penumbra to increase greatly in size. Experiments recorded in the paper already quoted have proved that the velocity of the molecules is greater as the vacuum gets higher, and consequently the trajectory of the molecules under deflecting action, whether of a magnet or of an insulated idle pole, is flatter at high than at low vacua.

An experiment is next described, having for its object to ascertain whether two parallel molecular rays from two adjacent negative poles attract or repel each other. It is considered that if the stream carries an electric current, attraction should ensue, but if they are simply streams of similarly electrified bodies, the result would be repulsion. Experiment proves that the latter alternative happens, lateral repulsion taking place between two streams moving in the same direction.

Many experiments are given to illustrate the law of action of magnets on the molecular stream, but the results are of too complicated a character to bear condensation without the diagrams accompanying the original paper.

The molecular stream is sufficiently sensitive to show appreciable deflection by the magnetism of the earth.

The author, after numerous experiments, has succeeded in obtaining continuous rotation of the molecular stream under the influence of a magnet, analogous to the well-known rotation at lower exhaustions. Comparative experiments are given with a "high vacuum" tube, where no luminous gas is visible, but only green phosphorescence on the surface of the glass, and a "low vacuum" tube, in which the induction spark passes in the form of a luminous band of light joining the two poles. These two tubes are mounted over similar electromagnets, the direction of discharge being in a line with the axis of the magnet. Numerous experiments, the details of which are given in the paper, show that the law is not the same at high as at low exhaustions. At high exhaustions the magnet causes the molecular rays to rotate in the same direction, whether they are coming towards the magnet or going from it; the direction of rotation being entirely governed by the magnetic pole presented to the stream. The north pole rotates the molecular discharge in a direct sense, independent of the direction in which the induction current passes. The direction of rotation impressed on the molecules by a magnetic pole is opposite to the direction of the electric current circulating round the magnet. These results offer an additional proof that the stream of molecules driven from the negative pole in high vacua do not carry an electric current in the ordinary sense of the term.

The author, after giving details of experiments in which platinum and glass are fused in the focus of converging molecular rays projected from a concave pole, describes observations with the spectroscope,

*Like the hands of a watch.

which show that glass obstinately retains at even a red heat a compound of hydrogen-probably water—which is only driven completely off by actual fusion.

The permanent deadening of the phosphorescence of glass is shown by projecting the shadow of a metal cross on the end of a bulb for a considerable time. On suddenly removing the cross, its image remains visible, bright upon a dark ground.

One of the most striking of the phenomena attending this research is the remarkable power which the molecular rays in a high vacuum have of causing phosphorescence in bodies on which they fall. Substances known to be phosphorescent under ordinary circumstances shine with great splendour when subjected to the negative discharge in a high Thus Becquerel's luminous sulphide of calcium has been found invaluable in this research for the preparation of phosphorescent screens whereon to trace the paths and trajectories of the molecules. It shines with a bright blue-violet light, and when on a surface of several square inches is sufficient to faintly light a room.

vacuum.

The only body which the author has yet met with which surpasses the luminous sulphides, both in brilliancy and variety of colour, is the diamond. Most diamonds from South Africa phosphoresce with a blue light. Diamonds from other localities shine with different colours, such as bright blue, apricot, pale blue, red, yellowish-green, orange, and pale green. One very beautiful diamond in the author's collection gives almost as much light as a candle when phosphorescing in a good vacuum.

Next to the diamond alumina and its compounds are the most strikingly phosphorescent. The ruby glows with a rich full red, and it is of little consequence what degree of colour the stone possesses naturally, the colour of the phosphorescence is nearly the same in all cases; chemically prepared and strongly ignited alumina phosphoresces with as rich a red glow as the ruby. The phosphorescent glow does not therefore depend on the colouring matter. E. Becquerel* has shown by experiments with his phosphoroscope, that alumina and many of its compounds phosphoresce of a red colour after insolation.

Nothing can be more beautiful than the effect presented by a mass of rough rubies when glowing in a vacuum; they shine as if they were red hot, and the illumination effect is almost equal to that of the diamond under similar circumstances.

Masses of artificial ruby in crystals, prepared by M. Ch. Feil, behave in the vacuum like the natural ruby.

In the spectroscope the alumina glow shows one intense and sharp red line less refrangible than the line B, and a faint continuous spectrum ending at about B. The wave-length of the red line is 6895.

* “Annales de Chimie et de Physique," 3rd series, vol. Ivii, p. 50.

The paper concludes with some notes by Professor Maskelyne, on the connexion between molecular phosphorescence and crystalline structure. The crystals experimented on have been the diamond, emerald, beryl, sapphire, ruby, quartz, phenakite, tinstone, hyacinth (zircon), tourmaline, andalusite, enstatite, minerals of the augite class, apatite, topaz, chrysoberyl, peridot, garnet, and boracite. Of these, the only crystals which give out light are diamond, ruby, emerald. sapphire, tinstone, and hyacinth. The light from emerald is crimson, and is polarised, apparently completely, in a plane perpendicular to the axis. Sapphire gives out a bluish-grey and a red light polarised in a plane perpendicular to the axis. The ruby light exhibits no marked distinction in the plane of its polarisation.

