rocks, schists, and cretaceous and tertiary strata, all of which have been carefully laid down on the map which accompanies the work. Respecting the origin, however, of the Dead Sea, M. Lartet comes to the somewhat surprising conclusion that it never communicated with the Red Sea. But no one looking at a physical map of this district can fail to be struck with the fact that the valley of the Jordan is evidently prolonged southwards to the Red Sea, the sole division being a comparatively small ridge running across the valley. Furthermore, the author's own map would seem to tell against him; for the ancient alluvials of these two seas almost join at this ridge, from the summit of which they were probably removed by denudation. The difference in the chemical analysis of the waters of the Asphaltites and Red Seas, which is one of the points brought forward, is scarcely to be wondered at when their different situations are taken into consideration, the one being land-locked and the other communicating with the ocean; nor do all analysts agree in their results as to the chemical composition of the waters of the Dead Sea.

Mr. S. B. J. Skertchley, of the Geological Survey, has found in the vicinity of Brandon, Suffolk, certain brick-earth deposits containing some fine specimens of paleolithic flint implements, associated with quantities of the broken bones of pleistocene mammalia, and a few freshwater shells.* These implements "are of the oval type, boldly chipped, but without any of the finer work which distinguishes the better made palæolithic implements." Mr. Skertchley maintains that these deposits are of pre-glacial age; but some doubts are held as to this being the case.

Two recently published books may also be alluded to here. "Field Geology," by W. H. Penning, F.G.S., will form a most invaluable guide to any wishing to map a district for themselves, and contains clear descriptions of all the appliances necessary for a geological survey, with instructions as to their uses, and the methods to be adopted in tracing out the different formations; whilst a section is added by Mr. Jukes Brown on the character, mode of occurrence, and way to collect the fossil remains. "The Geology of England and Wales," by H. B. Woodward, F.G.S., having been already reviewed in the pages of the ARGONAUT,† needs no further comment.

* Nature, 21st Sept. 1876.

+ Dec. 1876.

IT

ELECTRICITY AND LIGHT.

BY PROFESSOR W. F. BARRETT, F.R.S.E., &c.

T may startle some of our unscientific readers to be told that the opinion of the most eminent physicists of the present day inclines very strongly to the belief that light is an electro-magnetic phenomenon. That is to say, the propagation of light and the propagation of electricity both require the assumption of a medium whose properties are identical, and the impulse of a luminous, thermal, and electro-magnetic disturbance are transmitted through this medium with a velocity sensibly equal in each case. Hence it is more than probable that the two media are really one and the same, namely the substance to which the term ether has been given. Faraday's supposition, that if a material medium fills space it has probably other functions besides the conveyance of luminous and calorific radiations, is thus remarkably confirmed; and immense weight added to the conviction of the reality of the medium itself.

Hence it will be seen that an investigation of the points of contact between electricity and light is one of the most interesting and important inquiries which at present engage the attention of physicists. It will be our object in the present article to review some of these points of contact; for out of the scattered observations on this question, some will be found to have a higher significance when viewed in connection with the preceding remarks: especially is this the case when we come to the discoveries bearing on magnetism and light, to which we shall allude in the sequel.

It is well known that when a ray of light falls on a daguerreotype plate an electric current is produced, which can be readily made evident by a galvanometer. The molecular disturbance set up by light gives rise to electrical disturbance. In like manner, other chemical changes started by light, also start an electric current. Even if two strips of copper be taken, both immersed in water, but one kept dark by enclosing it in a porous cell, and the other exposed in a glass jar outside, a current is produced directly a ray of light falls on the exposed strip of copper. This current is much increased if the strips be strongly oxidised, and the maximum effect is obtained when red or yellow light is employed, the exposed strip

acting as the positive plate; whereas when the less refrangible rays were used a weaker reverse action takes place on the exposed copper strip. These are the results of some recent experiments of M. Ed. Becquerel.

Passing from inorganic to organic structures, thirty years ago Dr. Alfred Smee, F.R.S., made a discovery, not generally known, that when electrodes were inserted into the eye of an animal, an electric current was observed every time a light was brought in front of the

animal's eye.

Dr. McKendrick and Professor Dewar have recently discovered a somewhat analogous phenomenon. These careful experimenters used eyes of freshly killed animals, and even succeeded by simply placing the electrodes on the living eye in such a position as to be free from all pain: in fact, they experimented in part upon their own eyes. They found that every variation of light to which the eye is exposed gives rise to a corresponding variation in the electric condition of the retina. These experiments are still in progress, and the results are likely to be of the highest interest.

