XLIV. On the Electrical Light. By H. W. DOVE*.

INCE Fraunhofer first showed that the spectrum of the

SINCE

electric spark was distinguished from that of the light of the sun by a very bright line in the green, and a somewhat less luminous one in the orange, its prismatic analysis has been completed, especially by the investigations of Wheatstone, Masson and Ängströmt. Wheatstone has shown that the lines are different according to the nature of the metals between which the spark passes; and that when it is produced between two different metals, the spectrum exhibits combined the lines which are perceived when it is produced consecutively between similar balls of each of the metals. According to Angström, this also applies to similar balls formed of an alloy of the two metals. Masson has ascertained, that, leaving out of consideration the dissimilarity of the spectrum in the employment of different metals, determinate lines appear as common to it; and from this it has appeared the most natural opinion, that the light of the spark is mixed, consisting of a direct production of light in the medium in which the spark is produced, and glowing particles carried forward from the balls between which the spark is transmitted. Fresh proofs in favour of this view have been obtained, especially by Angström, by the prismatic analysis of the sparks evolved in different gases.

A composite phænomenon may be indirectly investigated by changing the constituents which enter into it, or by attempting to produce these in an isolated condition. Draper has shown that the spectrum of a glowing platinum wire contains no lines, so that it is white in the strict sense; whilst with regard to the true electrical light without phænomena of incandescence, we are only able to judge of its colour by the naked eye. The luminous phænomena, known under the names of electrical brush, glow, and interruption of the spark, are so constant and so feebly luminous as to render an exact prismatic analysis extremely difficult, and in many cases even almost impossible. The judgement of the colour of a homogeneous luminosity is, however, very delusive. The ordinary gas-flame which is yellow by day, and even the orange light of an oil-lamp, appear white in the dark. For this reason we may presuppose that the eye will only furnish an uncertain judgement as to the colour of the weaker electrical luminous phænomena. Many of these are so faint, that for their exact perception they require the exclusion of every other source of light; and Prevost, as is well known, has already observed, that with coloured illumination the Poggendorff's Annalen, No. 6. 1857. See Phil. Mag. vol, ix. p. 329.

brightest at last appears white. The colour of a source of light may, however, be investigated by allowing it to be absorbed by coloured dioptric media, or by investigating catoptric colours in its luminosity. I have availed myself of this method to compare the weaker electrical luminous phænomena with those of the spark.

The electrical brush may be produced in two ways: by attaching the point either to the positive primary conductor itself, or to a second conductor into which sparks pass continuously from the primary conductor. In the former case its rays are closer, but less branched and diffused; the brighter reddish-violet light, however, from which the rays are evolved is more intense, so that the whole brush appears to be illuminated by it. In the second case, the spark between the two conductors assumes, almost completely, the part of this bright basal point of the brush, the rays of which, however, are now much more branched. A similar difference is exhibited in the formation of the luminosity in a large exhausted electrical egg. If the superior conductor passing through the stuffing-box be in immediate contact with the primary conductor, the reddish-violet, perpendicularly descending stream of light is intense, whilst the diffused light of the rest of the space is weak; if, on the contrary, sparks be allowed to strike continually upon the superior conductor, the intensity of the perpendicular stream of light diminishes, whilst the whole space is filled with band-like whitish streaks of light, which incessantly change their form. This alone renders it probable that the perpendicular stream of light is the basal point of the brush which has become elongated in vacuo, and that the white bands correspond with its rays.

If the brush be looked at through a deep blue cobalt glass of half an inch thick which effaces the middle of the spectrum, its ramifications are still seen very distinctly, whilst they disappear completely in a red glass. A green glass which so obscures the red that when they are superimposed in ordinary daylight one seems to have a board before one's eyes, permits the passage of the rays, although more weakly than the cobalt glass. A blue picture on the red field appears to be brightly illuminated by the rays of the brush, upon a dark ground; a red picture on the blue field appears dark upon a bright ground, consequently just as when they are looked at in daylight through the deep blue glass. When looked at through an equilateral prism of Guinand's flint-glass, in which I can see several of Fraunhofer's lines of daylight with the naked eye, the rays of the brush appear nearly unchanged in colour, and only a little broader, whilst the bright basal point of the brush gives a spectrum in which red, green, and violet appear brilliantly, and which

scarcely differs from that of a small spark. The comparison is best made when the second conductor in contact with the point is alternately brought in contact with the conductor and removed from it. The spectrum recognized at the basal point of the brush is then transferred to the point of interruption, where the sparks pass. The phænomenon in the electrical egg is exactly analogous. The perpendicular stream of light gives a manycoloured spectrum, consisting of a very broad blue border, a broad green and a narrow red streak; it is faintly visible through a red glass, whilst the band-like streaks are seen very clearly through the cobalt glass, but are completely absorbed in the red glass.

The light of an exhausted glass tube containing a little mercury, which appears brilliantly white in the dark, and in daylight a bluish-green, is not visible through the red glass, but very distinct through a green one, and rather less bright through the deep blue. If the tube be held to the conductor, it shines for a long time uninterruptedly; and besides blue and green, the spectrum contains a slight trace of red. I have sketched coloured spirals upon a white ground, which, when looked at through certain glasses of the same colour, disappear in such a way, that, on turning them round, the perfectly white hinder surface cannot be distinguished from that on which the coloured spiral has been drawn. This experiment succeeds without any coloured glass in the case of a spiral sketched with Schweinfurt green, when illuminated in the dark by the mercurial tube; the light in the mercurial tube consequently has the colour of this spiral.

