From the above facts it would seem that certain bodies have the power of imbibing light and again emitting it, in certain circumstances, and that this power may remain for a considerable length of time. It is observed that the light which such bodies emit bears an analogy to that which they have imbibed. In general, the illuminated phosphorus is reddish; but when a weak light only has been admitted to it, or when it has been received through pieces of white paper, the emitted light is pale or whitish.—Mr. Morgan, in the seventy-fifth volume of the Philosophical Transactions, treats the subject of light at considerable length; and as a foundation for his reasoning, he assumes the following data:—1. That light is a body, and like all others, subject to the laws of attraction. 2. That light is a heterogeneous body; and that the same attractive power operates with different degrees of force on its different parts. To the principle of attraction, likewise, Sir Isaac Newton has referred the most extraordinary phenomena of light, Refraction and Inflection. He has also endeavoured to show that light is not only subject to the law of attraction but of repulsion also, since it is repelled or reflected from certain bodies. If such principles be admitted, then, it is highly probable that the phosphorescent bodies to which we have adverted have a power of attracting or imbibing the substance of light, and of retaining or giving it out under certain circumstances, and that the matter of light is incorporated at least with the surface of such bodies. But on this subject, as on many others, there is a difference of opinion among philosophers.5
9. Light is found to produce a remarkable effect on Plants and Flowers, and other vegetable productions. Of all the phenomena which living vegetables exhibit there are few that appear more extraordinary than the energy and constancy with which their stems incline toward the light. Most of the discous flowers follow the sun in his course. They attend him to his evening retreat, and meet his rising lustre in the morning with the same unerring law. They unfold their flowers on the approach of this luminary; they follow his course by turning on their stems, and close them as soon as he disappears. If a plant, also, is shut up in a dark room, and a small hole afterwards opened by which the light of the sun may enter, the plant will turn towards that hole, and even alter its own shape in order to get near it; so that though it was straight before, it will in time become crooked, that it may get near the light. Vegetables placed in rooms where they receive light only in one direction, always extend themselves in that direction. If they receive light in two directions, they direct their course towards that which is strongest. It is not the heat but the light of the sun which the plant thus covets; for, though a fire be kept in the room, capable of giving a much stronger heat than the sun, the plant will turn away from the fire in order to enjoy the solar light. Trees growing in thick forests, where they only receive light from above, direct their shoots almost invariably upwards, and therefore become much taller and less spreading than such as stand single.
The green colour of plants is likewise found to depend on the sun’s light being allowed to shine on them; for without the influence of the solar light, they are always of a white colour. It is found by experiment that, if a plant which has been reared in darkness be exposed to the light of day, in two or three days it will acquire a green colour perceptibly similar to that of plants which have grown in open day-light. If we expose to the light one part of the plant, whether leaf or branch, this part alone will become green. If we cover any part of a leaf with an opake substance, this place will remain white, while the rest becomes green. The whiteness of the inner leaves of cabbages is a partial effect of the same cause, and many other examples of the same kind might easily be produced. M. Decandolle, who seems to have paid particular attention to this subject, has the following remarks: ‘It is certain, that between the white state of plants vegetating in darkness, and complete verdure, every possible intermediate degree exists, determined by the intensity of the light. Of this any one may easily satisfy himself by attending to the colour of a plant exposed to the full day-light; it exhibits in succession all the degrees of verdure. I had already seen the same phenomenon, in a particular manner, by exposing plants reared in darkness to the light of lamps. In these experiments, I not only saw the colour come on gradually, according to the continuance of the exposure to light; but I satisfied myself, that a certain intensity of permanent light never gives to a plant more than a certain degree of colour. The same fact readily shows itself in nature, when we examine the plants that grow under shelter or in forests, or when we examine in succession the state of the leaves that form the heads of cabbages.’6
It is likewise found that the perspiration of vegetables is increased or diminished, in a certain measure by the degree of light which falls upon them. The experiments of Mr. P. Miller and others, prove that plants uniformly perspire most in the forenoon, though the temperature of the air in which they are placed should be unvaried. M. Guettard likewise informs us that a plant exposed to the rays of the sun, has its perspiration increased to a much greater degree than if it had been exposed to the same heat under the shade. Vegetables are likewise found to be indebted to light for their smell, taste, combustibility, maturity, and the resinous principle, which equally depend upon this fluid. The aromatic substances, resins, and volatile oil are the productions of southern climates, where the light is more pure, constant, and intense. In fine, another remarkable property of light on the vegetable kingdom is that, when vegetables are exposed to open day-light, or to the sun’s rays, they emit oxygen gas or vital air. It has been proved that, in the production of this effect, the sun does not act as a body that heats. The emission of the gas is determined by the light: pure air is therefore separated by the action of light, and the operation is stronger as the light is more vivid. By this continual emission of vital air, the Almighty incessantly purifies the atmosphere, and repairs the loss of pure air occasioned by respiration, combustion, fermentation, putrefaction, and numerous other processes which have a tendency to contaminate this fluid so essential to the vigor and comfort of animal life; so that, in this way, by the agency of light, a due equilibrium is always maintained between the constituent parts of the atmosphere.
In connection with this subject the following curious phenomenon may be stated, as related by M. Haggern, a Lecturer on Natural History in Sweden. One evening he perceived a faint flash of light repeatedly dart from a marigold. Surprised at such an uncommon appearance, he resolved to examine it with attention; and, to be assured it was no deception of the eye, he placed a man near him, with orders to make a signal at the moment when he observed the light. They both saw it constantly at the same moment. The light was most brilliant on marigolds of an orange or flame colour, but scarcely visible on pale ones. The flash was frequently seen on the same flower two or three times in quick succession; but more commonly at intervals of several minutes; and when several flowers in the same place emitted their light together, it could be observed at a considerable distance. The phenomenon was remarked in the months of July and August at sun-set, and for half an hour when the atmosphere was clear; but after a rainy day, or when the air was loaded with vapours, nothing of it was seen. The following flowers emitted flashes more or less vivid, in this order:—1. The Marigold, 2. Monk’s hood, 3. The Orange Lily, 4. The Indian Pink. As to the cause of this phenomenon, different opinions may be entertained. From the rapidity of the flash and other circumstances, it may be conjectured that electricity is concerned in producing this appearance. M. Haggern, after having observed the flash from the orange lily, the antheræ of which are at considerable distance from the petals, found that the light proceeded from the petals only; whence he concludes, that this electrical light is caused by the pollen which, in flying off, is scattered on the petals. But, perhaps, the true cause of it still remains to be ascertained.
10. Light has been supposed to produce a certain degree of influence on the PROPAGATION OF SOUND?—M. Parolette, in a long paper in the ‘Journal de Physique,’ vol. 68, which is copied into ‘Nicholson’s Philosophical Journal,’ vol. 25, pp. 28-39,—has offered a variety of remarks, and detailed a number of experiments on this subject. The author states the following circumstances as having suggested the connection between light and sound. ‘In 1803, I lived in Paris, and being accustomed to rise before day to finish a work on which I had long been employed, I found myself frequently disturbed by the sound of carriages, as my windows looked into one of the most frequented streets in that city. This circumstance which disturbed me in my studies every morning, led me to remark, that