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pletely polarized, and moreover not equal in every pari of the crystal. Examination with the microscope reveals many little flaws in certain parts of the trace; but I did not succeed in resolving the whole of it *.

I found in another seal of false topaz, from Brazil, the same properties, which are probably characteristic of this variety of quartz.

4. In smoky quartz, when it is not too dark, the trace is in general observed, but in very various degrees. When it is feeble, polarization is complete.

In a very fine brown specimen from Siberia, the defects of homogeneity manifest themselves, through transparence to diffused light, under the form of cloudy strata, indicating unequal distribution of the colouring-matter. With sunlight the trace is very marked, very incompletely polarized, and with a lens a multitude of defects and particles are distinguished. This incomplete polarization is a character always recognized when we operate on a liquid holding in suspension particles too numerous or of too great a volume.

5. A small specimen of amethyst cut in the shape of a prism with three sides, and with faces perpendicular to the edges, gave no visible trace in the greater part of the crystal; but in a few points small flaws are observed.

The study of these phenomena in quartz is often complicated by its property of chromatic polarization. Thus, by making the pencil of light pass, at a little distance from a face, parallel to the axis, and observing the trace with an analyzer, phenomena of coloration or depolarization can be seen, according to the orientation,

Rotatory polarization plays also an important part when the pencil is directed along the axis of the crystal. Here is a remarkable instance :-The second seal of false topaz above mentioned is cut so that its axis coincides with that of the crystal; it presents at its two extremities faces perpendicular to this axis. When a pencil of polarized light enters by one of these two faces to go out at the other, as the substance has a great power of illumination there is reproduced on a small scale the beautiful experiment realized by M. Lallemand wben he employed liquids possessing rotatory power. The phenomenon shows itself in coloured fringes which are displaced in the interior of the crystal when the Nicol is rotated that polarizes the incident light. The

* In truth, observation with the microscope is somewhat difficult : the luminous pencil, and consequently the trace, has always too large a section for the whole thickness to be brought to the point; the diffused light with which the entire field of the microscope is washed is opposed to the visibility of very small particles or defects.

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first fringes, on the side where the pencil enters, are very bright and distinct; they grow confused and are effaced towards the other extremity.

Calcareous Spar.-I have found, like M. Lalleinand, that fine specimens of Iceland spar present no power of illumination properly so called. The trace of a pencil of sunlight manifests itself solely by the slight red fluorescence of this substance; and no polarization is observed. But if we operate on certain crystals which exhibit minute defects, we can also obtain a white trace. The double refraction of the spar complicates the observation of the polarization-phenomena; yet, with some attention, it has appeared to me that the facts are in this case conformable to what could have been foreseen.

Rock-salt.— The only good specimen of rock-salt I have had at my disposal is a very fine prism belonging to the Physical Cabinet of the University of Geneva. In it a pencil of light produces a very slight trace, resolvable with the lens into small bubbles or cavities. The polarization is complete; and the defects are visible or disappear according to the orientation of the analyzer. This is the same as I have already stated for quartz.

In the other, less pure specimens which I have examined, the trace is visible, but unequal, and the polarization normal.

Diamond. I had in possession for a few days some large diamonds which form part of the collection of the Duke of Brunswick, and which had been obligingly lent me by the municipality of Geneva. Unfortunately the season was very unfavourable, and I was only able to avail myself, for the observations, of a few moments during which the sun shone. Besides, the diamonds being cut as brilliants renders the observation very difficult: it gives rise to much false light; and on account of the reflection of the facets, one may readily confound the luminous trace with one of its images.

(1) A white diamond of 39 carats gave rise to a trace very visible and nearly white. The greater part of the light emitted by it is due to fluorescence; but I feel certain that another part is due to illumination properly so called ; indeed the trace is partially polarized.

(2) Some yellow diamonds of 29, 31, 42, and 80 carats gave rise to a very visible bluish trace, principally due to fluorescence, but also presenting partial polarization*.

* To observe the polarization-phenomena, I operated in the following manner. A pencil of sunlight, concentrated by a quartz lens of long focus, entered the dark room; it was polarized in passing through a Nicol, and then fell upon a screen pierced with a small aperture. A slender pencil

I do not hesitate to attribute these phenomena of polarization to foreign particles. We know, indeed, from Dumas and Stas*, that diamonds leave a spongy residue after combustion ; and Sir D. Brewstert recognized with the microscope cavities of various shapes in their ivterior.

Ice.- When very clear and compact, it exhibits very little capacity of illumination. When floating in water in a large glass vessel through which the pencil of rays passes, the trace is very prouounced in the water, but almost invisible in the ice. Cutting a piece of ice into a suitable shape, I had no difficulty in distinguishing in it the trace, which, when viewed through a lens, is seen to resolve itself into minute particles. It is much less visible than that of the purest water I succeeded in obtaining by distillation I; its polarization is complete S.

Alum.--A crystal of alum, in which I had faces cut at right angles, gives a trace in the parts where flaws are present, but in other parts does not appear to have any capability of illumination.

Gypsum.- I cite from memory a specimen of gypsum from the collection of M. Alph. Favre. I examined it, a long time ago, with the oxyhydrogen light only, and did not recognize in it any trace.

The whole of these observations appear to me to well establish :

1st. That in non-fluorescent crystals the crystallizable matter by itself is destitute of illuminability.

