454 



NATURE 



[September io, 1891 



Friday was devoted to papers on electrical subjects. Prof. 

 Andrew Gray read a paper on the electro-magnetic theory of the 

 rotation of the plane of polarized light. Sir William Thomson's 

 explanation of the phenomenon rests on the supposition that the 

 ether has embedded in it a large number of small gyrostats. 

 Prof. Gray showed that the ordinary Maxwellian equations for 

 the phenomenon were obtainable on the supposition of the 

 existence of a closed chain of small magnets embedded in the , 

 undisturbed medium, which set themselves with their axes in the ; 

 direction of propagation of the ray as soon as the medium was 

 magnetized in that direction. 



This paper was followed by a most interesting communication 

 from the President, in which he gave an account of preliminary 

 experiments to ascertain if the ether is disturbed in the neigh- , 

 bourhood of a rapidly moving body — in other words, to ascertain 

 whether the ether behaves as a viscous fluid. Allusion was 

 first of all made to the experiments of Arago, in which he 

 endeavoured to determine whether or not the ether was stagnant 

 with respect to the earth by measuring the refractive index of a 

 glass prism at different times of the day, when the ether stream , 

 (if it exist) will flow in one direction or the opposite through 

 the prism. Arago found no such shift, indicating that the ether 

 was stagnant with reference to the earth. Fresnel, Fizeau, and 

 Michelson had also studied theoretically or experimentally the 

 ratio of so-called "bound" ether to "free" ether. The \ 

 problem which Prof. Lodge set himself to determine was 

 whether a disk moving with great rapidity would or would not 

 drag after it the ether in its immediate neighbourhood. Two 

 parallel co-axial disks of steel were arranged to spin at an 

 enormous rate. Rays of light from a single source were allowed 

 to fall on a glass plate feebly silvered so that about half the 

 light was transmitted and half reflected. By means of additional 

 reflectors the two beams passed in opposite directions several 

 times Tound in the space between the two disks, and were then j 

 observed in a common telescope and made to give interference 

 bands. In this way, assuming viscosity of the ether, the one 

 beam would have its velocity increased, the other would have its 

 velocity retarded, with the result that a shift of the interference 

 bands would be produced. So far, however, no such shift has 

 been observed. 



Prof. D. E. Jones gave an account of some experiments 

 made by him at Bonn on electric waves in wires. Measure- 

 ments of the electrical disturbance at different points of a wire, 

 in which stationary waves are set up, were made quantitatively 

 by putting a thermo-electric junction in the circuit at different 

 points, and noting the deflection of the galvanometer in its 

 circuit. Several curious results were recorded for which no 

 explanations were forthcoming. 



A communication was read from Lord Rayleigh, relating to 

 the reflection of polarized light from liquid surfaces. He finds 

 that the light reflected at the polarizing angle, from clean liquid 

 surfaces, is only very slightly elliptically polarized ; if, however, 

 the surface be ever so slightly contaminated, the amount of 

 elliptically polarized light in the reflected beam is enormously 

 increased. 



Saturday was devoted principally to the consideration of 

 papers on electrolysis. Mr. Shaw's report on the present slate 

 of our knowledge in electrolysis and electrochemistry included 

 a tabular compilation by Mr. Fitzpatrick of the electrical pro- 

 perties of soluble salts at different temperatures, and for different 

 concentrations. 



Mr. J. Brown read a paper on Clausius's theory of electro- 

 lytic conduction, and on some recent evidence for the dissocia- 

 tion theory of electrolysis, in which he gave an account of 

 experiments with so-called semi-permeable membranes. The 

 explanation of their filtering qualities simply depends on the 

 membrane acting as a conductor, 



Mr. Chattock gave an account of some important quantitative 

 experiments which he had made on the discharge of electricity 

 from points from which he finds that it is the air round the point 

 rather than the metal surface itself which offers resistance to the 

 discharge. 



On Monday the meteorological and allied subjects were taken. 

 The Reports of various Committees appointed to deal with 

 meteorological subjects were read. 



Dr. Johnstone Stoney read an interesting paper on the cause 

 of double lines in the spectra of gases. He assumes that the 

 molecules are vibrating in more or less complex harmonic curves, 

 and he illustrated the simple case of sodium vapour by means 

 of a pendulum oscillating to and fro, but with an apsidal motion. 



