214 



NATURE 



[December 27, 1894 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society. November 22. — " .\ Delerminalion of the 

 Specific Heat of Water in terms of the International Electric 

 Units." Uy Prof. .Vrthur Schuster, F. R.S., and William 

 Gannon, Exhibition (18511 Scholar, Queen's Colle(;e, Galway. 



This research was originally undertaken by Prof. Schuster 

 and Mr. H. Uadley, before the authors were aware that Mr. 

 E. H. Griffiths was engaged on a similar investigalion. After 

 a number of preliminary experiments, and just as the linal 

 arrangements for the conduct of the mea>uremenls were being 

 definitely made, Mr. Hadley, on his appointment to the head- 

 mastership of the School of Science and Art, Kidderminster, 

 had to leave Manchester. 



On Mr. Iladley's departure, Mr. W. Gannon took his place. 

 From the former gentleman we received a good deal of help in 

 the devising and construction of some important parts of the 

 apparatus. 



The principle of the method we have used is extremely 

 simple. The electrical work done in a conductor being 

 measured by / EG//, where E is the difference of potential at 

 the ends of the conductor, C the current, and t the time. We 

 keep the electromotive force constant, and measure / Qdt 

 directly by a silver vollame'er. We do not, therefore, require 

 10 knew the resistance of the wire, and we thus avoid the 

 difficulty of having to estimate the excess of temperature of the 

 wire over that of the water in which it is placed. We also gain 

 the advantage of not having to measure time, and thertfore to 

 be able to complete the experiments more quickly than we 

 could have safely done if the length of time the clirrent passed 

 had (o be measured with great accuracy. 



Our 6nal value is 



J = 41804 Joules on the mercury scale of hard French glass, 

 4'I905 on the nitrogen scale, 

 4'I9I7 on the hydrogen scale, 



at a temperature of ig"'!. 



In comparing our results with that of other observers, we have 

 in the first place to consider the value which Mr. Griffiths has 

 obtained in his very excellent series of measurements. His final 

 result (Roy. Soc. Proc. vol. Iv. p. 26 ; Phil. Trans, clxxxiv. A 



(1893) is 



J = 4'l982(i -0-00266 fl- 15) y. 10.' 



This refers to the nitrogen thermometer. .\t a temperature of 

 yf'l, the value would be reduced to 41936, which corresponds 

 to our 4' 1905 at the same temperature. Griffiths' value is to he 

 increased slightly, owing to the fact that he really measures the 

 difference between the specific heat of water and of air. This 

 would increase the value of J by "ooi i about, so that the value 

 of J at I9°"l would be raised to 41947 y. 10", which is exactly 

 one part in a thousand larger than ours. The difference is small, 

 but must be due to some systematic error, as bnih Giiffiths' 

 value and our own agree so well with each other, that ordinary 

 observational errois and accidental disturbances could not have 

 produced so large a difference in our results. The least satis- 

 factory part of a calometric measurement must always be the 

 applicali 'n of the cooling correction, and «e have considered 

 it of great importance to reduce that correction as much as 

 possible. The uncertainty of the cooling correction does not 

 necestarily depend on its value ; thus we can much diminish it 

 bjr starting, as we have done in the third series, with the initial 

 temperature of the calorimeter about as much below that of the 

 water jarket as the final temperature is above it; yet the 

 uncertainly of the correction does not seem to us to be 

 dimini-hcd by thai process. We may reasonably estimate the 

 uncertainty due to the cooling correction, by calculating what 

 the error m the observed rate of cooling, either at the beginning 

 or the end of the experiment, must have been in order to pro- 

 duce a difference of one p.irt in a thousand in the final result. 

 We find in our own experiments that the error must have 

 amounted to more than 15 per cent. We consider it unlikely 

 that so large an error occurred always in the same direction. 

 Apart from the co ding correction, however, it is dilficult to see 

 how a difference one-lcnih per cent, in our result can be pro- 

 duced unless by the accumulation of a number of small errors 

 The difference between our value of the equivalent and tha! 

 NO. I313. VOL. 51] 



of Mr. Griffiths are, however, of sm.iller importance than the 

 difference which exists between them and the equivalent as 

 determined directly by Joule, Rowland, and Miculescu. 

