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have received more, and at the points corresponding to the hollows it will have received less; hence, then, there will result a kind of image, but generally invisible. If a vapour be then condensed on this polished surface, we see that it is then under the conditions which I just now mentioned, and that the condensation will take place in a manner visibly different upon the different points, that is to say, the invisible image will become visible." * This very simple method of accounting for these phenomena, has been adopted by Professor Grove, on the strength of some experiments which show that these images may be very readily formed by the action of any volatile body; and a very eminent authority states, that he has found many of the phenomena ascribed to latent Light, or to heat, are owing to the absorption of matter in the state of vapour or minute particles, passing from the object to the surface of the glass or metal upon which the image of that object is impressed; and by this means we have obtained very fine pictures upon glass which are positive when seen by reflection, and negative when seen by transmitted Light."

(450.) Many of the phenomena may be produced in the way above stated; but M. Fizeau is mistaken in applying his results in explanation of the whole class. I have proved in the most incontrovertible manner, that these images may be produced, when by processes of boiling and heating every trace of organic matter must be removed from the surface. And when we consider that these impressions are, under certain circumstances, formed to a considerable depth beneath the surface, we must admit that, although this may be one of the causes, yet that is only one amongst many others, and indeed one of the least energetic of the lot. All these facts, however, substantiate my position that LIGHT is not the agent which acts in darkness, consequently, that Moser's

* Scientific Memoirs, vol. iii. p. 489.

MOLECULAR FORCES.

269

position is untenable. At the same time we must not too hastily dismiss the question, as it is probable that amongst the operations of those diversified powers, we may be enabled to detect the existence of one more important than either, which may enable us to trace out the influence which disposes the molecular structure of bodies, and the relations of matter as displayed in the phenomena of chemical affinity.

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CHAPTER VIII.

ON PHOSPHORESCENCE.

(451.) WE have thus far been considering Light as an emanation from the sun: how far this view is borne out by the facts discovered remains to be decided. There are circumstances which might lead us to consider Light as an essence independent, and universally diffused, and amongst these the most striking are the phenomena of phosphorescence. It has already been stated that Benvenuto Cellini observed it in gems; and a considerable degree of attention was bestowed upon phosphori by the Honourable Robert Boyle. Many minerals have the property of emitting Light when rubbed or broken, and Sir David Brewster has observed the phenomena in upwards of fifty minerals, when they are exposed to a heat below redness in the dark. From these facts we might argue that this essence was capable of existing in an invisible state for any period of time; and that it would, when properly excited, produce the effect of Light.* The eminent authority just now quoted has stated that the phosphoric Light of minerals has the same properties as the direct Light of the sun. This statement must, however, be received with some consideration, if Sir David Brewster means the undecomposed sun-beam. We have no satisfactory evidence which shows that any heat ever accompanies phosphorescent Light, and we have never discovered that it is capable of producing chemical change. By the prism we detect the same number of colours in a phosphorescent beam as in the sun's rays;

Edinburgh Phil. Journal, vol. i.

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and this is all, I presume, that this talented philosopher intended to imply.

(452.) Many bodies after exposure to the solar rays give out Light when in the dark: this is particularly the case with some flowers, as the Nasturtium; and if the human hand is held in the sunshine for half an hour, it will emit Light for some minutes in the dark. The bodies, however, which exhibit this peculiarity in the most remarkable manner are the Bolognian stone, a sulphuret of barium; and Canton's phosphorus, which is prepared by calcining oyster shells and sulphur together. If these substances are exposed to the solar rays, they acquire the property of shining in the dark so strongly, as to enable the observer to distinguish the printed letters on the white page of a book. There are some other substances which exhibit these phenomena: Homberg's phosphorus-the melted chloride of calcium- Baldwin's phosphorus melted nitrate of lime the sulphuret of strontian, &c.

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(453.) Some of the elder natural philosophers particularly examined the solar phosphori, especially Beccaria, who stated that the violet ray was the most energetic, and the red ray the least so, in exciting phosphorescence in these bodies.

Dessaignes has remarked that the solar-phposhori emit the same character of rays, whatever may have been the kind of Light to which they were exposed.* This is not, however, very easily reconciled with M. E. Becquerel's experiments. It has been stated by M. Grotthouss that in some diamonds the most efficacious exciting Light is different from that excited. This observer also noticed that electrical discharges restored the property of phosphorescence in cases where it was destroyed by violent heat.

(454.) M. Edmond Becquerel has particularly examined

* Mem. Inst., tome xi.

the action of the spectrum upon the solar phosphori. The result of his inquiries has been the determination with considerable exactness of the spaces occupied by the rays which impart phosphorescence. He states, that the sulphuret of calcium is rendered phosphorescent by the rays which extend from the indigo to a little beyond the violet; and that two points of maximum intensity are to be detected-one within the visible violet rays, and the other beyond it. The sulphuret of barium exhibits the action of the same rays, but shows only one maximum point, which is situated beyond the violet. Those exciting rays, this observer is inclined to think, have a definite action, and hence he would distinguish them as the "Phosphorogenic rays." M. E. Becquerel has also observed that the rays below the indigo unto the red, or a little beyond it, have the power of destroying the phosphorescence which the more refrangible rays have excited.

(455.) Many of the experiments of M. E. Becquerel are instructive. Paper being covered with gum Arabic is dusted over with the sulphuret of calcium, and exposed to the action of the spectrum. Upon examining the paper in the dark, two luminous bands are visible, a, b, c, d: these spaces correspond with the violet rays, and

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the "invisible chemical rays" so called; a dark space exists between these bands, which mark the region occupied by the extreme violet rays, and the lavender rays of Sir John Herschel. If this action is allowed to

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