fore of the light, is increased in proportion as the carbons are consumed. Such inequality of illumination is not experienced in the Rapieff system. Nor is it experienced in using the ingenious regulators of M. Lontin, in which the resistance is kept constant irrespectively of the length of the carbon rods.

In another of M. Rapieff's lamps, the two pairs of carbon rods are placed not one above the other, but side by side. The arc is produced at the junction of the four points, and the effect is considerably increased by the presence of a cylinder of lime, which is placed above the light, and contributes by its incandescence to increase the intensity of the light by as much as forty per cent.

In the various forms of electric lamp hitherto described in this article, the carbon pencils have been separated to a certain distance across which the voltaic arc is produced. A form of lamp has, however, been recently devised by Mr. Richard Werdermann, in which the electric light is produced while the carbons are in direct contact. The lamp is therefore reduced to extreme simplicity of construction. In the ordinary arrangement in which the two pencils are of equal sectional area, the end of the positive carbon is worn into a crater-like shape, and from this pole the greater part of the light is emitted; on the other hand, the negative carbon is formed into a cone and becomes but slightly luminous. Werdermann found that by increasing the sectional area of the negative electrode, its consumption is diminished, and if it be sufficiently large it suffers no appreciable loss during the passage of the current. He therefore uses in his lamp two carbons which are extremely different from each other both in size and in shape. The negative carbon is a disc flat on one side and curved on the other, its shape being not unlike that of a bun. The diameter of this disc is about two inches, and its thickness one inch. The curved surface of the disc is directed downwards, and against this surface the positive carbon is pressed. This carbon is in the form of a thin pointed pencil, three millimetres in diameter. It is held by means of a spring collar in a metallic tube, in which it slides vertically up and down. Constant pressure against the curved surface of the negative electrode is secured by an arrangement of cords passing over pulleys and attached to a counterpoise. On the passage of the current, a very small electric arc is produced, but it is remarkable for steadiness and for purity of colour. It may be obtained from an electro-motor of very weak power. Indeed in some experiments recently made at the British Telegraph manufactory in the Euston Road, London, the electricity was obtained from a small Gramme machine, driven by an engine of two horse-power and having an electro-motive force equivalent to only about four Daniell's cells. Yet with this weak power, ten electric lamps were placed in circuit at once. The light, even with large lamps of 300 candles, is of so soft a character that it appears unnecessary to protect it with globes

of opal glass. This light is to be used experimentally in front of the Mansion House and the Royal Exchange.

A lamp, not altogether unlike Mr. Werdermann's in principle, has been constructed by M. Regnier. In this apparatus a carbon. pencil presses directly against the edge of a circular disc of carbon, which revolves in a vertical plane. The pencil forms the positive electrode, and the current enters not far from the pointed extremity in contact with the disc. As the carbon burns away it is urged forwards by simple mechanism, and thus contact is never interrupted. The residuum, or ash, left by the combustion of the positive carbon is continuously removed by the rotation of the negative disc. It is said that this lamp gives a clear light with only a small electromotive force, and that several lamps may be successfully operated by the same current.

A novel form of electric lamp has been patented by Mr. W. Wallace, of Ansonia, Connecticut, and has been recently exhibited in London with much success by Messrs. Ladd and Co., the British agents for the apparatus. The peculiarity lies mainly in the shape of the carbons, which, instead of being either pencils or circular discs, as in other lamps, take the form of rectangular slabs, each about nine inches in length and three inches in breadth. The thickness varies in the two electrodes, the positive carbon, which is placed above, being about half an inch in thickness, while the negative carbon, placed below, is only about a quarter of an inch thick. These two slabs of carbon are in contact only along one edge. As soon as the electric current passes through them it brings into play an electro-magnetic arrangement, which pulls the carbons apart to the extent of about one-eighth of an inch. Across this space the voltaic arc is established, and a light is produced at the point of least resistance between the carbons. The position of the luminous focus may, however, be determined at any desired spot by the momentary insertion of a metallic conductor between the two plates. At the luminous focus the space between the plates gradually widens, in consequence of the combustion of the carbons, and the resistance therefore increases at that spot. A time is soon reached when the current refuses to cross this resistance, and prefers to establish itself at a neighbouring point which offers less resistance. In this way the light slowly travels from end to end along the edges of the carbons; but when it reaches the last point it makes a turn, and slowly marches from point to point in the opposite direction. The distance between the two edges is maintained constant by appropriate mechanism, and the light is thus kept of uniform intensity. It is said that the light can be maintained for one hundred hours without requiring any change of carbon.

