A system of chemistry, in four volumes (Volume 1).

  • Thomas Thomson
Date:
1818
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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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    ; BOOK I. DIVISION 1. 200,000 miles in a second. The discovery of Roemer has been still farther confirmed and elucidated by Dr. Bradley's very inge- nious theory of the aberration of the light of the fixed stars.* 2. While a ray of light is passing through the same medium, or when it passes perpendicularly from one medium to another, it continues to move without changing its direction; but when it passes obliquely from one medium to another of a different density, it always bends a little from its old direction, and assumes a new one. It is then said to be refracted. When it passes into a denser medium, it is refracted towards the perpendicular; but when it passes into a rarer medium, it is refracted/rom the perpendicular. In general, the quantity of refraction is proportional to the density of the medium; but if the medium be combustible, the refraction is greater than it would otherwise be.f In the same medium the sines of the angles of incidence and of refraction have always the same ratio to each other. It has been the general opinion of philosophers since the days of Newton, that the refractive power of the same body in different states is proportional to its density. But M. M. Arago and Petit have lately shown by a set of experiments, that when a liquid body is converted into vapour, its refractive power diminishes at a greater rate than its density. Thus the refractive power of sulphu- ret of carbon, while liquid, when compared to that of air, is a little greater than 3 ; while that of the same substance in the state of va- pour, being likewise referred to air, does not surpass 2. The liquids tried by these philosophers were sulphuret of carbon, sulphuric ether, and muriatic ether.:}: This newly discovered fact, constitutes one of the strongest objections to the Newtonian theory of light that has yet been advanced. 3. When a ray of light enters a transparent medium, as a plate of glass, with a certain obliquity, it continues to move on till it comes to the opposite surface of the glass; but then, instead of passing through the glass, it bends, and passes out again at the same surface at which it entered; just as a ball would do if made to strike obliquely against the floor. The ray is then said to be re- fected. The angle of reflection is always equal to the angle of in- cidence. When the surface of a medium is polished, as glass or mirrors, oblique rays do not enter them at all, but are refectedwhen they approach the surface of the body. All surfaces are capable of reflecting a greater or smaller number of oblique rays. Rays are only reflected at surfaces. Newton has explained these phenomena by supposing an attrac- tion to exist between light and the medium through which it is * Phil. Trans, xxxv. 637, and xlv. 1. [2 Smith's Optics, uh. sup. and S. Gravesand's Phys. Elem. Math. lib. ii. cap. 1. p. 708.—C.] ■f It was the knowledge of this law that led Newton to suspect the diamond to be com- bustible, and water to contain a combustible ingredient.— Optics, p. 72. * Ann. de Chimie et Physique, i. I.