 Joule's latest value, which is the only one which needs con- 

 sideration, is 77265 foot-pounds, at 617 F. The number 

 refers to the degree as measured by Joule's mercury thermo- 

 meter. RowKind adds to this a correction to the air thermo- 

 meter of about 3, and another small correction for a change in 

 the heat capacity of the apparatus, which brings the value up 

 to about 776. The correction to the air thermometer has been 

 obtained by means of a comparison made by Joule himself with 

 one of Rowland's thermometers. Joule's original thermometers 

 have been temporarily placed by Mr. B. A. Joule in the hands 

 of Prof. .Schuster, in order that an accurate comparison may be 

 instituted between them and modern thermometers. .\ full 

 description of the comparisons made will be given on another 

 occasion. The result arrived at shows that the correction is less 

 than that assumed by Rowland, and would bring his value up 

 only to 775 at the temperature indicated. 



Great weight must be attached to Rowland's determination, 

 which at the temperature to which Joule's number applies is 

 7776, and at I9°'i, 7761, corresponuing to our 77S5. 



Ei}iiivaleiil in foot-pounds at Greenwich at ig"'! referred to the 

 "Paris" Nitrogen Thermometer. 



GrifTiths. 



Schuster and 

 Cinnon. 



Joule. Rowland. 



774 776-1 779-1 77S'S 



We now turn to an investigation of .Miculescu {.innales de 

 Cliiinie ct de Physiipie, vol. 27, 1892), in which the mechanical 

 equivalent of heat is meaaured directly by what seems a very 

 excellently devised series of experiments. Its result is 

 41S57 X 10'. 



In order to compare Miculescu's value with that of others, 

 we must apply a temperature correction which is somewhat 

 doubtful ; but taking the mean of Rowland's and Griffiths' 

 values as the most probable at present, we obtain at 15' the 

 following table : — 



Equivalent in foot-pounds at Greenwich at 15° referred to the 

 "Paris" Nitrogen Thermometer. 



Joule. Rowland. Miculescu. Griffiths. ^'cannon!"'' 



775 778'3 7766 780-2 779-7 



If we remember that Rowland's number referred to the 

 "Paris" nitri'gen thermometer would probably be smaller by 

 one unit, we are struck with the fair agreement there is, on the 

 one hand, between the results of Joule, Kowland, and Miculescu, 

 and on the other hand between Grilliihs and ourselves. 



As far as we can draw any conclusions from the comparison, 

 it seems to point to a difference in the value obtained by the 

 electrical and direct methods. Whether this difference is due 

 to some remaining error in the electrical units, or to some 

 undiscovered flaw in the method adopted by Mr. Griffiths and 

 ourselve-, remains to be decided by further investigation. 



Linnean Society, December 6. — Mr. C. U. Clarke, F.R.S., 

 President, in the chair. — .Mr. E. M. Holmes exhibited and 

 made remarks upon a small collection of Japanese marine alga;, 

 some of which were of considerable rarity in European collec- 

 tions. — Prof. D. Campbell brought forward some illustrations 

 of the relations of vascular crypioganis, as tleiluced from their 

 development. His remarks, which were li-.lened to with great 

 attention, gave rise to an interesting discussion, in which Prof. 

 Bower, Dr. D. H. Scott, Mr. Carruthers, and Prof. Marshall 

 Ward took part. — "A new revision of the DipUrocarfe^r," was 

 the title of a paper by .Sir Dietrich Hrandi>, K.C.l.E, who 

 gave an excellent account of this ordir of forest trees, their 

 structure and mode of growth, together with a survey of the 

 literature relating to them, and a clear expo.iiion of his views 

 concerning clasMfication. He pointed out ihat the order 

 Dipterucarpeif consists almost entirely of large trees which do 

 not llower until they have attained a great si/e, wiih a spread- 

 ing crown on a branchless stem often more than 100 feet high. 

 Hence it is difficult to obtain complete spccimcr.s in llower .ind 

 fruit ; and this explains why a lirge piopuriiun of the genera 

 and species have only of late year- became iiccuiatcly known. 

 Iv irthals in 1840 knew 34 species; A. .le Caiidolle in 1868 

 de cribcd 126; Mr. Thiselton Kjcrin 1874 e-tiii.aitd the order 

 at 170. Sir D. Urandis now consuler-. tnai there ate 320 well- 