When a long conducting wire which conveys an electric current is suddenly broken, a brilliant spark momentarily appears at the point of separation. This spark is produced with a feeble current, even with a current much too weak to sustain an appreciable arc,

and therefore to be used for the electric light. However, according to the Journal of the Franklin Institute, Professors Elihu Thompson and E. J. Houston have availed themselves of this spark for purposes of illumination. A pair of carbon pencils is mounted vertically, but while the positive carbon, as usual, is fixed, the negative carbon is capable of vibration. At first the two pencils are in close contact, and the current passes, of course, through them; but the movable rod by its motion breaks contact, and a spark immediately appears. Before the impression made by this spark upon the retina has faded away, the oscillating carbon springs back, whereby contact is momentarily renewed, and as momentarily broken; another spark then appears, and as these sparks succeed each other with great rapidity, they give rise to a continuous sensation of light. An electric light may thus be obtained with a battery much too feeble to produce the light in its ordinary form.

Many of the recently devised systems of electric illumination, which have been briefly described in this article, promise to accomplish, more or less successfully, that great object which has so often proved a stumbling-block to the inventor-the divisibility of the light. It seems paradoxical to say that the great disadvantage of the electric light lies in its excessive brilliancy. Yet that is really the case. You get either more light than you want, or an insufficient light. To temper the intensity of the electric light it is common to use shades of ground or opaline glass; but the production of an intense light to be afterwards deadened is obviously a wasteful process. It is not until the strong light can be economically divided into several lights of moderate intensity that it stands a chance of becoming the domestic light of the future.

But if an electric light of moderate power can be cheaply obtained, its advantages over gas, as at present burnt, are beyond question. The electric light, for instance, does not vitiate the surrounding atmosphere as ordinary combustion vitiates it. The carbon-points burn away, it is true, and thus consume oxygen, and produce carbonic-acid gas; but the action is insignificant compared with that which takes place during the production of the same amount of light from candles or oil or gas. Moreover, the electric light, if necessary, can be produced in a closed vessel from which air is excluded; and thus the surrounding atmosphere may be kept perfectly free from contamination. Again, the electric light is recommended by its exceptional purity. In a gas-flame the yellow rays predominate, and hence it becomes impossible by gaslight to distinguish, say, a bluish-green from a greenish-blue. But by the electric light colours are much more accurately discriminated. Hence its great value in picture galleries, in dye-works, or in warehouses and shops where coloured goods have to be examined at night.

Any general sketch of electric illumination would be incomplete without a passing reference to another mode of obtaining light by

electricity, which without promising to become of economic importance is nevertheless of great theoretical interest. It is well known that when an electric discharge takes place through a gas or a vapour in a rarefied condition, luminous effects of singular beauty are produced. The phenomena are best seen in Geissler tubes-so named after an artist in Bonn who originally devised them. These are hermeticallysealed glass tubes enclosing various gases in a highly attenuated state, through which the sparks from an induction coil can be passed by means of platinum electrodes fused into the glass. On the passage of the current a soft and delicately-tinted light streams through the tube from pole to pole. As this light is due to incandescence of the molecules of gas, it is not surprising that it should diminish if the rarefaction be carried too far. Many of the tubes contain fluorescent substances, such as uranium-glass, which glow with great beauty during the discharge, and thus contribute to the luminosity. Although the electric light in the Geissler tubes is too feeble to be employed for ordinary purposes of illumination, it has yet received special applications which are not without interest. Thus, it has been used to a limited extent by medical men in examining the condition of any cavity of the body into which it becomes possible to introduce a properly-shaped tube. It has also been suggested to use it in coalmines where fiery seams are being worked; and for this purpose an ingenious lamp has been constructed by MM. Dumas and Benoit. The light, however, is much too weak, and the apparatus much too fragile, to be used for ordinary purposes; though it is possible that an electric lamp on this principle might, under special circumstances, occasionally be of service. It has also been proposed to employ Geissler tubes in gunpowder factories, and, to a limited extent, for submarine illumination.

