486 



NATURE 



[March 24, 1892 



about twenty-five years ago, being a very fine collection of many 

 thousand ornithological specimens, with a quantity of interest- 

 ing correspondence with Mr. J. Thompson, of Knowsley 

 Aviary, Mr. Reid, of Doncaster, R. Dunn, of Hull, and 

 many other naturalists of that period ; these explain upon what 

 terms he obtained the egg and a very fine specimen of the Great 

 Auk. Albert F. Calvert. 



63, PatshuU Road, Kentish Town, March 19. 



Superheated Steam. 



Lord Rayleigh (p. 438) rebuts my objection to the state- 

 ment regarding the efficiency of a vapour-engine in which pure 

 water is replaced by a saline solution, pointing out " that 

 Maxwell's exposition of Carnot's engine applies wtthout the 

 change of a single word, whether the substance in the cylinder 

 be water, mercury, or an aqueous solution of chloride of calcium." 

 The latter italics are mine. In the statement objected to by me 

 the aqueous solution of chloride of calcium was in the boiler, 

 and what was in the cylinder was superheated steam, which is 

 not included in the above list, so that the application of Max- 

 well's exposition is somewhat difficult. The greater part of the 

 fresh water supplied to passengers in steamships is now produced 

 by condensing the superheated vapour of a saline solution, and 

 the culinary experience is that the substance which was in solu- 

 tion has all been left in the boiler. My contention, therefore, 

 still stands— the saline mixture is not the working substance, 

 and Carnot's law refers to the working substance only, and not 

 to anything left in the boiler. 



" In each case there is a definite relation between pressure 

 and temperature." This is evidently merely a slip of the pen, 

 the writer having for the moment forgotten that he was dealing 

 with superheated steam, for which there is not a definite relation 

 between pressure and temperature. The condition of super- 

 heated steam is completely defined when both pressure and 

 temperature are given ; but pressure is here a function of tem- 

 perature-and-something-else, and temperature is here a 

 function of pressure-and-something-else. That something- 

 else may be volume or it may be energy, or, preferably, it 

 may be entropy, but it must be something which cannot be 

 predicated from pressure alone or from temperature alone. 



" (So far as the substance is concerned), all that is necessary 

 for the reversible operation of the engine is that the various 

 parts of the working substance should be in equilibrium with 

 one another throughout." No; for, in addition, it is necessary 

 that the working substance should have only one pressure con- 

 sistent with any given temperature. For this reason, super- 

 heated steam, however it may have been produced, can never 

 be the working substance in a Carnot's engine. In the reversed 

 cycle, when the steam is raised from a saline solution, from the 

 beginning of the higher isothermal, the pressure would go on in- 

 creasing until it became that due to saturated steam at the tem- 

 perature of the superheat. This might be double the maximum 

 pressure in the original cycle. 



"The various parts of the working substance should be in 

 equilibrium with one another throughout." The writer seems 

 to say that the steam of a saline solution is a stable saturated 

 vapour. It is HgO at a given pressure and temperature, and 

 the condition of the substance is by this definition completely 

 determined, and there is no alternative ; but it is not stable. 

 Say that the steam-space of the boiler is increased by adding a 

 vertical cylinder alongside the boiler, open to it. On the 

 bottom of that vessel the steam might condense — pure water — 

 and the temperature of the steam immediately over this water 

 would be that of saturated vapour at the same temperature, 

 and from there all through the steam-space to the surface of the 

 saline solution in the boiler the temperature would increase, and 

 all would have the same pressure. There would be mechanical 

 equilibrium, but not thermal equilibrium. 



"At the upper limit, all the heat is received at the highest 

 point of temperature," — but just as it would be if the evaporation 

 were from a film of water upon a nearly bare combustion 

 chamber crown. The plate is left in the boiler, and so is the 

 salt, and in neither case would the steam exhibit a "state of 

 things strongly contrasted with that which obtains when vapour 

 rising from pure water is afterwards superheated." I have 

 stated in my previous letter that the heat of evaporation is all 

 received ai identically the same temperatures as when it is raised 

 from pure water at the same pressure, and the contrast is only 

 as strong as that between occult and obvious. I have now 



NO. I169, VOL. 45] 



shown that the vapours must be identical from either beginning ; 

 and unless each carried a certificate of birth, I do not now see 

 how it would be possible to tell one from the other. 

