120 



NATURE 



[Dec. 14, 1871 



" Gay Lussac has proved by experiment that when 

 an elastic fluid streams from one receiver into a 

 second exhausted one of equal size, the first vessel is 

 cooled, and the second one heated, by exactly the same 

 number of degrees. This experiment, which is distin- 

 guished for its simplicity, and which, to other observers, 

 has always yielded the same result, shows that a given 

 weight and volume of an elastic fluid may expand to 

 double, quadruple, in short, to several times its volume 

 without experiencing, on the whole, any change of tem- 

 perature ; or, in other words, that for the expansion of 

 the gas of itself {'Z/i iind fiir sick), no expenditure of 

 heat is necessary. But it is equally proved that a gas 

 which expands under pressure suffers a diminution of 

 temperature. 



" Let a cubic inch of air at i", and under the pressure 

 of 30 inches of mercury, be warmed by the quantity of 

 heat .r to 274'' C, its volume being kept constant ; this 

 air, on being permitted to stream into a second exhausted 

 vessel of the same size, will retain the temperature of 

 274°, and a medium surrounding the vessel will suffer no 

 change of temperature. In another experiment, let our 

 cubic inch of air be kept, not at constant volufnc, but 

 under the constant pressure of the 30-inch mercurial 

 column, and heated to 274°. In this case a greater 

 quantity of heat is required ; let it be .i- -f y. 



"In comparing these two processes, we see that in both 

 of them the air is heated from 0° to 274°, and at the same 

 time permitted to expand from one volume to two 

 volumes. In the first case the quantity of heat necessary 

 was = .r, in the second case = x-\- }'■ In the first case the 

 mechanical effect was = o, in the second case it was equal 

 to 1 5 lbs. raised one inch in height." 



He then proceeds with his calculation. 



Here it will be seen that Mayer was quite awake to the 

 importance of the considerations dwelt upon by Prof. Stokes 

 — that he knowingly chose for his determination a sub- 

 stance which, ati und fiir sich, in expanding, consumes no 

 heat. Hence, when by its expansion HL^aiiis/ pressiire\\^n.\. 

 is consumed, no part of that heat is lost in producing " a 

 change of state in the matter operated upon." The heat con- 

 sumed is, therefore, the pure equivalent of the work done. 



With regard to Dr. Joule, I have, to my regret, vainly 

 endeavoured to find a mislaid document written a year ago, 

 in which I ventured to describe his labours,* and to express 

 the esteem I entertain for them. Supposing him to have de- 

 rived his inspiration from Mayer's papers, that they had even 

 caused him to prosecute his experiments on the mechanical 

 equivalent of heat, he would still have rendered immortal 

 service to science, and more than merited the honours 

 bestowed upon him last year. For, wanting his work, the 

 mechanical theory, however strong the presumption?, and 

 however concurrent the evidence in its favour, could not 

 be regarded as completely demonstrated. But Joul.; was 

 not stimulated by Mayer. His work is his own, being 

 practically contemporaneous with that of Mayer. He not 

 only demonstrated experimentally the mechanical theory 

 of heat, but in its completer form he was an independent 

 creator of that theory. And so impressed was the Council 

 of the Royal Society last year with the magnitude of his 



• Thanks to the friendly eflforts of Dr. Sharpey, this document re.iched my 

 hands just as the proof of this paper \v.as being returned for pre i. With 

 the permission of the Editor of N.'iTUEE I will publish the docui;icnt, with 

 some additional matter, next week. J. T. 



merits, that they actually added to the Rumford Medal 

 already bestowed upon him, the final distinction of the 

 Copley Medal. If England rated him as highly as I do, 

 his reward would not be confined to mere scientific recog- 

 nition. 



As regards the latter, however, I do not think that the 

 possibility suggested by Prof. Stokes represents any real 

 danger. I do not imagine that the eyes of Science are in 

 the least degree likely to be " shut to the merits of our 

 own countryman." And I believe that the Royal Society, 

 by stamping in two consecutive years these two men with 

 the highest mark of its approval, will have strengthened 

 that confidence in its impartiality which, throughout the 

 whole scientific world, it has so long and so justly enjoyed. 



John Tyndall 



AIRY ON MAGNETISM 

 A Treatise on Magnetism. By G. B. Airy, Astronomct 

 Royal. (Macmillan and Co.) 



THIS is a book written upon the true scientific prin- 

 ciple expressed by Newton when he said " Hypo- 

 theses non fingo." The elementary laws of magnetism 

 are deduced by rigorous induction from particular cases 

 and are then applied to explain phenomena. The book 

 contains the substance of a series of lectures delivered by 

 the Astronomer Royal at the University of Cambridge. 

 One great element of e.xcellence in the book is that the 

 mathematics employed throughout arc of a simple 

 character, sd that the first principles of magnetism are 

 thus thrown open to one who has gone no great way in 

 mathematical reading. 



Formula; having been obtained in the early sections for 

 the action of one magnet on another, and the law of the 

 inverse square having been established by a comparison 

 of calculation with experiment, the great bulk of the 

 volume is occupied in investigations which bear more 

 directly on terrestrial magnetism and the magnetism of 

 iron ships. The methods of determining the values of 

 the magnetic elements at any place are carefully explained 

 and illustrated, and the necessary formuUc deduced from 

 the theory established in the preceding sections. We 

 would especially recommend to the reader's attention the 

 articles on the theory of the dipping needle. One chapter 

 of extreme interest is devoted to " Theories of Terrestrial 

 Magnetism," and the beautiful theory of Gauss is sketched 

 out. We sincerely hope that that theory which was carried 

 by Gauss to the fourth order of approximation will be 

 before long carried to a higher order. Data now exist 

 for this advance, as it requires accurate determinations of 

 only eleven more elements. 



The subject of the deviation of the compass in iron 

 ships is one upon which the Astronomer Royal is peculiarly 

 justified in speaking or writing. All the sections relating 

 to the disturbance of compass needles are full of most 

 important and suggestive matter. One section is devoted 

 to the continuous registration of small changes in terres- 

 trial magnetism , and the concluding section just touches on 

 the subject of the relation between galvanic currents and 

 magnetic forces, without entering into any calculations. 



The book supplies a distinct want which has hitherto 

 existed in the list of our mathematical text-books, and is 

 a most valuable contribution to the diffusion of physico- 

 mathematical science. James Stuart 



