May 30, 1895] 



NATURE 



1(9 



SOCIETIES AND' ACADEMIES. 

 London. 

 Physical Society, May 10. — Captain \V. de W. Abney, 

 President, in the chair. — Mr. Herroun read a paper on 

 the iodine voltameter. After referring to the usual methods 

 of determining the value of the small currents used in cali- 

 brating galvanometers and other apparatus for measuring 

 small currents, and discussing the errors to which they are subject. 

 the author gave his reasons for selecting iodine. He did this 

 since, with the exception of mercur)' in the mercurous state, 

 iodine has the largest electro-chemical equivalent, and in 

 addition, by titration with sodium thiosulphate, it is possible to 

 determine the quantity of iodine liberated with a greater accuracy 

 than can be obtained by weighing a deposit of copper or silver 

 with the balance. The solution employed in the voltameter 

 contains 10 to 15 |)er cent, of zinc iodide. If care is taken to 

 leave a small piece ol metallic zinc in this solution, no free iodine 

 is liberated on keeping, unless the solution is exposed to a strong 

 light for sonie time. The anode consists of a plate of platinum 

 at the bottom of a tall and fairly narrow baaker. The wire 

 Ifeading the current to the anode is encased in a glass tube, so 

 that the iodine is only liberated at the bottom of the beaker, 

 where, on account of its great density, it tends to collect. The 

 kathode consists of an amalgamated zinc rod, which, to 

 prevent loose |)articles of zinc falling down into the iodine, is 

 surrounded by a piece of filter-paper or vegetable parchment. 

 In an electrolysis lasting for as long as two hours, none of the 

 iotline is found to diffuse up to the part of the solution near 

 the zinc kathode. Where, on account of the extreme feebleness 

 of the current employed, it is necessary to allow the electrolysis 

 to continue for longer than two hours, a U-tube is used with 

 two small plugs of asbestos at the bend, the anode being in one 

 limb and the kathode in the other. With this form of voltameter, 

 even after the current has flciwed for several days, no signs of 

 iodine have been found in the limb containing the kathode. On 

 account of the production of electric convection currents, the 

 iodine voltameter does not seem to be quite so suitable for the 

 accurate measurements of strong currents. After the current is 

 stopped the zinc electrode is immediately removed, the solution 

 stirred, and the amount of iodine liberated determined by titra- 

 tion with sodium thiosulphate. The author finds that a con- 

 venieni strength of the thiosulphate solution is one in which 

 one c.c. corresponds to the amount of iodine liberated by five 

 coulombs of electricity. This solution contains 12-8375 grnis. 

 of pure recrystallised sodium thiosulphate per litre. It is pos- 

 sible to perform the titration to within 0"i c.c, which corresponds 

 to 0'5 coulomb, or, if the electrolysis lasts one hour, to 1/7200 

 ampere. In a comparison made with a sil\er voltameter, the 

 current as deduceil from the silver was 0'0264 ampere, and 

 that deduced from the iodine 0'0266. The author considers 

 that part of the difference may be due to the effect of 

 oxygen dissolved in the solution of silver nitrate. Prof. Carey 

 Foster consideceil this process for mea.suring currents a most 

 valuable one. The idea of using a volumetric method for 

 measuring currents was to him new. He did not, however, see 

 the advantage of using a substance with a high electro-chemical 

 equivalent if a volumetric method was going to l>e employed to 

 estimate, the quantity of the substance liberated. It would be 

 possible to use a chloride, though in this case the titration would 

 ])rol)ably be less accurate. Prof. Silvanus Thompson said he 

 thought the methotl would be lery valuable, but he would like 

 to know if any error was likely to arise if too great a current 

 dcn-sity was employed. The numlier the author had assumed 

 for the atomic weight of silver (loS) was only approximate ; if 

 the nuire accurate value (1077) were used, the agreement be- 

 tween the results obtained with the silver and iodine voltameters 

 would be improved. . Mr. Trotter asked what was the largest 

 current that could be accurately measured. Mr. Enright said 

 he had iLsed porous diaphragms in iodine voltameters, and foimd 

 that the iodine collected in the positive compartment, while the 

 water was driven. over into the negative compartment. With 

 strong, currents it Wiis possible to get almost pure iodine left in 

 one compartment. Mr. Watson thought that, since the value for 

 the electro-chemical equivalent of iodine used by the author was 

 deduced from Rayleigh's value of the electro-chemical equivalent 

 of silver, and that Rayleigh's experiments were performed in air, 

 the difference obtained with the .silver and iixline voltameters 

 could hardly be due to the cause suggested. Mr. Elder warned 

 the menxbers that volumetric measurements were not so accurate 

 or easy as they seemed. He particularly mistrusted a solution 



NO. 1335, VOL. 52] 



