Licence: Public Domain Mark
Credit: A system of chemistry, in four volumes (Volume 1). Source: Wellcome Collection.
Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)
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![posed to it two hours without undergoing any sensible alteration. But when exposed to the first pencil, which was much less bright and less hot, it was blackened in less than six minutes.* 15. M. Morichini, Professor of Chemistry at Rome, announced, in 1813, that when steel needles are exposed to the action of the violet ray of light they become magnetic.f But when these experi- ments were carefully repeated by Professor Configliachi of Pavia,:j: and by M. Berard of Montpellier,§ they did not succeed. Hence we may conclude that Morichini deceived himself by using needles already possessed of magnetic properties. 16. Such are the properties of light as far as they have been ex- amined. They are sufficient to induce us to believe that it is a body, and that it possesses many qualities in common with other bodies. It is attracted by them, and combines with them precisely as other bodies do. But it is distinguished from all the substances hitherto described, by possessing three peculiar properties, of which they are destitute. The first of these properties is the power which it has of exciting in us the sensation of vision, by moving from the object seen, and entering the eye. The phenomena of colours, and the prismatic spectrum, indicate the existence of seven different species of light; but to what the difference of these species is owing, has not been ascertained. We are altogether ignorant of the component parts of every one of these species. The second peculiar property of light is the prodigious velocity with which it moves whenever it is separated from any body with which it was formerly combined. This velocity, which is but little less than 200,000 miles in a second,|| it acquires in a moment; and it seems to acquire it too in all cases, whatever the body be from which it separates. The third, and not the least singular of its peculiar properties, is, that its particles are never found cohering together, so as to form masses of any sensible magnitude. This difference between light and other bodies can only be accounted for by supposing that its particles repel each other. This seems to constitute the grand dis- tinction between light and the bodies hitherto described. Its par- ticles repel each other,^[ while the particles of the other bodies at- * Annals of Philosophy, ii. 165. f Gilbert's Annalen der Physick, xlvi. 367. t Ibid. p. 337. § Annals of Pilosophy, iv. 228. II [See p. 27, note of the editor.—C] U fit is very possible that the particles of light repel each other. The following sugges- tions, however, bear upon this point. 1. The phenomena of mirrors and of lenses, where light, as well as caloric, is so highly concentrated. The attractions of the substances where- of these instruments are formed, are manifestly stronger, than the repulsive spheres that surround the particles of heat and caloric : analogous to this is the electricity accumulated round a main conductor. The repulsive force then, cannot be very strong. 2. By the con« elusions drawn by Roemer from the immersion of Jupiter's satellites, and by Bradley from the parallax of the fixed stars, the velocity of light is about 200,000 miles per second of time. When this is compared with M. D'Arcet's experiments on the continuous sensation pro- duced by a ray of light, it will appear, that there may be continuous vision, though the parti- cles of light be 20,000 miles from each other; a distance which renders repulsion between](https://iiif.wellcomecollection.org/image/b21159610_0039.jp2/full/800%2C/0/default.jpg)
