524 



NA TURE 



[March 31, 1892 



might easily develop their language in the vicinity of the Altai 

 Mountains and the Baikal. 



As to the Manchus, they have forgotten their early occupa- 

 tions since coming to China, and they attend now only to the 

 duties of the puhlic service or to military training. The 

 language, like the Mongol, is rich with the spoils of antiquity. 

 All the various forms of culture, whether belonging to 

 Shamanism, Confucianism, or Buddhism, with which they have 

 become successively familiar, have contributed a share. To 

 these must be added the vocabulary of the huntsman, the fisher- 

 man, and the shepherd, and all the terms necessary for the 

 feudal relationship as well as those of the trades and occupations 

 of the old civilization. 



SOCIETIES AND ACADEMIES. 

 London. 

 Physical Society, March ii. — Prof. A. W. Riicker, 

 F.R.S., Vice-President, in the chair.— Mr. H. M. Elder read 

 a paper on a thermodynamical view of the action of light on 

 silver chloride. In the decomposition of silver chloride by lij^ht, 

 chlorine is given off", and a coloured solid body of unknown 

 composition (sometimes called- " photochloride ") formed, the 

 reaction being indicated by the formula « AgCl = Ag,tCl„_i + ^Clj. 

 If the experiment be carried out in a sealed vacuum, the chloride 

 is darkened up to a certain point, but regains whiteness when 

 left in the dark. These facts have led the author to believe 

 that the pressure of the liberated chlorine is a function of the 

 illumination or intensity of light falling upon the chloride, in 

 the same way as the pressure of a saturated vapour is a function 

 of the temperature. Since illumination is a quantity in many 

 respects analogous to temperature, he considers it not unreason- 

 able to apply thermodynamic arguments, and regard chlorine in 

 presence of silver chloride and "photochloride" as the working 

 substance in a "light engine." He therefore supposes a 

 Carnot's cycle to be performed on the substances at constant 

 temperature, the variables being pressure, volume, and illumina- 

 tion. Since the cycle is strictly analogous to Carnot's, except 

 that illumination is written for temperature, he infers that the 

 efficiency is a function of the two illuminations. It also follows 

 that just as Carnot's cycle is used to determine an absolute 

 scale of temperature, so this cycle may be applied to determine 

 an al)solute scale of illumination. It only remains to determine 

 an empiric scale analogous to the air thermometer, and to com- 

 pare it with the photodynamic scale, provided a method of 

 making the comparison can be devised. Assuming the axioms 

 applied to Carnot's cycle are true when illumination is written 

 for temperature, the author shows mathematically that/ oz \Pl^, 

 where p is the pressure, I the illumination, T the absolute 

 temperature, and p the heat of combination per gramme-molecule 

 of chlorine evolved. If P be the heat of formation of silver 

 chloride, the fraction p/P may be considered as expressing the 

 fraction of the total chlorine that can be removed by the action 

 of light upon it, supposing the gas removed so as to keep the 

 pressure below that corresponding to the illumination. The 

 chemical equation might then be written — 



P/pAgCl- Agp/^Clp^^_^ + ACl2; 

 thus the formula for "photochloride" would be Agp, Clp, _^ 

 Prof. Riicker read a letter from the President (Prof. Fitz- 

 gerald) on the subject of the paper. He inquired what axiom 

 corresponding with the second law of thermodynamics was 

 employed. He was not sure that the engine was perfectly rever- 

 sible, and felt doubt on the subject of phosphorescence mentioned 

 in the last operation of the cycle. Nevertheless, the paper was a 

 most interesting one, and very suggestive. Prof. Herschel 

 pointed out that Becquerel's phosphoroscope showed that all 

 kinds of light produced phosphorescence, and thought that, in 

 considering the subject, the non-thertnal character of photo- 

 genic light should be kept in view. Mr. Baker said 

 he had been working on silver chloride for several years, 

 and found that no darkening whatever took place if kept 

 dry and in vacuv. He considered oxygen necessary to the 

 action. Dr. C. V. Burton, referring to the mo'ivity of the 

 system, said that only a small fraction of the energy of the 

 illumination was actually made use of. He also thought it 

 necessary to consider how far the second law of thermodynamics 

 could be treated as an axiom. He himself had been led to be- 



lieve the law did not hold for mixtures of substances differing 

 in a finite degree from one another. Some time ago he experi- 

 mented on a solution of sodium sulphate placed in a dialyzer 

 kept at constant temperature. The more acid portion passed 

 through the membrane, and on mixing a rise of tempera- 

 ture was observed ; the dialyzer thus acted like Maxwell's 

 demons, and the mixing increased the motivity of the system. 