of sodium thiosulphate, since he had found a solution of this 

 substance to change even in twenty-four hours. The difficulty 

 of accurately reading the burette might be overcome by weighing 

 the burette and its contents before and after the titration. The 

 author in his reply said that with the size of electrodes he used 

 (about 9 sq. cm. surface) O'l ampere was the maximum cjrrent 

 it was safe to us;. The only subiiance likely to bs produred by 

 too great a current density was periodate, which, since it was 

 insoluble, would immediately be noticed. The influence of the 

 dissolved oxygen was only appreciable with small currents where 

 the electrolysis lasts some time, while in Rayleigh's experiments 

 large currents were employed. The chairman, while returning 

 thanks to the author for his paper, mentioned that in his experi- 

 ence he had found zinc salts to be ver)' untrustworthy. — Mr. A. 

 Sharp read a paper entitled a new method in harmonic analysis. 

 The author, in this paper, applies the principle of the form of 

 harmonic analysis for giving direct readings of the amplitude and 

 epoch of the various constituent harmonic terms, previously de- 

 scribed by him, to the performance of harmonic analysis without 

 the itse of an instrument. The kinematic principle is as 

 follows : Let the cur\e to be analysed be drawn with a scale of 

 abscissa such that the period is 2ir. Let a wheel w roll on 

 the paper and be connected with a tracing-point P in such a 

 manner that as P moves uniformly in the -v direction the axis of 

 the wheel -jj turns uniformly counter clockwise in a horizontal 

 plane, and the distance rolled through during any short interval 

 is equal to the corresponding displacement of the tracer P in the 

 y direction. The curve traced out by w the author calls the 

 roller curve, and from the vector joining the initial and final 

 points of this curve the amplitude and epoch can be determined. 

 Suppose the periodic curve consists of a portion of the cur\'e 

 ^ = a„ + (Zj-v -7- a»»- -*• -f amx'" repeated over and over again. Then, 

 if the tracer is taken round this periodic curve you get a rolled 

 curve which may be called the first rolled curve. If now the 



curve whose ordinates are -?' is traced out, the roller curve 

 dx 



obtained is the evolute of the first, and so on for— ;-!!, &c. The 



dx' 

 author gives two worked examples, and compares the values of 

 the coefficient obtained with those given by the harmonic 

 analyses of the Guilds Central Technical College. Prof. 

 Henrici said he had not received the paper in time to thoroughly 

 master it, but he thought that, at any rate for curves where no 

 discontinuity occurred, the relation found by the author between 

 the roller curves was always true, the last evolute being a 

 point, and the one before that a circle. The interesting point 

 was whether the method was capable of being used for practical 



I purposes, for it occupied a ])lace with respect to harmonic 

 analysis similar to that occupied by .Simpson's rule in planimetry. 



I Prof. Silvanus Thompson asked if the author had devised a 

 form of mechanism capable of fulfilling the kinematical conditions 

 given at the commencement of the paper. The author in his 

 reply said he had devised such a mechanism, and that it was 

 described in his previous paper. In addition he had since 

 invented a more practicable form which he had patented. The 

 chairman said the Society ought to congratulate itself on the 

 large number of important papers dealing with harmonic 

 analysis and planimetrj- that had lately been communicated. 



Malacological Society, May 10. — Prof. G. B. Howes, Pre- 

 sident, in the chair. — On behalf of Miss de Buigh specimens 

 were shown illustrating the variation of Columbdla menatoria, 

 Linn. — Mr. Da Costa exhibited a collection of univalve mollusca 

 from Lakes Tangan)-ika and X'ictoria N)-an7.a, and pointed out 

 the entirely different characters of the molluscan fauna of these 

 two lakes. — On behalf of Mr. C. S. Cox were exhibited living 

 specimens of Glaitdina fnim Italy. — Mr. E. .\. Smith ex- 

 hibited an almost complete collection of the land and fresh- 

 water molluscaof St. Vincent, W.I. — Mr. E. R. Sykes exhibited 

 specimens of AchatineUa variabilis, Newc, and allied forms, 

 from the Island of Lanai. The following communications were 

 read : — Notes on Trochonaiiina and other genera of the land 

 mollusca, with reference to the animals of Martensia Mozani- 

 iiiensti, Pfr. , .and other species, by Lieut. -Colonel H. H. 

 Godwin-.\usten. — Report on the land and freshwater shells 

 collected by Mr. H. H. Smith at St. Vincent, W.L, by E. A. 

 Smith. — Note on the larval oyster, by M. F. Woodward. 



Victoria Institute, May 6. — Dr. Chaplin in the chair. — A 

 |)aper on the so-called Pitlutatilhropiis of Dr. E. Dubois was 

 read by Prof. E. Hull, LL.D., K.R.S., after which a paper by 