Among positive crystals tinstone glows with a fine yellow light, polarised in a plane parallel to the axis of the crystal. So far the experiments accord with the quicker vibrations being those called into play. and therefore in a negative crystal the extraordinary, and in a positive crystal the ordinary, is the ray evoked. Hyacinth, however, introduces a new phenomenon, being dichroic, the colours, in three different crystals, being pale pink and lavender-blue, pale blue and deep violet, and yellow and deep violet-blue, polarised in opposite planes.

The only conclusion arrived at is, that the rays, whose direction of vibration corresponds to the direction of maximum optical elasticity in the crystal, are always originated where any light is given out. As yet, however, the induction on which so remarkable a principle is suggested, cannot be considered sufficiently extended to justify that principle being accepted as other than probable.

VI. "Note on a Direct Vision Spectroscope after Thollon's Plan, adapted to Laboratory use, and capable of giving exact Measurements." By G. D. LIVEING, M.A., Professor of Chemistry, and J. DEWAR, M.A., F.R.S., Jacksonian Professor, University of Cambridge. Received April 3, 1879. Having seen in the "Journal de Physique" for May, 1878, the account of M. Thollon's ingenious direct vision spectroscope, it occurred to us that by a little modification we could adapt his plan so as to produce an instrument well fitted for the work in which we were engaged, combining the advantage of excellent definition, which his plan secures, with the means of getting exact measurements with the least possible chance of errors of adjustment or inequalities in the working of the automatic system. The principle consists in having two prisms only (half prisms as M. Thollon calls them), of which one is fixed, and receives the light from the collimator by a reflecting

prism and transmits it in a plane at right angles to the axis of the collimator to the second prism.

This second prisim is moveable about an axis parallel to its edge and to the axis of the telescope, and has a right angled reflecting prism attached to it, so that the light after traversing this prism twice passes the second time through the fixed prism and so by reflection into the telescope. The lever carrying the second prism with its reflecting prism is moved by a micrometer screw, by the head of which the movement of the prism is read.

We placed the design in the hands of Mr. Hilger, some time since, and we now exhibit the instrument to the Society.

In the last number of the "Journal de Physique," M. Thollon describes some modification of his instrument, but it does not seem that his modified plan is so well adapted to the ordinary use of a chemical laboratory as ours.

The accompanying diagram represents a section through the prisms at right angles to the axis of the collimator and telescope.

April 24, 1879.

THE PRESIDENT in the Chair.

The Presents received were laid on the table and thanks ordered for them.

The Right Hon. Richard Assheton Cross, Secretary of State for the Home Department, was admitted into the Society.

I. "On the Nature of the Fur on the Tongue." By HENRY TRENTHAM BUTLIN, F.R.C.S. Communicated by J. BURDON SANDERSON, F.R.S., Professor of Physiology in University College, London. Received March 26, 1879.

[PLATES 10-13.]

The fur on the tongue is generally stated to consist chiefly of epithelial cells, usually sodden and granular. But several observers have described fungi as existing in it, or in the buccal mucus. Robin, for instance, describes a form of Leptothrix (L. Buccalis) in the mouth, and particularly in and between the teeth. Kölliker mentions, as of constant occurrence, masses or dark-brown bodies (which had previously been described by Miquel and Neidhardt, as occasionally present) having a granular aspect, which he believed to be of the nature of a fungus, similar if not identical with the fungus affecting the teeth. Billroth speaks of finding in the white fur of himself and of several patients, exquisite palmelloidal forms of Ascococcus and Glæcoccus colonies.

The object of this paper is to show that schizomycetes form the essential constituent of the fur, and to explain, as far as possible, some of the laws which govern the formation of fur.

The tongue is kept clean by free movement and by being rubbed against the interior of the mouth, the gums, and the teeth; but fur almost always exists upon its surface, both in health and in disease. The fur is generally thickest in the morning before food is taken, and during illness, when the necessary cleansing is not properly performed. It occurs, too, most abundantly in the centre and back part of the tongue, covering a triangular area immediately in front of the circumvallate papillæ, for this part of the tongue is most difficult to keep clean. It occupies the papillary surface of the tongue, scarcely ever extending beyond it, and is, therefore, not found posterior to the circumvallate papillæ. It does not form a continuous layer unless it is exceedingly thick, but lies upon the tops of the filiform and some of the fungiform papillæ. In children the fungiform papillæ are usually quite free from fur, but in adults the difference between the fungiform and filiform papillæ is not nearly so well marked, and, with the exception of those situated near the apex of the tongue, the fungiform papillæ are frequently coated. Fur forms upon the filiform papillæ, because these papillæ are rough and possessed of longer or shorter epithelial processes, to which foreign matters cling readily. and from which it is very difficult to dislodge them. The fungiform papillæ, on the contrary, are usually smooth and rounded on the summit, and even when large are easily kept clean.