The electric spark is distinctly visible through any coloured glasses, with the colour of the latter. Catoptric colours momentarily illuminated by it appear distinctly, as also do colours of interference when I concentrate the sparks of a self-discharging Leyden flask, by the object-lens of my polarizing apparatus, upon the aperture of the polarizing Nicol's prism; and the calcspar plate, which appears colourless when the Nicol's prism is rapidly rotated in continual illumination, then exhibits the annular system distinctly, and therefore behaves exactly like the coloured sectors of a rotating colour-circle. Whilst the nature of the metals exerted an influence upon the spectrum of the spark, the absorption-phænomena of the rays of the brush remained unaltered, when I developed it from gold, platinum, iridium, nickel, iron, bismuth, tin, zinc and copper, or from a drop of water sprinkled on the conductor; this is in accordance with Faraday's observations. Whilst the introduction of a moist thread essentially modifies the light of the spark, the brush produced by a conductor united with the primary conductor by a wet thread remains unchanged. On the other hand, the lumiPhil. Mag. S. 4. Vol. 14. No. 94. Nov. 1857. 2 C

nosity of a uranium glass occurs with equal vividness with the brushes and sparks. I have found no essential difference between the luminosity of a Ruhmkorff's apparatus and that of an electrical machine, both as regards the sparks and brush in the air, and the luminosity in the electrical egg.

The spark of an electrical machine often appears interrupted at one spot by a weaker violet or reddish light. This interrupted spot generally lies nearest to the negative end; and by removing to an appropriate distance a non-insulated conductor placed near the principal conductor, a stream of sparks may easily be obtained which appears white at the primary conductor, and coloured at the conductor standing near it. This less luminous part is, however, very distinctly visible through a red glass, so that it is distinct from the light of the brush.

The preceding experiments, in connexion with the results of the prismatic investigation of the spark, appear to me to lead to the following conclusion.

A wire becoming red-hot by heat is first red, then orange, and lastly white, so that it behaves like the combination of light which is obtained when a screen is drawn away from the spectrum concealed by it in such a way that the red end first becomes visible, and to this the violet is finally added. The increase of brilliancy from the slightly luminous brush to the bright spark behaves quite otherwise. In this case it is as if the screen removed first set free the violet end, and then the other colours. This distinction of itself renders it improbable that the phænomena of electrical light in the state of less brilliancy can be ascribed to a gradually increasing ignition of solid particles. They rather resemble the weakly luminous flame of hydrogen, which becomes white by solid ignited carbon in the so-called gas-flames, or by other solid matters, as in the Drummond light. The true electrical light is produced at great distances in the surrounding, isolating, aëriform medium, when the latter is attenuated. With this coloured light belonging to the strongly refrangible part of the spectrum, phænomena of ignition may be combined, by particles torn away from the positive and negative bodies. If these particles be only at a red heat, the impression of a violet light is produced by their mixture with the electric light. To this class belong the column of light in the electrical egg, and the basal point of the brush, and lastly, the indented reddish sparks of an electrical machine, at distances to which a white spark does not pass. If particles at a white heat come together, the whole is white, as in the sparks of Leyden jars; in opposition to the bright light of incandescence, the less strongly luminous electric light disappears in the same way as the weak bluish lower part in a gas-flame appears black in opposition to

the bright mass of light, whilst with the small brilliancy of a wax-light the latter betrays its colour even without optical aids of absorption. Only prismatic analysis and the action upon uranium glass indicate the presence of the electric light also. If the particles at a white heat do not reach each other, the spark acquires a spot of interruption, which, however, still shows red light besides the true electric light, when the particles previously at a white heat have become cooled to redness. The basal point of the brush, which retrogrades in proportion to the larger field in which the electric light becomes visible, is to be compared with the spot of interruption of the spark; the particles of the solid body which are here still red-hot may, on reaching a greater distance, be completely extinguished, so that then the electric light alone prevails. The brush could not be coloured by a spirit-flame coloured yellow with chloride of sodium held under it, as it then becomes converted into a spark. The phænomena of the exhausted tube with mercury indicate the modification which the electric light undergoes in media other than atmospheric air.

XLV. Proceedings of Learned Societies.

ROYAL SOCIETY.

[Continued from p. 314.]

March 19, 1857.- Dr. W. A. Miller, V.P., in the Chair.

HE following communications were read:

THE

"On the Action of Aqueous Vapour in disturbing the Atmosphere." By Thomas Hopkins, Esq.

In this paper it was maintained that the great disturber of the equilibrium of atmospheric pressure is the aqueous vapour which is diffused through the gases. These gases, when ascending, cool (say 5°) through expansion by diminution of incumbent pressure, whilst the vapour that is within them cools only 1°; and a consequence is, that when a mixed mass ascends, the vapour is condensed by the cold of the gases. It is well known that condensation of vapour gives out much heat, and this heat warms and expands the gases when they are forced to ascend, taking vapour with them; and the process being repeated and continued, an ascending current is produced in the atmosphere, cloud is formed, the barometer sinks, rain falls, and winds blow towards the part.

This was shown to take place in all latitudes, producing disturbances great in proportion to the amount of vapour condensed. In tropical regions, where the aqueous material is abundant, the disturbances are great, but take place principally in the higher regions of the air. The diminution of atmospheric pressure within the tropics at the surface of the earth, as measured by the barometer, extends over large surface, but is not great in any one place. In