The blue tints are strongest upon the angles of the fragments, and upon the solid angles of the fissures.

And I may call attention to a similar distribution of colours to be observed in large crystals and specimens of massive dark purple fluor, which have their colours unequally conferred upon the surface, some portions being nearly white, other parts having faint tints of violet, purple, or blue; while towards the edges and solid angles of the crystals, the colours increase to intensity.

If massive dark fluor be broken into fragments, some of those may be selected which are scarcely tinted, except upon the edges and surfaces of the differently crystallized portions just separated, and upon these parts intense colour resides.

I took a large mass of purple fluor, weighing several pounds, and a portion was broken from a large cubic crystal, which was deep purple in the solid edges and angles, while the internal part near to the centre of the external planes was nearly white, the crystals having a mottled appearance; the white portion was highly phosphorescent: calcined in a crucible to redness, and subjected to electricity, no colour was produced, although it became highly phosphorescent.

Fluor spars with different colours were electrified in their natural state, but no alteration or addition of colour was remarked, excepting the dark purple fluor, whose depth of tint was increased.

It is a curious circumstance, that those portions of fluor which are naturally the most coloured, are also, when rendered white by heat, recoloured with the greatest facility by electricity; and as the latter power would appear to confer colour only by modifying in some way the arrangement of the particles, may not natural fluors owe their colours to structure? And may we not be allowed to suppose that nature used the same means, and that ELECTRICITY confers colour and phosphorescence in the first instance? Both the natural and the induced colours are destroyed by heat; and the colour, like the phosphorescence, may be repeatedly restored by electricity. I may now, perhaps, venture to draw the following conclusions from the experimental details advanced, which have proved electricity to be efficient in the restoration of phospho

rescence.

From the very feeble phosphorescent effects obtained by exposing substances to the intense light of the discharge, and also to the constant current of voltaic electricity, it is inferred that light, and great quantity of electricity, are not essentially necessary, but that the effects are due to electricity of great intensity, and hence the influence of the discharges of ordinary electricity.

As electricity itself does not permeate glass, the effects upon substances hermetically sealed up may be thus explained: when the outsides of the glass tubes are electrified by the intenseness of the discharge, a corresponding state is simultaneously induced upon the interior surface, and the contiguous substances are rendered phosphorescent by the so excited electricity.

The colours of bodies, generally, are believed to be due to peculiar structures capable of decomposing light, and reflecting particular coloured rays.

Since by experiment I have shown that colorific structure obtains in certain varieties of colourless fluors, as the result of intense electrization; and as electricity, under various conditions, manifestly commands the relations of molecules and masses of matter, by effecting, destroying, or suspending their combinations, may it not be advanced, that when matter (such as calcined fluor) which is not phosphorescent, is exposed to electric discharges, that they cause vibrations of the particles, which, being repeated with every discharge, gradually modify the structure, and bring it into a peculiar state? May not the action of heat allow this state to return to what it was originally; and from the vibrations of the atoms of matter in changes of structure proceed the undulations fitted to produce light?

This explanation appears to me to be in perfect conformity with the received laws and actions of light, heat, and electricity; and also with the conditions of the earthy substances.

Other causes, competent to these alternating changes of structure, may exist besides heat and electricity, but the above view seems to apply to the phenomena of phosphorescence generally. The alteration of phosphoric colours after some time may be regarded as consequent to the variations of atmospheric temperature having been sufficient so far to alter

the position of the particles, that when heat is ultimately applied, the vibrations produced are fewer and comparatively weaker,

Note. Since my previous communication, I have been informed of a work devoted to phosphorescence, and also of an article in Gmelin's Chemistry, both in the German language*. On referring to the abstract contained in parts of the latter work, it appears that electricity has been employed with phosphori; and that certain bodies, phosphorescent by heat, whose property had been destroyed by calcination, had the property restored by electric shocks: any doubt upon the subject might perhaps be decided by consulting the original authority. My attention has also been called to some experiments by Mr. Skrimshire (Encycl. Metrop., Art. Electricity, §. 177), in which transient phosphorescence was conferred upon different substances by drawing sparks from them, or passing electrical discharges over them. The eyes were kept closed until the sound of the discharge was heard, and the light then observed. I am not acquainted with the detail of these experiments, and my own train of investigation was conducted independent of them, and was nearly concluded before I became aware of any similar inquiry.

ON THE DARKNESS BETWEEN THE PRIMARY AND SECONDARY RAINBOWS.

BY MR. AINGER.

[In a Letter to M. FARADAY, Esq., F.R.S., &c.]

MY DEAR SIR, 10, Doughty-street, Oct. 1830. IN consequence of your remark a few days since, that you had not seen a satisfactory explanation of the darkness between the primary and secondary rainbows, I have referred to several of the most accessible works on the subject, and I find that the phenomena of the rainbow are in general very imperfectly, and in many cases very incorrectly, described. I do not discover that the darkness in question, though sufficiently obvious, is ever alluded to; nor does it appear to me

* Placidus Heinrich, Phosphorescenz der Korper, vol. iv. Gmelin's Handbuch der Chemie, part i.