March 5, 1896] 



NATURE 



431 



value of the light taken from the same • part of the 

 arc, and taken at any assigned instant during the period. 

 Thus the arc itself was its own standard, and difficulties 

 due to slow variations in the mean light of the arc dis- 

 appear. The facts observed may be summed up as follows : — 

 The purple light of the true arc undergoes a periodic 

 variation, and, as far as the eye can judge, is completely ex- 

 tinguished for a certain interval during the phase ; it has equal 

 maxima values during the period, at instants slightly lagging 

 behind the instants of maximum power expenditure in 

 the arc. On the other hand, the illuminating power of 

 the carbon crater varies between a minimum value and two 

 unequal maxima ; the greater maximum occurring when the 

 carlx)n is positive, and an instant slightly lagging behind the 

 instant of maximum power expenditure in the arc. The second 

 part of the paper consisted of a comparison of the efficiency of 

 the alternate current arc regarded as a light-giving agent, as 

 compared with that of a continuous current arc taking the same 

 mean power. Using two arcs, which may be regarded as typical 

 if those used in practice, the mean spherical candle-power was 

 impared for equal expenditure of power in the arcs ; and it was 

 found that for the alternating current arc employed the total 

 mean spherical candle-power was always less than that of the con- 

 tinuous current arc. Lowering the frequency seemed to decrease 

 the efficiency of the alternating current arc. Prof. Ayrton said 

 the behaviour of the alternate current arc was of great interest, 

 for the power supplied could not be measured by simply 

 multiplying the current by the electro-motive force, since the 

 current lags behind the volts. The resistance, i.e. the ratio of 

 the current to the E.M.F., also lags, but the authors do not 

 appear to have made any attempt to measure the true resistance. 

 The authors were to be congratulated on the guarded tone they 

 had adopted as to the bearing of these experiments on the 

 question of the relative efficiency of the alternating and con- 

 tinuous current arcs. In a previous communication, one of the 

 authors had stated that the alternating current arc must 

 necessarily be a less efficient light-producing agent than the 

 continuous current arc. Although the last set of curves given 

 in the paper might appear to support this supposition, he (Prof. 

 Ayrton) felt that the difference obtained was probably due to the 

 fact that the alternating current arc was not being worked 

 under proper conditions. The quality of the carbons and the 

 length of the arc have a most important influence on the 

 efficiency of an arc. At present our knowledge is not sufficient 

 to allow of our stating definitely whether or not an alternating 

 current arc can be made as efficient as a direct current arc, but 

 there is no doubt that it will be possible to get much better 

 results than are at present attainaljle. Prof. S. P. Thompson 

 .lid that when the fact of the existence of the difference in 

 ; base between the current and volts in an alternating arc was 

 first published, he had made some experiments which showed 

 that there was a lag and not a lead, i.e. that the arc acted as if 

 it possessed self-induction. The resistance also lagged, and he 

 thought this lag might be due to a thermal lag. The tempera- 

 ture of the arc will lag behind the current, both when it is 

 increasing and when it is decreasing, and if the resistance of the 

 arc depends on the temperature of the vapour in the arc, then 

 the resistance would also lag behind the current. It was not 

 possible from ct priori reasoning to say whether or no an alter- 

 nating current arc could ever be obtained of an efficiency equal 

 to that of the direct current arc. With suitable carbons, length 

 of arc, current and volts, it seemed to him that it might be 

 possible to obtain an equal efficiency. The light-giving process 

 in an arc is not merely an irreversible degradation of electric 

 energy into heat, for the difference of potential between the 

 carbons maybe written \ — a ■¥ bl, where a maybe regarded as 

 a back electro-motive force and bl as a true resistance. The 

 first term of this expression does not vary with the length of the 

 j arc (/), but the second term does. Multiplying through by the 

 I current (C), the equation : Watts expended = Ca + Cbl, is ob- 

 I tained. It is the first of the terms on the right-hand side, which 



is a reversible effect, and corresponds to the power expended in 

 driving the current against a back electro-motive force, on which 

 the light given out chiefly depends, due to something occurring 

 at the crater surface. Mr. Blakesley asked whether Prof. 

 Thompson's idea of the light being due to the reversible part of 

 the process was not a strong argument in favour of the direct 

 current arc. Prof. Ayrton said that in two communications 

 made to the congress held at Chicago, it was shown that even 

 with direct current arcs there was a certain length of arc for 



NO. 1375, VOL. 53] 



which the efficiency was a maximum. Mrs. Ayrton had quite 

 recently found that the efficiency of arc-lamp carbons altered 

 with time. Prof. Thompson's suggestion as to a thermal lag 

 was a valuable one. Prof. Fleming, in his reply, said that when 

 comparing two agents where there were so many variables it was 

 practically necessary to restrict the investigation. In their case 

 they had kept the mean power constant, and had left the other 

 variables to take care of themselves. — The Society then 



adjniirnpfl U\\ Mirrh IJr ' 



Paris. 



