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. In these allied phenomena we have effects which are evidently dependent upon several dissimilar causes. The phosphorescence of the living animal is due, without doubt, to nervous excitation: that of the living vegetable to solar luminous influence; and in the case of the mosses of caverns, &c. to the chemical agency of the sun’s rays, which appears to be capable of conduction. In the dead organic matter we have a purely chemical action developing the light, and in the inorganic bodies we have peculiar molecular constitution, by which an absorption of light appears to take place. The subject is one of the greatest difficulty; the torch of science is too dim to enable us to see the causes at work in producing these marvellous effects. The investigation leads, to a certain extent, to the elucidation of many of the secrets of luminous action; and the determination of the question, whether light is an emanation from the sun, or only a subtile principle diffused through all matter, which is excited by solar influence, is intimately connected with the inquiry.