He stated that the application of astronomical methods of 

 calculation to molecular motions of sodium vapour gives rise 

 to a double D line instead of to a broadening of the line 

 as might at first sight be imagined. In the discussion which 

 followed, Mr. Webster stated that Prof. Michelson, who was 

 endeavouring to determine the metre in terms of the wave- 

 length of light emitted by a vibrating atom, had found by 

 the interference method that all the mercury lines are 

 double. 



Dr. Copeland exhibited a model to explain the probable 

 nature of the bright streaks on the moon. He attributes the 

 appearance of the streaks to the existence of transparent spheres 

 on the moon's surface, which reflect the light from the posterior 

 surface so as to be only visible in the line of light. 



During the morning the President interpolated some observa- 

 tions dealing with the effect of light in modifying the effect of 

 the gravitational attraction of the sun on small particles. When 

 sunlight falls upon a body, a very small repulsive effect is produced, 

 amounting to about 67 dynes per square metre. Thus, for 

 example, during an eclipse of the moon about 1000 tons are 

 suddenly applied, but this small force is incapable of producing 

 any observable effect on the motion of our satellite. The 

 smaller the body, the larger, of course, the surface exposed 

 relatively to the mass, and therefore the greater should be the 

 effect produced. For a certain size of particle (about that of a 

 grain of dust) the gravitational attraction and light repulsion 

 should balance one another. The effect is clearly independent 

 of distance. 



On Tuesday, after the Report of the Committee on Electrical 

 Standards, read by Prof. Carey-Foster, and an account of an 

 elaborate research by Mr. Swinburne on the causes of varia- 

 tion of Clark cells, there was arranged a joint discussion with 

 Section G, on "Units and their Nomenclature," which was 

 opened by the President, who suggested that the discussion 

 should, as far as possible, be confined to electrical units, and 

 that the mechanical units should be left to a later period. He 

 discussed at some length the relative advantages and disadvan- 

 tages of the various names for the unit of self-induction, secohm, 

 quadrant, henry, &c., and expressed himself as of opinion that 

 the quadrant, which was really an angular measure, but which 

 was frequently used as a linear measure, was very objectionable 

 in that it indicated that the unit of self-induction was a length, 

 when it was perfectly well known not to be a length. He 

 was, therefore, of opinion that some name with a less 

 obvious meaning, such as that of a person, was very desirable. 

 He thought also that the secohm was too large for practical 

 purposes, and that some sub-multiple such as xrinr would be 

 preferable. 



The President was followed by Mr, Preece, who referred to 

 the work of the British Association Committee on Electrical 

 Standards, which had lasted now for thirty years, and expressed 

 the opinion that it would be undesirable to interfere in any way 

 with the old standards now about to be legalized by the Board 

 of Trade. 



Prof. Stroud read a paper on some revolutionary suggestions 



on the nomenclature of electrical and mechanical units, in which 



he advocated selecting 10" cm. as the unit of length, lO"'' gm. 



as the unit of mass, and i sec. as the unit of time to form the 



•• basis of a new practical system of units. He also explained the 



i details of a system of automatic nomenclature for C,G.S. a.nd 



{ other units, which he thought should be taken into consideration 



I before any fresh names were authorized. The special feature of 



1 the system is that every label is self-explanatory. 



Dr. Johnstone Stoney thought the old system should remain 

 intact, and that the proper way to deal with the subject of 

 j nomenclature was to indicate sub-multiples by numerical pre- 

 fixes ; e.g. he would call a microfarad a sixth farad, and the 

 capacity of a Leyden jar would be about a tenth farad. He 

 1 suggested that the name for the unit of magnetism should be a 

 j Gilbert, and that of the unit magnetic field a Gauss, 



Prof. Carey Foster thought that if the volt and ampere were 

 made ten times as great, fresh names, such e.g. as "gal," 

 I from Galvani, should be introduced. 



j Prof. Riicker laid stress on the importance of recognizing the 

 j fact that we possessed at present no definite knowledge 

 ' as to the absolute dimensions of any electrical or magnetic 

 \ unit, and therefore it was undesirable to introduce names 

 I (such e.g. as quadrant) implying the possession of such 

 knowledge. 



Prof. S. P. Thompson drew attention to the desirability of 



NO. 



114 1, VOL, 44] 