Although the electric light has lately eclipsed most other scientific subjects, it would be wrong, in chronicling the progress of science, to overlook the recent announcement by M. Marc Delafontaine of his discovery of two metals. These have been obtained during his examination of the Samarskite of North Carolina, which contains the associated oxides of several rare metals, such as yttrium, erbium, and terbium. In the course of an elaborate investigation carried on for the last two or three years, Delafontaine has obtained an oxide which in colour and in density stands intermediate between yttria and terbia, and which he feels justified by his spectroscopic researches in regarding rot as a mixture of these earths but as the oxide of an independent metal. I therefore announce definitely,' he writes, the discovery of the oxide of a new metal.' For this metal he proposes the name of Philippium in honour of M. Philippe Plantamour, of Geneva, the friend and pupil of Berzelius. A solution con

'On a new metal-Philippium,' Chemical News, October 25, 1878, p. 202. 'On Decipium: a new metal from Samarskite,' ibid., November 8, p. 223.

taining philippium gives a magnificent absorption-band in the indigo, which is not seen in solutions containing terbium, erbium, or yttrium -the metals with which philippium is likely to be confounded.

The second new metal of this group has been christened Decipium, from the Latin decipiens. The decipium has not yet been completely separated from the associated oxide of didymium; but several of its salts have been prepared. The nitrate of decipium gives an absorption-spectrum composed of three bands in the indigo and the blue, differing in position from those of any other known metal.

The Schwendenerian theory of the structure of lichens has probably been the exciting cause of more controversy of late years than any other subject in scientific botany. According to this bold and happy generalisation, for which we are indebted to De Bary and Schwendener, lichens are not autonomous organisms, forming a group intermediate between alga and fungi, but are true ascomycetous fungi, exhibiting the extraordinary peculiarity of being invariably parasitic upon, or rather commensal with, some species of alga. Thus the hyphae of the fungus itself—the colourless tissue of the lichen thallusafford protection to their hosts; while the latter-the green gonidia of the lichen-supply nutriment for the fungus as well as for themselves, in virtue of the chlorophyll they contain.

Of the objections to the theory perhaps the most important was the statement of several authors that they had observed the budding of the green gonidia from the colourless hyphæ. But this and minor objections seem to be completely set aside by the recent researches of Stahl, who may fairly be said to have proved the theory. His observations show, first, that the spores of lichens produce the hyphal or fungoid portion of the plant, and that only; and, secondly, that the hitherto unknown process of sexual reproduction is wholly independent of the gonidia. The more important facts upon which his conclusions are based are the following:

The spores of lichens are found in spore-cases or asci, occurring in the well-known cup or saucer-shaped receptacles, and separated from one another by layers of hypha, among which often occur peculiar goridia of a smaller size and lighter colour than those of the rest of the plant, known as hymenial gonidia. When the spores are expelled from the asci, each is found to be surrounded by a number of these bodies, mechanically adherent to it. Stahl observed directly the germination of the large many-celled spores of Endocarpon under the microscope, and found that many of the numerous hyphæ emitted from the spore came in contact with the hymenial gonidia,

Beiträge zur Entwickelungsgeschichte der Flechten. A detailed account of Stahl's work, together with a critical summary of the more recent phases of the controversy on the Schwendenerian theory, will be found in two papers by Mr. Sidney H. Vines, in the Quarterly Journal of Microscopical Science for April and October of the present year.