The pale light emitted by phosphorus in the dark is well known; and this is evidently only a species of slow combustion, a combination of the phosphorus with the oxygen of the air. Where there is no oxygen, phosphorus will not shine; its combustion in chlorine or iodine vapour is a phenomenon of a totally different character from that which we are now considering. This phosphorescence of animal and vegetable matter has been regarded as something different from the slow combustion of phosphorus; but, upon examination, all the chemical conditions are found to be the same, and it is certainly due to a similar chemical change.
The luminous matter of the dead whiting or the mackerel may be separated by a solution of common salt or of sulphate of magnesia; by concentrating these solutions the light disappears; but it is again emitted when the fluid is diluted. The entire subject is, however, involved in the mystery of ignorance, although it is a matter quite within the scope of any industrious observer. The self-emitted light of the carbuncle of the romancer is realized in these remarkable phenomena.
The phosphorescence of some plants and flowers is not, perhaps, of the same order as that which belongs to either of the conditions we have been considering. It appears to be due rather to an absorption of light and its subsequent liberation. If a nasturtium is plucked during sunshine, and carried into a dark room, the eye, after it has reposed for a short time, will discover the flower by a light emitted from its leaves.
The following remarkable example, and an explanation of it by the poet Goethe, is instructive:—
“On the 19th of June, 1799, late in the evening, when the twilight was deepening into a clear night, as I was walking up and down the garden with a friend, we very distinctly observed a flame-like appearance near the oriental poppy, the flowers of which are remarkable for their powerful red colour. We approached the place, and looked attentively at the flowers, but could perceive nothing further, till at last, by passing and repassing repeatedly, while we looked side-ways on them, we succeeded in renewing the appearance as often as we pleased. It proved to be a physiological phenomenon, and the apparent corruscation was nothing but the spectrum of the flower in the complementary blue-green colour. The twilight accounts for the eye being in a perfect state of repose, and thus very susceptible, and the colour of the poppy is sufficiently powerful in the summer twilight of the longest days to act with full effect, and produce a complementary image.”[110]
The leaves of the œnothera macrocarpa are said to exhibit phosphoric light when the air is highly charged with electricity. The agarics of the olive-grounds of Montpelier have been observed to be luminous at night; but they are said to exhibit no light, even in darkness, during the day. The subterranean passages of the coal mines near Dresden are illuminated by the phosphorescent light of the rhizomorpha phosphoreus, a peculiar fungus. On the leaves of the Pindoba palm, a species of agaric grows which is exceedingly luminous at night; and many varieties of the lichens, creeping along the roofs of caverns, lend to them an air of enchantment by the soft and clear light which they diffuse. In a small cave near Penryn, a luminous moss is abundant; and it is also found in the mines of Hesse. According to Heinzmann, the rhizomorpha subterranea and aidulæ are also phosphorescent.
It is but lately that a plant which abounds in the jungles in the Madura district of the East Indies was sent to this country, which, although dead, was remarkably phosphorescent; and, when in the living state, the light which it emitted was extraordinarily vivid, illuminating the ground for some distance. Those remarkable effects may be due, in some cases, to the separation of phosphuretted hydrogen from decomposing matter, and, in others, to some peculiar electric manifestation.
The phosphorescence of the sea, or that condition called by fishermen brimy, when the surface, being struck by an oar, or the paddle-wheels of a steamer, gives out large quantities of light, has been attributed to the presence of myriads of minute insects which have the power of emitting light when irritated. The night-shining nereis (Nereis noctiluca) emits a light of great brilliancy, as do several kinds of the mollusca. The nereides attach themselves to the scales of fishes, and thus frequently render them exceedingly luminous. Some of the crustaceæ possess the same remarkable property;—twelve different species of cancer were taken up by the naturalists of the Zaire in the Gulf of Guinea.[111] The cancer fulgens, discovered by Sir Joseph Banks, is enabled to illuminate its whole body, and emits vivid flashes of light. Many of the medusæ also exhibit powerful phosphorescence.[112] These noctilucous creatures are, many of them, exceedingly minute, several thousands being found in a tea-cup of sea water. They float near the surface in countless myriads, and when disturbed they give out brilliant scintillations, often leaving a train of light behind them.[113] By microscopic examination no other fact has been elicited than that these minute beings contain a fluid which, when squeezed out, leaves a line of light upon the surface of water. The appearance of these creatures is almost invariably on the eve of some change of weather, which would lead us to suppose that their luminous phenomena must be connected with electrical excitation; and of this, the investigations of Mr. C. Peach, of Fowey, communicated to the British Association at Birmingham, furnish the most satisfactory proofs we have as yet obtained.
Benvenuto Cellini gave a curious account of a carbuncle which shone with great brilliancy in the dark.[114] The same thing has been stated of the diamond; but it appears to be necessary to procure these emissions of light, that the minerals should be first warmed near a fire. From this it may be inferred that the luminous appearance is of a similar character to that of fluor spar, and of numerous other earthy minerals, which, when exposed to heat, phosphoresce with great brilliancy. Phosphorescent glow can also be excited in similar bodies by electricity, as was first pointed out by Father Beccaria, and confirmed by Mr. Pearsall.[115] These effects, it must be remembered, are distinct from the electric spark manifested upon breaking white sugar in the dark, or scratching sulphuret of zinc.
In the instances adduced there is not necessarily any exposure to the sunshine required. It is probable that two, if not three, distinct phenomena are concerned in the cases above quoted, and that all of them are distinct from animal phosphorescence, or the luminous appearance of vegetables. They, however, certainly prove, either that light is capable of becoming latent, or that it is only a condition of matter, in which it may be made manifest by any disturbance of the molecular forces. We have, in answer to this, very distinct evidence that some bodies do derive this property from the solar rays. Canton’s phosphorus, which is a sulphuret of calcium, will, having been exposed to the sun, continue luminous for some time after it is carried into the dark; as will also the Bolognian stone,—a sulphuret of barium. This result appears to be due to a particular class of the solar rays; for it has been found, if these sulphurets, spread smoothly on paper, are exposed to the influence of the solar spectrum for some little time, and then examined in the dark, that luminous spaces appear, exactly corresponding with the most refrangible rays, or those which excite chemical change; and one very remarkable fact must not be forgotten—the dark rays of the spectrum beyond the violet produce a lively phosphorescence, which is extinguished by the action of the rays of least refrangibility, or the heat rays—whilst artificial heat, such as a warm iron, produces a very considerable elevation of the phosphorescent effect.[116] It is not improbable, that the fluorescent rays of Mr. Stokes may be materially concerned in producing the phenomena of phosphorescence: experiments are, however, required to prove this.