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final result by cλ. In this case the forces causing the rotational phenomena are the same for all directions of propagation of the wave, but they only produce a visible effect (unless e is much larger than one could expect) on waves propagated nearly in the directions of the axes. The two axes will possess similar properties. If we replace To by a linear vector function of V, we must replace y by a linear vector function of λ, say X, in the final result. The axes will now in general have different properties. If Sa0a and SB08 have opposite signs, the rotations of plane-polarized beams proceeding along the axes will be in opposite senses.

8. We may expect to find phenomena of the kind discussed. in this paper in biaxial crystals of substances that possess rotatory power when in a state of solution. I have examined plates of cane-sugar cut perpendicular to an axis and from ·4 to 8 cm. thick. The plates were held between two nicols, one of which was provided with a graduated circle and index, and examined by sodium light without using a converging system of lenses (a better arrangement would be-polarizer, plate, object-glass of telescope, eyepiece, and analyser, since with the thicker plates used the rings are inconveniently small when viewed with the naked eye). The brush was not black where it passed the axis; but this part of the brush could be made black by turning the analyser in one direction, while turning it in the opposite direction made this part of the brush fainter. The angle of rotation necessary to make the brush black at the centre was read off the circle, and the rotation per centimetre calculated. The results are, for that axis which is approximately perpendicular to the cleavageplane 22° 2° to the left, and for the other axis 64° 6° to the right. The same coefficient has values 217° for quartz, 31° for sodium chlorate, and 10° 2 to the right for amorphous sugar (by extrapolation from concentrated solutions); so that the rotation observed for crystallized sugar is more of the order of magnitude of that found in crystals than that found in amorphous solids or in liquids. In the former case the rotation can only be attributed to some spiral arrangement of the molecules in the crystal (cf. Reusch's artificial quartz composed of films of mica arranged spirally). In the case of the sugar crystal this arrangement is unlikely, from the nature of the symmetry of the monoclinic system of crystals. A possible explanation both of the greater rotatory power of the crystal and of the difference of sign in the case of the two axes, is that the sugar molecule acts differently on polarized light which falls on it in different directions. These differences could be seen when the molecules are arranged in an orderly

manner in a crystal; but in solution we have to deal with the average effect for all possible directions. This average will naturally be much less than the maximum value.

If the plate of sugar is examined by circularly polarized light, a single spiral is seen, resembling one of the two seen in the corresponding experiment with quartz. With a plate 17 inch thick, cut perpendicularly to the axis with the stronger rotatory power, half a turn of the spiral was seen, resembling the written letter c. With a thicker crystal a complete turn was seen. The spiral can be seen equally well in white or in monochromatic light.

The method with a mirror, often used for the observation of Airy's spirals in quartz, is unavailable without modification, for, since the sugar crystal is not symmetrical about an optic axis, the two rays, before and after reflexion, must pass in the same direction through the plate, not merely in directions equally inclined to the axis which is under observation. This difficulty can be overcome and the spiral observed by the following arrangement of apparatus, the pieces being named in order from above downwards :-Nicol prism with its axis vertical, glass plate inclined to the horizontal at the polarizing angle, plate of sugar, lens of about 1 inch focus, horizontal glass mirror with its plane accurately passing through the principal focus of the lens. The plate of sugar should be approximately at the other principal focus of the lens, and the inclined glass plate and the nicol should be as close to the plate of sugar as possible. The course of the light is the same as in the Norremberg doubler. The spiral was seen with perfect distinctness when a plate 17 inch thick was used. With a thicker plate the spiral was hardly better seen, the decrease in apparent size due to the increased thickness apparently counterbalancing the improvement due to the increased rotation. The phenomenon was equally clear in white and in monochromatic (sodium) light. The spiral could only be seen inside the first ring, and was like a perverted S. Faint brushes commenced at the first ring.

This method of observation can with advantage be used with quartz also. The lens mentioned above must be replaced by one of shorter focus, or by a converging system of lenses, and a new lens (or system) must be placed over the quartz. The quartz can then be held in the hand and inclined as desired in order to see the various parts of the system of spirals to the best advantage.

Sugar crystals are liable to be irregularly crystallized; in the above experiments crystals that did not show the rings as complete circles in monochromatic light were rejected.

Similar experiments were made with Rochelle salt, and the spirals observed with a plate 1.75 cm. thick. Monochromatic light was found to be essential. The rotation was 12 deg. per cm. to the right. The value calculated from the specific rotation of a 20-per-cent. solution is 4 deg. per cm. in the same sense. On account of the symmetry of the crystal no difference between the two axes is to be expected.

XXXVII. The Spectra of Hydrogen, and some of its
Compounds. By Prof. JOHN TROWBRIDGE.
[Plate VI.]

Na late paper t I expressed the conviction that the so

apart from the spectrum of water-vapour; and that one can never be sure that one is observing, with a condenser discharge, a pure spectrum of hydrogen. I am convinced from further experimentation, that this conclusion is correct; and I am also led to the conclusion that a certain amount of watervapour is essential in all electrical discharges through gases.

Just as aqueous vapour seems to play an important rôle in most chemical reactions, so, it seems to me, its presence in rarefied gases, contained in ordinary glass tubes, enables a dissociation to take place which determines the strength and character of the electrical discharges.

