus importante et qu'il a formulee avec le plus de 

 precision. Je commence par en reproduire le texte : — 



" Even the elaborate thermo-electric experiments of Sir W. 

 Thomson, Magnus, &c., are altogether ignored. What else can 

 we gather from pas'^ages like the following? — 



" ' . . . Si I'effet Thomson a pu etre mis en evidence par 

 I'experience, on n'a pu jusqu'ici constater I'existence des forces 

 electromotrices qui lui donnent naissance. . . . '" 



Rappelons d'abord que, dans I'etude des phenomenes elec- 

 triques et thermiques qui se produisent au contact de deux 

 metaux, il faut soigneusement distinguer trois choses : — 



(i) Le phenomene calorifique connu sous le nom d'effet 

 Peltier. Dans le cas d'un metal unique mais inegalement 

 chauffe, le phenomene correspondant s'appelle effet Thomson 

 et se manifeste par un transport de chaleur. 



(2) La difference de potentiel vraie ou force electromotrice de 

 contact. 



(3) La force electromotrice apparente ou difference de poten- 

 tiel entre les couches d'airvoisinesde la surface de deux metaux. 