 Prof. Riicker expressed his doubts as to whether the cycle 

 described in the paper was strictly analogous to that in Carnot's 

 problem. In the latter case the parts of the working substance 

 only diff"ered infinitesimally from one another, whilst in the 

 former the working body was a mixture of two solids and a gas. 

 In order that the increased illumination should not alter the 

 temperature, heat must be carried av\ay. According to the 

 paper, the first part of the cycle must be both adiabatic and 

 isothermal. This seemed hardly possible. If the chlorine 

 alone be considered, it could not be true, and it could only hold 

 if the chloride absorbed all the heat given out by the compres- 

 sion of the chlorine. This seemed improbable, but, if true, it 

 would be very important. Captain Abney saw another difficulty 

 in the fact that at low temperatures silver chloride is not acted 

 on even by violet light, whereas heating greatly increases the 

 action. In his opinion the conclusions arrived at required con- 

 firmation, but the paper would form a starting-point for many 

 new experiments. Mr. Elder, in reply to Prof. Fitzgerald, said 

 the axiom corresponding to the second lav as stated by Clausius 

 might be formulated thus : Energy cannot of itself pass from a 

 less bright to a brighter body. In the paper he had assumed 

 that the energy given out during compression at the lower 

 illumination was of the same quality as that absorbed at the 

 higher. The whole question depended on comparisons of in- 

 tensities of illuminations of difTerent wave-lengths. In the 

 expression p oc P'"^, p was probably a function of T, and 

 Captain Abney's objection was not necessarily fatal. Speaking 

 of the presence of oxygen being essential to decomposition, he 

 believed some sensitizing body was necessary, but judging from 

 experiments he had seen, an infinitesimal quantity would prob- 

 ably be sufficient, for the action seemed to be of a catalytic 

 nature. He felt the weight of Prof. Riicker's objections, but 

 thought they might possibly be met. — A paper on choking 

 coils was read by Prof. Perry, F.R.S. Regarding a choking 

 coil as a transformer with one primary and many secondaries 

 represented by the conducting ma-ses, he pointed out that all 

 the secondaries might be replaced by a single coil of « turns, 

 and resistance r ohms, short-circuited on its-rlf. Assuming.no 

 magnetic leakage, the equations for the two circuits at any 

 instant are V = RC -f N0I, and O =' re + «9I, where N and 

 n are the turns, R and r the resistances, I the total induction (in 

 lo*^ C.G.S. lines), and C and c the primary and secondary cur- 

 rents respectively. Since the exciting current, C, is all-important 

 in choking coils^ and its value depending on the law of mag- 

 netization, the equations are treated in a different manner from 

 that adopted in ordinary transformer calculations. Expressing 

 the magnetic law as a Fourier series, 1 = 2 A^o-j sin ix, the 

 value of A (viz. NC + nc) is deduced, and when V or I is 

 given as a periodic function of the time, C may be calculated. 

 Assuming V = Vo. sin kt, the author finds 



Ta r Jn-2<?sin/-t-<f'-'.sin ] /tZ-go-i-tan 

 N-ff/C'L ^ 



tan/-f- 



b cos yzt - m cos ^kt I, 



where ^ = n'-akir, f is the hysteresis term, and and w con- 

 stants depending on. the law of magnetization. For ordinary 

 transformer magnetizations, b = O'Z, and m = 0-05. From the 

 above expression it will be seen that if there is no hysteresis 

 (i.e. f = o), the efifect of the eddy currents, e, is to increase the 

 amplitude of the important term, and to produce a lead of 

 90° - cot-V, whereas the effect of hysteresis without eddy 

 currents is to leave the amplitude unaltered, and produce a 

 lead/. Putting /= ogives results in accordance with experi- 

 mental observation, hence the author is inclined to believe that 

 there is no hysteresis in transformers. He also points out that 

 the higher harmonics must exist, and thinks it probably that a 

 choking coil with finely divided iron may prove a method of 

 increasing frequency by mere magnetic means. Taking the 

 case of a 1500-watt transformer (2000 volts) unloaded, in which 

 the loss in eddies was 40 watts, it is shown that a secondary of 

 2 turns, and resistance 1-9 ohms, would replace the eddy 



NO. 



II 70, VOL. 45] 