2nd. That crystals which possess the faculty of being illuminated by returning polarized light laterally, owe that property either to the presence of foreign particles or to defects of crystallization.

of polarized light was thus obtained, which I caused to enter into the diamond through the large front face of the brilliant, and to emerge through the parallel facet (the collet) which generally terminates the rear of a brilliant. The trace was then viewed with a Nicol, through one of the inclined faces that form the posterior pyramid. In these conditions, on turning the Nicol, differences of intensity are observed if the trace is partially polarized.

* Annales de Chimie, 1844, vol. i. p. 15.
† Trans. of Royal Society of Edinburgh, vol. xxiii. p. 1.

For the purification of water by distillation, see Archives, 1870, vol. xxxvii. p. 146.

f I am here speaking of the naturally formed ice of ponds; there are great differences, according to the specimens. I recall here that Tyndall indicated water proceeding from the melting of ice as the most exempt from particles and illumination-power that he could obtain.

VII. On the Spectrum of the Aurora. (Extract from a Letter from Professor A.S. HERSCHEL to R. H. Scott, Esq., F.R.S.*)

THE paper by Ångström, noticed in ‘Nature' (vol. x.

No. 246, July 16, 1874), was a rare legacy of his last days to spectroscopists of auroras. Nothing more conclusive and satisfactory, that I know of, has been written on the subject; and little more will be done now, I expect, by future observers than to verify his conclusions and to extend the research in the direction that he points out, in its more numerous details.

The spectrum of the aurora is no doubt in the main the same as that of the pale blue light round the negative pole in an air or nitrogen vacuum-tube, with the induction-spark passing through it. There are so many well marked lines in this spectrum that, looking at Angström's representation of them, it is probably owing to the insignificant appearance of that part of the vacuum-spark that its proper spectrum has not been more frequently studied with reference to the aurora, as Angström seems to have done by an experiment specially adapted for the purpose.

There are several forms or modes (apparently four or five) of electrical discharge through rarefied gases t. When very much rarefied, air transmits the electricity so as to discharge the Rubmkorff poles without a spark. In that state there is still a glow of heated air round both poles, which increases in size and length along the tube as the air-pressure is increased, faster round the positive than round the negative pole. This has been accounted for by showing that the air offers far greater resistance to the passage of electricity when it surrounds a cathode or negative, than when it surrounds an anode or positively electrified pole. The difference becomes more obvious as the pressure and density of the gas are increased. The negative glow shrinks into a very small space, while the positive brush extends through nearly the whole length of the tube, abolishing at last the dark space that at very low tensions separates the two lights from each other. At pressures not exceeding one or two millimetres the positive glow is stratified; but if the pressure is increased it becomes continuous; and if the air-pressure amounts to that of

or an inch of mercury, or upwards, it again gathers into

* Communicated by R. H. Scott, Esq.

† An examination of these with revolving mirrors, by A. Wüllner, at Aix la Chapelle, appeared inothe “ Jubelband” of Poggendorff's Annalen this year at the same time as Angström’s paper in that volume, which also contains some other tracts (by A. de la Rive and others) tracing the effects of magnets and of metallic vapours in augmenting the discharge through air.

Phil. Mag. S. 4, Vol. 49. No. 322. Jan. 1875, F

somewhat larger light clouds; and at about 1 or 2 inches of barometric pressure a spark passes between the poles. This spark is red; it scarcely diminishes the strength of the concomitant glow discharge; and it is far less luminous than the white spark which begins to appear at 5 or 6 inches of pressure, and may often be seen at first broken up along its length into parts which are alternately white and red. The spectra, like the general appearances, of these two forms of the spark are quite distinct.

I do not know if these several phases of the positive part of the discharge have all been examined spectroscopically. They pass into each other according to the shape and size of the tube or flask, as well as the air-pressure; and it is difficult to say how much of each is concerned in those observations which have been made of air-spark spectra in comparisons with the aurora.

No one, so far as I know,, has compared with it the negative-glow spectrum so fully as Angström has now done; and it seems very probable that its peculiar fitness for the comparison has been overlooked-the feat of filling a bottle with the negative glow discharge being certainly a novelty; if it is really true that he succeeded in obliterating the positive brush entirely in its favour.

The next remarkable novelty in the paper is the way in which be proposes to account for the “citron” line of auroras ; for there is evidently nothing of the kind in the negative glow, however well that answers to all the secondary facts of faint blue, red, and greenish lines. If oxygen and its compounds are (as has, I believe, been lately shown) strongly fluorescent, Tait and Dewar have also proved, as shown by some of their experiments this year, that they also possess powers of phosphorescence

-Geissler tubes shining for some time after the spark has passed through them, from the production of ozone during the discharge. When one of the globes of a phosphorescent "garland” tube was heated over a Bunsen flame, that globe which was heated did not shine after the spark had passed, apparently because, as we know, a very little heat is sufficient to destroy ozone. Whatever the way may be in which the ozone or otherwise electrified gas remains self-luminous after the discharge, it seems very reasonable to suppose some action of the same kind (perhaps, as Angström says, simply fluorescence) as common in all auroras, and that this produces the well-known auroral line.

Pocket spectroscopes can, of course, do nothing further to fix the position of the citron line; nor can they alone fix very exactly the places of any of the fainter ones. But as every aurora shows this strong monochromatic light, it might be used to bring out a row of punctures transverse to the slit, as a divided scale in the field of view whereby to map the fainter lines, or at any rate

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