Elements of chemistry, including the recent discoveries and doctrines of the science / by Edward Turner.

  • Turner, Edward, 1798-1837.
Date:
1833
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, including the recent discoveries and doctrines of the science / by Edward Turner. Source: Wellcome Collection.

    45/1026 (page 17)
    Leslie employed a hollow tin cube filled with hot water as the radiating substance. The rays proceeding from it were brought, by means of a concave mirror, into a focus, in which the bulb of a differential thermometer was placed. He found that certain substances radiate caloric much more ra- pidly than others, and that the nature of the surface of a heated body has a singular influence upon its radiation. By adapting thin plates of different metals to the sides of the tin cube, and turning them successively towards the mirror, he found a very variable effect produced upon the thermometer. A bright smooth polished plate of metal radiated caloric very imperfectly ; but if its surface were in the least degree dull or rough, the radiating power was immediately augment- ed. Or if the metallic surface were covered with a thin layer of isinglass, paper, wax, or resin, its power of radiation increased surprisingly. It follows from these researches that velocity of radiation depends more on the surface than the substance of a radiating body :—that the most imperfect ra- diators are to be sought among those bodies which are highly smooth and bright, such as polished gold, silver, tin, and brass; but that these same metals radiate freely when their smoothness and polish are destroyed, as by scratching their surfaces with a file, or covering them with whiting or lamp black. Leslie next examined the power of different substances in reflecting caloric, and he soon arrived at the interesting conclusion, that those surfaces which radiate least reflect most powerfully. A polished plate of tin or brass is an excellent reflecting surface, but a bad radiating one; by removing the polish in any way, its reflecting power is diminished in the same proportion as its radiating power is increased. His ex- periments, indeed, justify the conclusion, that the faculty of radiation is inversely as that of reflection. There are only two modes by which calorific rays, falling upon a solid opaque body, can dispose of themselves :—they must either be reflected from it, or enter into its substance. In the latter case caloric is said to be absorbed. Now it is manifest, that those rays which are reflected cannot be ab- sorbed; and that those which are not reflected, must be ab- sorbed. . Hence it follows that the absorption of heat in the same body is inversely as its reflection ; and since the pro- c