 London, March 12. J. Macfarlane Gray. 



Phoronomy. 



I THINK it will be admitted by all, that precision of 

 language is of great importance in scientific terminology ; and 

 the letter of Dr. Besant, which appeared in your issue of last 

 week (p. 462), certainly suggests strong reasons for employing 

 the word pfu.ronomy in the place of kinematics. 



The word may at first sight appear strange to the present 

 generation of mathematicians ; but if it becomes acclimatized, 

 its employment will appear as natural as the phrases kinetic and 

 potential energy, in the place of such meaningless phrases as 

 vis-viva And force-function. 



When the medical profession require a new word, they almost 

 always have recourse to the Greek language ; and mathematicians 

 and physicists would do well to follow their example, and in 

 cases of doubt or difficulty to consult some eminent classical 

 scholar. I must confess, that I have no sympathy with the 

 attempts, which have occasionally been made, to introduce short 

 words of Teutonic origin into scientific nomenclature, as such 

 words have always appeared to me to be singularly deficient in 

 point. A good example of this is furnished by the word spin, 

 which Clifford attempted to introduce in the place of the phrase 

 molecular rotation. The latter phrase, although a little long, 

 exactly expresses the idea which it is intended to convey, viz. 

 that the molecules of the fluid possess a motion of rotation as 

 well as a motion of translation. The word spin, on the other 

 hand, does not express any such idea, but is strongly suggestive 

 of the juvenile, though not altogether unscientific, pastime of 

 spinning peg-tops. A. B. Basset. 



322 Oxford Street, W., March 18. 



I HAVE before me the second edition of F. Redtenbacher's 

 " Principien der Mechanik und des Maschinenbaues" (Mann- 

 heim, 1859), of which the first section is entitled "Die Bewe- 

 gung als Erscheinung(Phoronomie)." Whether the term occurs 

 already in the first edition (1852), I cannot affirm, but I remem- 

 ber very well that Redtenbacher, in his lectures in Carlsruhe, 

 in 1858, insisted upon that term being distinct from "Dynamik" 

 and "Kinematik." I conclude, therefore, that the majority of 

 the 786 students of that year — among them many foreigners — 

 as also those of other years, were conversant with the term. 



M. AM Ende. 



Westminster Chambers, 5 Victoria Street, 

 London, S.W., March 19. 



Some other correspondent is pretty sure to be mentioning that 

 Mr. VV. H. Besant will find in Kant's Metaph. Anfangsgriinde 

 der Naturwissenschaft all the authority he could desire for his 

 proposed use of the word "phoronomy." Kant regularly uses 

 the word in the sense of the later "kinematic" ; and he was 

 man of science enough to justify anyone in following his lead. 



G. C. R. 



The Tudor Specimen of Eozoon. 



In reference to the remarks made by Sir J. W. Dawson 

 (Nature, March 17, p. 461) on my paper on the Tudor 

 specimen of Eozoon {Quart. Journ. Geol. Soc, vol. xlvii. pp. 

 348-55), I should like to say that the whole point of that paper 

 was that it was based on Sir J. W. Dawson's original type. 

 The figure of this specimen has been repeatedly republished by 

 Sir J. W. Dawson, and, in the absence of illustrations or details 

 of other specimens from Tudor, upon its evidence alone rests 

 the asserted occurrence of Eozoon in the Tudor limestone, and 

 the great claims based thereupon. The value of other speci- 

 mens from this locality was not rated very highly by Sir J. W. 

 Dawson so recently as September 1888, when he remarked, 

 " Without additional specimens,^ and in the case of creatures so 

 variable as the Foraminifera, it would be rash to decide whether 



I And he previously refers only to " the specimen," " this very interesting 

 specimen," "the fine specimen from Tudor," &c. 



