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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.

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.

(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

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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continue during a certain time, a quarter of an hour for instance, the diffused Light impresses the remainder of the surface over nearly all parts, so that on examining the paper in the dark, almost the whole surface appears luminous, the parts a, b, c, d being the brighest, but the space from the least refrangible indigo to the lowest edge of the red rays is completely dark. The lower part of the preceding wood-cut shows the effect produced. It would appear from this that the rays of less refrangibility than the indigo have the power of preventing phosphorescence, which exactly accords with Beccaria's

statement.

(456.) If, before exposing the phosphorescent surface to the action of the spectrum, it is exposed for a few seconds to the solar rays, or diffused Light, it becomes luminous in all its parts. If now we project a spectrum upon it for a few minutes, and then examine it in the dark, it will be found that every part remains luminous except the part d, e, which has become dark. If the temperature of the surface is raised by a spirit lamp, all the parts previously luminous become vividly phosphorescent, whilst this part remains completely dark.

(457). By the aid of coloured glass-screens, this is rendered very evident. If a piece of paper prepared with sulphuret of calcium is exposed to daylight, it becomes luminous. By placing immediately on the surface a card which partially covers it, and then a red glass permeable only to the red and orange-coloured luminous rays, and the chemical rays which accompany them, and exposing this arrangement to the sun for a minute, it will be found, on examining the paper in the dark, that the part which has been acted upon by these rays has entirely lost its phosphorescence, whilst every other part is still luminous. The same effect takes place if sulphuret of barium (Bologna phosphorus) is used.

(458.) These experiments distinctly prove that the obscure rays of the spectrum produce phosphorescent

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