LETTER vegetation. JII. Hence the previous atmosphere, which contains in its composition that portion, was indispensable, as was also some water on the soil where they were to grow. This exact placing of the vegetable plants. His conclusions are, Solar light favors in them the assimilation of carbonic acid gas-facilitates their verdure, and the formation of their volatile and aromatic principles-It is essential to florification and fructification. We cannot obtain ripe seeds from plants reared in darkness-Many become more lax and watery as the light is lessened-Lamp or torch light but imperfectly supplies that of the Sun-They incline to it, and those near it harden to dryness; but it keeps those green which in total darkness would become pale.' Bull. Un. 1829, p. 54. M. Martius observed the milky juice of the Euphorbia Phosphorea to be luminous-Mr. Prinsep found that light exerts a great influence in the change of colour in plants in Autumn. Its privation prevented any alteration, tho in light the leaf passed from green to yellow, and sometimes to red. Ib. p. 436. Treviranus distinguishes the light and heat developed from plants, independently of their life, from the light and heat which are intimately connected with their existence and increase with their vigorous health. He doubts if those flowers are really luminous which have been marked as such, and concludes, from his experiments, that neither heat nor light are given out during the life of vegetables. Bull. Un. 1330, p. 257. In this conclusion I cannot concur, for I have at times remarked, after sunset, a perceptible glow in the colours of flowers, unusual to them in their common state, as if coloured light of their own colour was issuing from them. I have seen the difference in the Sweet-Williams, Geraniums, Marigolds, Hearts-ease, and Pinks, at various parts of the day, when the Sun was not visible. It has been most apparent to me in the red flowers; next, in the yellow. It resembled an actual secretion of light, additional to their usual show. The presence of the Sun upon them lessens the effect. It is most perceptible in his absence. I have noticed the same occasional appearance on the bricks in a country path. In some states of the atmosphere, when the Sun was clouded, they have had a peculiar glow, without any visible causes. 6 The most favourable proportion is thought to be one-fifth of oxygen, or that of the common air. Pure oxygen gas accelerates vegetation, but makes the plant feeble. In less than one-eighth of oxygen, germination will not take place. There is a remarkable connection between water and plants. Madden observed, in the deserts he traversed, that wherever there is water, no matter in what part of the wilderness, there vegetables are found.' Trav. in Turkey. III. formation and first germination, is another test of the LETTER authenticity of the Hebrew cosmogony, which random fiction could not have stood. I was considerably affected in my younger days by the long-standing objection, that Moses made light to exist before the creation of the Sun; as books then usually taught, what some still fancy, that there could not have been light without this luminary. But not chusing, on such an important point, to attach my faith to any general assertion, I sought to find out if any investigator of the nature of light had perceived any distinction in its qualities or operation, which made it a fluid, or matter, independent of the Sun. It was not easy, before the year 1790, to meet with the works of any student of nature on such a subject, as it had been little attended to: but I at length saw the fact asserted by Henckel, a German of the old school, of some value in his day; and soon afterwards some experiments were announced in England, which confirmed the supposition. It has been a favourite point of attention with me ever since; and no truth in philosophy seems to be now more clearly ascertained, than that Light has a distinct existence, separate and independent of the Sun. This is a striking confirmation of the Mosaic record; for that expressly distinguishes the existence and operation of light from the solar action upon it, and from that radiation of it which is connected with his beams and presence. By Moses, an interval of three days is placed between the luminous creation, and the appearance and position of the Sun and Moon. Light was therefore operating by its own laws and agencies, without the Sun, and independently of his peculiar agency, from the first day to III. LETTER the fourth of our terrestrial fabrication. But from the time that the Sun was placed in his central position, and his rays were appointed to act on our Earth, they have been always performing most beneficial operations, essential to the general course of things. They have also been ascertained, by Dr. Herschel, to have a power of heating, distinct from their production of light and colour,—an interesting discovery, connected with more consequences and inferences than have yet been noticed." The glory of Sir Isaac Newton began by his discovering that light was not simple and homogeneous; but that it consisted of seven rays of different colours and of different and invariable degrees of refrangibility. The same degree of this belonged always to the same colour, and