I am led, moreover, to the conclusion that pure hydrogen is a perfect insulator; and that the passage of electricity through a gas depends upon the dissociation of the hydrogen and oxygen, by means of which change in the distribution of energy the gases are made luminous. Before proceeding to an account of my experiments, I will state some of the grounds upon which I base my belief that pure hydrogen is an insulator of electricity.

V. Schuman, in an important paper ‡, has shown that a column of pure hydrogen at atmospheric pressure transmits the ultra-violet rays as well as the most perfect vacuum he has been able to obtain. Now Maxwell's electromagneti theory of light demands that the space between us and the sun, or, in other words, the vacuum of space, should be perfect insulator, otherwise the electromagnetic waves woul be completely absorbed, and the earth would remain i darkness. This observation of Schuman seems to me on

*Communicated by the Author.

† Phil. Mag. Sept. 1900.

Ann. der Physik, 1901.

of the most important in physical science; for it proves, I believe, incontestably that hydrogen cannot be a conductor. Professor Dewar has also shown that liquid hydrogen is an insulator. The experiment sometimes shown, in which a wire, rendered incandescent by a current of electricity and surrounded by an atmosphere of carbonic dioxide, is suddenly diminished in brilliancy by supplanting this atmosphere by one of hydrogen, can be explained, in my opinion, not by the better conductibility of hydrogen for heat, but by the increased resistance of platinum due to the occlusion of this gas by platinum. A palladium wire increases often as much as 50 per cent. by the occlusion of hydrogen; and a platinum wire also shows a similar increase of resistance.

The increased length of the electric spark in an atmosphere of hydrogen is not due to an increased conductibility, but to a dissociation of water-vapour which is analogous to the dissociation which takes place in a voltaic cell.

These are some of the facts which lead me to believe that hydrogen is an insulator, and that water-vapour therefore plays a controlling part in the passage of electricity through gases. I am conscious that the conclusions in this paper are somewhat radical, and I have, therefore, worked assiduously during the past three years to test them in every way which my mind suggested; for it is not probable that many investigators have at present twenty-thousand storage-cells which would enable them to repeat my experiments. The strength of currents and the voltage I have employed have certainly reached the limit of glass tubes to withstand such powerful discharges. The form of tube figured in my previous article * is the only one which I have found capable of withstanding steady currents of one-tenth to one-fifth of an ampere, and instantaneous condenser discharges of many hundred amperes. The great advantage of the use of a storage-battery over the employment of a Ruhmkorff coil in the study of the ionization and molinization of gases is now generally recognized. This advantage is forcibly seen in the first experiment which I will bring forward in support of my view of the importance of the rôle played by water-vapour in the passage of electricity through gases. A wide tube, of the type I have referred to, the narrow portion being approximately 1 centimetre, was provided with massive copper ring electrodes, 1 inch in outside diameter and one-eighth of an inch thick, which were heavily electroplated with copper in order to avoid the impurities of commercial copper. The glass tubes.

* Phil. Mag. Sept. 1900.

were then exhausted and filled with hydrogen made by the electrolysis of distilled water and phosphoric pentoxide. The gas was sent through tubes filled with caustic potash and many drying-tubes filled with phosphoric pentoxide. The gas was kept in the drying-tubes many hours, and its flow was delayed by partitions of glass-wool: more than a litre of the gas was used in the process of flushing out the spectrumtubes, so that the entire pump and connecting-tubes were for several hours presumably filled with hydrogen gas.

When the tubes, having been exhausted to the most luminous stage, were excited by a condenser-discharge and were examined by a straight-vision spectroscope, the ordinary fourline spectrum of hydrogen alone seemed to be present. When, however, the invisible portion in the violet was photographed, the bands beginning approximately at wave-lengths 3900 and 4315 were invariably present, unless the tube had been maintained, during the process of filling, at a temperature of more than 350° C. After such a process of heating the spectrum became that represented in fig. 2 (Pl. VI.), while before heating it was that shown in fig. 1. In both figures the normal spectrum is above the gaseous spectra. Further toward the ultra-violet under all conditions there were also faint nitrogen bands. Long heating diminished the strength of these bands. This process of experimentation shows that mere eye-inspection of glass tubes filled with rarefied gases is generally fallacious; we might conclude from this eyestudy that the presence alone of the four-line spectrum of hydrogen denoted that we had this gas in a pure state; whereas the photography of the invisible portion would show that this was far from the truth.

When the glass tubes filled with rarefied hydrogen were submitted to the influence of a steady current of electricity, it was found that perfectly pure copper was deposited in a lustrous state on the glass walls of the tube which surrounded the negative terminal, while an olive-green oxide of copper covered the walls around the positive terminal. When the same tube was excited by a Ruhmkorff coil, no difference could be detected in the deposits around both terminals: they were both rusty-green, with here and there it may be streaks of pure copper. The mirrors produced by a strong, steady current at the negative terminal were very lustrous, and showed no trace of an oxide of copper. It was evident that the current had dissociated water-vapour in the presence of an excess of hydrogen, and had reduced the copper at the negative pole, and had set free oxygen at the positive pole which had, in turn, combined with copper. The rarefied gases thus acted like a voltaic cell.

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