Considerable inquiry has of late been made in the remarkable deportment of selenium when exposed to light. Selenium is a quasi metal, a body having some affinities with sulphur on the one hand, and tellurium on the other. It exists in different molecular states. It may, like sulphur, be either amorphous or crystalline. Now amorphous or vitreous selenium has so very high an electric resistance that it cannot be measured; it is, in fact, an excellent insulator, just like sulphur. But if this amorphous selenium be heated to the temperature of boiling water, and kept hot for some time, it becomes partly crystalline, or granular, and its resistance at this temperature becomes measurable; but on cooling the resistance increases something like 100 per cent. The curious point is, that now it shows some slight increase of conductivity, i.e. decrease of resistance, when exposed to light. This discovery of the effect of light on crystalline selenium was accidentally made by Mr. May, a telegraph clerk in Valentia, and it has formed the subject of an investigation by Lieut. Sale and others. Among the earliest inquirers to attack the subject, especially the chemical aspect of the question, were two Irish chemists-Mr. Draper and Mr. Moss-whose recent papers before the Royal Irish Academy show that they have arrived at conclusions

as the light emerged from the substance, the plane of polarisation was observed, by noting the position of the analyser when it cuts off the ray. A powerful magnetic force was then made to act, so that the lines of force within the substance coincided with the direction of the ray. Under these conditions Faraday discovered that the effect of the magnetic field was to turn the plane of polarisation through a small angle, round the direction of the ray as an axis; the direction of rotation was the same as that of the positive current in the electro-magnet. Besides heavy glass, numerous other transparent bodies have been tried with a similar, though less effect; in the case of certain salts of iron the rotation is, however, the reverse of that just stated.

The physical explanation of Faraday's discovery is doubtless to be found in the molecular change impressed on the transparent body submitted to the magnetic force. A plane polarised ray may be regarded as consisting of two circularly polarised rays moving in opposite directions, and of these two rays that moving in the righthanded direction has, by the act of magnetisation, performed a greater number of vibrations in the same period than the lefthanded ray. In other words, the index of refraction of the glass for one of these circularly polarised rays has been altered by the molecular stress produced by the magnetic force. For if the heavy glass be suspended between the poles of the magnet, it is repelled, as Faraday subsequently discovered, from both poles, so that it sets across the lines of force that stream from pole to pole: what is true of the glass as a whole must be equally true of the molecules of the glass, hence the production of the molecular stress to which we have referred.

Many of our readers are probably aware of the fact that there are certain substances which, independently of magnetic force, rotate the plane of polarisation as the ray passes through the substance. There is this radical difference, however, between such natural rotation, and that induced by magnetic action, viz. in the former case the direction of rotation is reversed when the direction of the ray is reversed, and in the latter case the direction of rotation is exactly the same, whichever way the ray passes along the substance, the direction of rotation only being reversed by reversing the magnetic poles. Hence it is possible, by reflecting a ray to and

fro along a diamagnetic body, to increase the amount of rotation considerably.

M. H. Becquerel has recently shown that the phenomena of magnetic rotation are not only a function of the wave length of the light used, but also of the index of refraction, and of the chemical composition of the bodies employed.

A new and surprising discovery in connection with this subject was published by the Rev. Dr. Ker, at the last meeting of the British Association. Dr. Ker's discovery consists in this, that he has acted upon a polarised light by magnetism, without the intervention of any solid or liquid medium. By simply reflecting a ray of plane polarised light from a polished surface of iron, Dr. Ker has obtained an effect analogous to Faraday's discovery, whenever the iron mirror is intensely magnetised. By proper precaution, Dr. Ker has prevented the reflected ray from being elliptically polarised by the act of metallic reflection; and he finds that at oblique incidence, when the iron is made a south pole the ray is turned to the right, when a north pole to the left; the current which magnetises the iron carrying, as it were, the plane of polarisation with it.

This is Faraday's effect over again, only with reflection from a surface, instead of transmission through a body. Hence it is probable the effect is due to a molecular change, impressed on the iron by the act of magnetisation. If this be so, gilding or silvering the magnetic pole should prevent the occurrence of the phenomenon, though it would not prevent the free action of magnetic force upon the polarised ray, if simply due to this cause. This crucial experiment has just been tried by Mr. Fitzgerald, of Trinity College, Dublin, with precisely the results anticipated by the writer. Mr. Fitzgerald has also given, but not yet published, an explanation of Dr. Ker's phenomenon, based upon a change in the index of refraction of the iron by the act of magnetisation. Experiments will shortly be in progress for the examination of the effect of perpendicular incidence upon the magnetic pole-a point of much theoretic importance. Meanwhile we await with much eagerness Dr. Ker's fuller publication of his discovery, and his further researches therein, and in conclusion join our congratulations to those which Dr. Ker has already received for the invaluable contributions to natural knowledge which he has already given to the world.