the same colour to the same degree of refrangibility." Red, yellow, and blue, are the primary colours; white light, their compound. An opposing theory to this has been gradually growing up from the time of Des Cartes, and is now maintained by several men of no small name and powers in science," which considers light to be an 86 The rays of solar light possess several remarkable physical properties: they heat; they illuminate; they promote chemical combination; they effect chemical decompositions; they impart magnetism to steel; they alter the colours of bodies; they communicate to plants and flowers their peculiar colours; and are, in many cases, necessary to the development of their characteristic qualities.' Dr. Brewster, Life of Newton, p. 90. "From his experiments, Dr. Herschel' drew the important conclusion, that there were invisible rays in the light of the Sun, which had the power of producing heat; and which had a less degree of refrangibility than red light.' These results were confirmed by Sir Henry Englefield. Dr. Brewster's Optics, p. 89. 10 Brewster's Newton, p. 43. "Dr. Hooke, and Huygens, in Newton's lifetime, urged the undulatory theory, which Des Cartes had first suggested. Newton answered them. But Euler and others revived it. New observa tions 13 III. an undulating vibration of an ethereal medium univer- LETTER sally diffused, and not, as Newton thought, an emanation of particles direct from the Sun. 12 La Place preferred the opinion that Light is an emanation from a luminous body." But the newer system comes nearest to the Mosaic fact, that light was a distinct production anterior to the Sun; and appears to be gaining ground in philosophical minds." Perhaps some harmonizing combination of both theories may reconcile all the phenomena, and best explain the true nature and operation of light. It seems most probable that light is an ethereal fluid now. universally diffused, and pervading all things, and not an emanation from the Sun; but that this luminary has a direct and additional agency upon it, whose effects we daily see. It may not be impertinent to suggest, that light seems, like heat, to have two states,-active and latent. The active state causes its visible phenomena, and our sensation of daylight. When this subsides, by the Sun's departure, into its latent state, our sense tions induced Dr. Young to adopt it, with very scientific illustrations. Since he wrote, Dufresnel, A. L. Cauchy, and M. Pouillet, have enforced it. Dr. Ure explains its modern shape at some length, in his Geology. 12 Dr. Brewster thus briefly contrasts the two systems. In the Newtonian theory, light is supposed to consist of material particles emitted by luminous bodies; and moving thro space with a velocity of 192,000 miles in a second. In the undulatory theory, an exceedingly thin and elastic medium, called Ether, is supposed to fill all space, and to occupy the intervals between the particles of all material bodies.' Dr. Brewster's Optics, p. 134. 13 Syst. vol. 2, p. 91. 14 Each of these two theories of Light is beset with difficulties peculiar to itself: but the theory of Undulation has made great progress in modern times; and derives such powerful support from an extensive class of phenomena, that it has been received by many of our most distinguished philosophers.' Dr. Brewster's Optics, p. 135. III. LETTER of darkness or night, is produced. The solar rays again emerging on it, have the power of changing its latent state into its active visibility.15 Light has also the property of being absorbed by, and, I would add, of combining with, all substances; 16 with some wholly, which are then black; with others, the most numerous cases, only in part; and then, that portion of them, which is not so absorbed, emanates from the substance in the colour which comes from them to After having for many years attended to the phenomena of Light, I cannot but consider it to be an universally diffused fluid. Thus far the idea would accord with the Undulatory theory; but many facts lead me also to conclude that it actually enters into the composition of all or of most substances, and, like 15 Fresnel says, on the new theory, If the phenomena of light be considered as the vibrations of an elastic fluid, we may infer that the direction of these vibrations is perpendicular to that of the luminous ray. This is supposed on the idea that the different parts of an elastic fluid act on each other by alternate compression and dilatation.' Bull. Univ. 1830, p. 108. 16 One of the most curious properties of bodies is their power of absorbing light. Charcoal is the most absorptive of all. Even the most transparent bodies in nature, air and water, when in sufficient thickness, are capable of absorbing a great quantity of light.' Dr. Brewster, Optics, p. 137. 17 Sir I. Newton' concluded that the colours of natural bodies are not qualities inherent in the bodies themselves, but arise from the disposition of the particles of each body to stop or absorb certain rays; and thus to reflect more copiously, the rays which are not thus absorbed.' Dr. Brewster's Life of Newton, p. 46. Bodies absorb light in different degrees, in this order: Black hornblende, Obsidian, Rock crystal, Charcoal, Coal of all kinds, Mica, Water, Ib. Opt. 137. |