Academy of Sciences, February 24. — M. A Cornu in the 

 chair. — On partial differential equations of the second order 

 with imaginary characteristics, by M. E. Picard. — On the 

 radiations emitted by phosphorescence, by M. H. Becquerel. 

 Ar experimental proof of the fact that light emitted by phos- 

 phorescent substances possesses the power of affecting a photo- 

 graphic plate through a screen which is opaque to sunlight. — 

 On the carbide of manganese, by M. IT. Moissan. This carbide, 

 MujC, which is easily prepared in the electric furnace from 

 Mn:,04 and sugar charcoal, differs from other metallic carbides 

 in the simplicity of its reaction with water, which proceeds thus, 



MnjC + 6H2O = 3Mn(OH)2 -f- CH4 + IL. 

 Neither acetylene nor liquid hydrocarbons being formed. — Study 

 of the borides of nickel and cobalt. The borides, NiB, CoB, 

 are easily obtained in the crystalline state from the metal and 

 boron at 1200°, their properties being very similar to those 

 of the iron boride previously described. — On the estimation 

 of arsenic, by M, A. Gautier. A criticism of a paper by 

 MM. Engel and Bernard upon the same subject. — On a 

 colloidal substance elaborated by the lymphatics in the 

 normal state, by M. L. Ranvier. — The relation between 

 muscular work and the albuminoid materials of the body, by 

 M. A. Chauveau. — An experimental study of the question as to 

 whether albuminoids take part in the production of external 

 work. The results show that the amount of nitrogen secreted 

 is practically the same whether the animal is doing external 

 work or not, and thus confirming the conclusion drawn by the 

 author from previous experiments, that it is by the combustion 

 in the muscle of carbohydrates that the energy necessary for 

 external work is produced. — On the corals of the Gulf of Lyons, 

 by M. de Lacaze-Duthiers. A communication from the Arago 

 Biological Laboratory.— On the method employed for conferring 

 immunity from the venom of serpents, from documents fur- 

 nished by M. de Serpa Pinto, by M. d'Abbadie. A question 

 of priority. — Observations of Venus on Mount Mounier, by 

 M. Perrotin. The results of these observations, which were 

 made under very favourable conditions at a height of 

 9000 feet above sea - level, are consistent with the 

 slow rotation discovered by M. Schiaparelli. — On the con- 

 version of dextro-rotatory camphoric acid into dextro-rotatory 

 camphor, by M. A. Haller. — Analysis, by volumetric methods, 

 of a mixture of chlorides, hypochlorites, and chlorates, by 

 M. A. Carnot.— Analysis of a mixture of chlorides, chlorates, 

 and perchlorates, by M. A. Carnot. — Observations on the new 

 Perrine comet (1896, Feb. 15) made at the Observatory of 

 Lyons, by M. G. Le Cadet. — Observations on the same, made 

 at the Observatory of Toulouse, by M. F. Rossard.— On the 

 production of Rontgen silhouettes, by M. C. V. Zenger.— On 

 the action of the X-rays on the diamond, by MM. A. Buguet 

 and A. Gascard. The transparency of the diamond and of jet 

 to the Rontgen rays renders it very easy to distinguish them from 

 their imitations, the latter being opaque. — On the cause of the 

 invisibility of the Rontgen rays, by MM. Dariex and de Rochas. 

 It was found that the media of the eye, although perfectly 

 transparent to ordinary light, are nearly opaque to the X-rays. — 

 On the Rontgen rays, by M. G. Meslin. —On some properties of 

 the X-rays of M. Rontgen, by M. H. Dufour. The phenomena 

 are compared to the silent discharge. — On the emission of the 

 Rontgen rays by a tube containing a fluorescent substance, by 

 M. Piltchikof. The length of exposure may be much reduced 

 under these conditions. — On some properties of dark light, by 

 M. G. Le Bon.— On photography through opaque bodies, by 

 MM. A. and L. Lumiere. The authors, in attempting to 

 repeat the experiments of M. G. Le Bon, obtained entirely nega- 

 tive results.— On the properties of metals extracted from their 

 amalgams, by M. Guntz (see Note, p. 423). — Action of some 

 hydrogen compounds on sulphurj'l chloride, by M. A. Besson. 

 The reactions with hydrogen sulphide, bromide, iodide and 

 phosphide were studied.— On the dry distillation of wood by 



