December 7, 1905] 



NA TURE 



1 2V5 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



The Second Law of Thermodynamics. 



The point which Mr. Browne (p. 102) raises is covered 

 by \ oigt (" Thermodynamic, " vol. ii., § 69, pp. 209 

 et seq.). Ordinary diffusion of two gases at equal pressure 

 and temperature is an irreversible process involving loss 

 of available energy, but when the diffusion takes place 

 through porous membranes this available energy can be 

 utilised in a greater or less degree in the form of work, 

 and this is the case in Mr. Browne's experiment. By 

 introducing the conception of " semi-permeable partitions," 

 Voigt obtains a reversible method of mixing or separating 

 gases. In this case the partial pressure of the mixture is 

 equal to the sum of the partial pressures of the components. 

 In ordinary diffusion the volume of the mixture is equal 

 to the sum of the volumes of the components. The work 

 of expansion from the former to the latter final state can 

 be utilised if a reversible transformation is employed. It 

 is lost in the case of ordinary diffusion. An equal amount 

 of work must, however, be supplied from without to 

 separate the gases. The results are fully in accordance 

 with the second law. G. H. Bryan. 



Mr. M. A. Browne's letter (p. 102) raises an interesting 

 and difficult Question which at some period of his career 

 must be faced by every student attempting to grasp for 

 himself the significance of the second law of thermo- 

 dynamics. As I, with difficulty and without much help 

 from the text-books, extricated myself from a similar 

 dilemma, perhaps the steps in the train of reasoning which 

 helped me may interest others. 



There is no need to take the complicated case chosen by 

 Mr. Browne of the diffusion of hydrogen and nitrogen 

 through a palladium septum. A precisely analogous 

 difficulty exists in the simpler case of an ordinary cylinder 

 of compressed gas doing external work on expansion either 

 at the expense of its own heat or of the heat of uniform 

 temperature of its environment. The gas expands and does 

 an amount of external work W, while the equivalent H 

 units of heat flow into the gas from the surroundings, so 

 that the result of the process is that H units of heat at 

 the uniform temperature of the surroundings have been 

 quantitatively converted into external work. This is, no 

 doubt, contrary to many of the earlier statements of the 

 second law. 



The test, of course, is to compress back the gas into 

 tlie cylinder, when at least W units of work have to be 

 converted back into heat during the process. Moreover, 

 this must be done whether or not the gas did work on 

 expansion. Although a gas expands freely into a vacuum, 

 doing no work, and, as Joule has shown, experiencing no 

 appreciable change of total energy in the process, to get it 

 back again into the cylinder, at least W units of work must 

 be converted into heat. I know of no better way of intro- 

 ducing thermodynamical considerations to the chemical 

 student than by commencing with the concrete case of a 

 gas cylinder. The extension of the same considerations to 

 all processes naturally occurring, the flow of heat from a 

 hot to a cold body, the diffusion of gases through septa, 

 the change of one allotropic form of element into another, 

 all follow as illustrations of the " majestic " and 

 " universal " law that that mysterious something which is 

 not energy, but an abstraction of energy — its availability for 

 work — tends always to a minimum, or, as others have it, 

 the entropy increases. The student passes his examin- 

 ation, no doubt, but if he is a philosopher he may prefer 

 to meet his difficulties singly, and not have theni " con- 

 centrated in a phrase." It is possible that he mav like to 

 think sometimes of a gas as expanding, because it is its 

 nature to. The reason is easily understood on mechanical 

 or kinetic considerations. But the attempt to replace these 

 NO. 1884, VOL. J$~] 



considerations by the two ex cathedra statements — (1) the 

 entropy of a gas increases during increase of volume, 

 (2) the entropy of the universe tends to increase — and to 

 deduce from them the direction of natural tendency in the 

 case of a gas changing in volume, seems to the writer to 

 involve the thermodynamical equivalent of the fallacy of 

 " putting the cart before the horse." We cannot escape 

 mechanical and molecular considerations. 



The University, Glasgow. F. Soddy. 



Atomic Disintegration and the Distribution of the 

 Elements. 



With reference to the association of uranium and 

 radium, would you permit me to put on record a point that 

 must have occurred to many, though possibly not to some, 

 of those who are speculating so brilliantly about uranium 

 and its disintegration products. I refer to the extra- 

 ordinary conjunction in nature between silver and lead. 

 This conjunction is so frequent that it can hardly be casual. 

 A lead mine is a silver mine and a silver mine a leadl 

 mine all the world over, and yet the chemical attraction 

 between silver and lead is slight, and the two metals are 

 not sufficiently common to concur by chance. It is to be 

 noted also that the concurrence, if the word may be used 

 in this sense, is usually of the order of ounces for silver 

 and tons for lead, and that the atomic weight of lead is 207 

 and of silver 108. Hence there appears to be some ground 

 for the suspicion that silver is a disintegration product of 

 lead. • Lead also happens to present special facilities for 

 experiment to test this surmise. It is cheap, and it is a 

 comparatively inexpensive matter to free ten tons of lead 

 from all traces of silver by the usual crystallising process, 

 and then put it aside for ten years and test again for 

 silver by the same process. 



There are several other curious groupings of elements in 

 nature that seem to be worthy of consideration from the 

 transmutation point of view. One of these is the frequent 

 concurrence of copper and gold. In the Great Cobar 

 copper mine in New South Wales the gold occurs in the 

 ratio of about four ounces to the ton of copper. Such con- 

 junctions as gold and quartz are, of course, easily ex- 

 plained by chemistry and coincidence, and chemical forces 

 also sufficiently explain the concurrence of sulphur with 

 silver and lead, but the giant deposits of silver, lead, and 

 zinc, with smaller quantities of copper and still smaller 

 quantities of gold at Broken Hill, in Australia, to sav 

 nothing of similar vast deposits in many other countries, 

 can hardly be due entirely to chemical and casual forces. 

 Anyone interested in the subject will find much statistical 

 and other information in the annual report of the Broken 

 Hill Proprietary Company. This document affords con- 

 siderable food for reflection, and a visit to the mine itself 

 is absolutely awe-inspiring. Walking through galleries of 

 glittering grey crystals of silver, lead, and zinc sulphide — 

 solid ore — for 300 feet across the lode, which is a mile or 

 more in length and of unknown depth, is one of the experi- 

 ences of a lifetime. Donald Murray. 



3 Lombard Court, London, E.C., November 30. 



Zoology at the British Association. 



In your account, under the above heading, of the pro- 

 ceedings of Section D of the British Association at 

 Johannesburg, you state (p. 40) that in my paper on 

 Cephalodiscus I " gave a preliminary account of the new 

 species discovered in African seas by Dr. Gilchrist." 



I shall be obliged if you will allow me to say that my 

 communication to Section D consisted of an abstract of 

 the results which were published, last July, in mv report 

 on "The Pterobranchia of the Siboga Expedition," and 

 that it did not include any account of Dr. Gilchrist's 

 specimens of Cephalodiscus. S. F. Harmer. 



King's College, Cambridge, November 14. 



[Our contributor was unfortunately engaged in the 

 committee room during Dr. Harmer's introductory remarks, 

 and this led to the misunderstanding to which Dr. Harmer 

 directs attention. — Editor.] 



