assisted by telescopic views, which plainly favour the same opinion, we do not hesitate to infer, that the SUN, also, is richly stored with inhabitants. No. XLIV. ON THE NATURE OF LIGHT. Fairest of beings! first created light! The sparkling gem, the vegetable race, The nobler worlds that live and breathe, their charms, From thy unfading source of splendour draw! MALLET. WE experience, every moment, the utility of that light which invests our globe; but there is not a subject, concerning the nature of which such a variety of opinions have been formed. Light is defined to be that sensation occasioned in the mind by the view of luminous bodies; or, that property in bodies, whereby they are fitted to excite those sensations in us. The Greeks considered light as an accident, or property, resulting from the first principles of things; and Des Cartes defines it to be a globulous matter diffused through the universe; which being impelled by the sun, strikes upon our eyes, in the same manner as a staff, that is pushed at one end, presses in the same instant at the other. But Des Cartes, and the ancient philosophers, in particular, spoke the language of hypothesis and conjecture: our immortal Newton had recourse to experiment and observation. The rays of light, according to him, are small corpuscles, or particles of matter, emitted with exceeding celerity from the luminous body; with a velocity so immense, indeed, as to enable them to move at the inconceivable rate of 11,000,000 miles in a minute, Mr. Roemer, a Danish philosopher, was the first who showed, that light is about eight minutes in its passage from the sun to the earth. This idea was first suggested to him, by observing the eclipses of Jupiter's moons, by which it appeared, to a demonstration, that the motion of light is not instantaneous, but progressive, or such as would carry it through a space equal to the radius, or semi-diameter of the earth's annual orbit in about eight minutes of time: so that if the sun were annihilated, we should see him for eight minutes afterward, and if he were created again, it would be eight minutes before we should see him. The minutest particles, which are thrown off from the body of the sun, move through a space of 95,000,000 miles in eight minutes, which is about a million of times swifter than the motion of a cannon-ball, when it is first projected from the mouth of the piece: a rapidity too great for the imagination to follow, or the mind to comprehend. The wonderful divisibility of the parts of matter is no where more apparent, than in the minuteness of the particles of light. Dr. Nieuwentyt has computed, that an inch of candle, when converted into light, becomes divided into 269,617,040 parts, with 40 ciphers annexed; at which rate there must issue from it, when burning, 418,660 particles, with 39 ciphers more, in the second of a minute; which is vastly more than a thousand times a thousand million times the number of sands the whole earth can contain; reckoning ten inches to one foot, and that 100 sands are equal to one foot, It must be acknowledged, that many difficulties and objections have been urged against the materiality of light, or the hypothesis of light, consisting of small particles emitted from luminous bodies; and that many eminent philosophers, both foreigners and English, have recurred to the opinion, that light consists of vibrations propagated from the luminous body, through a subtile etherial medium. Among these are the names of Euler and Franklin, whose arguments have been combated by Dr. Horsley, Mr. Melville, and others. But an account of their reasonings on this subject would necessarily lead me into a disquisition much too abstruse for the popular and intelligible manner in which I wish to conduet this paper. Sir Richard Blackmore, in his Creation, thus alludes to the difficulties which attend the subject: Behold the light emitted from the sun! What more familiar, and what more unknown? That light, however, is a real substance, notwithstanding the objections that have been urged against this hypothesis, seems to be established by the phenomena of the Bolognian stone, and of other substances, which possess the remarkable property of imbibing light, of retaining it for some time, and afterward of emitting it'. The doctrine of the materiality of light is further confirmed by those experiments, which demonstrate, that the colour and inward texture of some bodies are changed, in consequence of their being exposed to the light. Among various observations of this kind, was that made by M. Duhamel, who found that the juice of a certain shellfish in Provence contracted a fine purple colour when exposed to the light of the sun, and that the stronger the light, the more splendid was the colour. Pieces of cloth, dipped in this liquor, and exposed to the sun, be came red, although they were inclosed in glass; but they acquired none of this colour in the same exposure, if they were covered with the thinnest plates of metal. 1 Phosphorescent-stone is found on different eminences around Bologna in Italy, and especially on Monte Paderno,' loose and scattered about between gypseous stones in a marly earth. It is found most readily after heavy rains, in the streams which run down the sides of the hill. To render it capable of shining in the dark, a piece particularly heavy, fo liaceous, and pure must be selected. After being made redhot, it is pounded and reduced to a fine powder, which, by means of a solution of gum-tragacanth, becomes a kind of paste, and is then converted into small cakes. When these are dried, they are brought to a state of ignition between coals, and then suffered to cool; after which they are preserved from the air and moisture in a close vessel. If one of these cakes be exposed a few minutes to the light, and then carried into a dark place, it will shine like a burning coal. This power of emitting light becomes lost in the course of time; but it may be restored at first by heating, and afterward by exposure again to ignition. It was discovered at the begin ning of the seventeenth century by a shoe-maker of the name of Vincentio Casciarola. See further in Beckmann's Inventions, vol. iv. p. 418. 2d edit. The expansion or extension of any portion of light is inconceivable. Dr. Hook shows, that it is as unlimited as the universe; proving it from the immense distance of some of the fixed stars, the light of which becomes sensible to the eye, by means of a telescope. He adds, that they are not the great bodies only of the sun or stars that are thus liable to disperse their light through the vast expanse of the universe, but the smallest spark of a lucid body must do the same, even the smallest globule struck from a steel by a flint. Sir Isaac Newton observes, that bodies and light act mutually upon each other; bodies on light, in emitting, reflecting, refracting, and inflecting it; and light on bodies, by heating them, and putting their parts into a vibrating motion, in which heat principally consists. For all fixed bodies, he observes, when heated beyond a certain degree, emit light, and shine; which shining, &c. appears to be owing to the vibrating motions of their parts; and all bodies, abounding in earthly and sulphureous particles, if sufficiently agitated, emit light, which way soever that agitation be effected. Thus, seawater shines in a storm; quicksilver, when shaken in vacuo; cats, or horses, when rubbed in the dark : and wood, fish, and flesh when putrefying. Light proceeding from putrescent animal and vegetable substances, as well as from glowworms, is mentioned by Aristotle. Bartholin names four kinds of luminous insects, two with wings, and two without; but it is asserted by travellers, that in hot climates, they are found in much greater numbers, and of different species. In particular, on the river Menam, which runs through Siam, a vast number of those insects, called fire-flies, make a beautiful appearance in the night: On Menam's orient stream, that nightly shines THOMSON. |