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

  • Turner, Edward, 1798-1837.
Date:
1834
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.

    49/1106 (page 21)
    HEAT: Al Pictet and Prevost. According to the former, bodies of equal temperature do not radiate at all; and when the temperature is unequal, the hotter give calorific rays to the colder bodies till an equilibrium is established, at which moment the radiation ceases. Prevost, on the contrary, conceived radiation to go on at all times, and from all substances, whether their temperature were the same or different from that of surrounding objects. (Recherches sur la Chaleur.) Consistently with this view, the temperature of a body falls whenever it radiates more heat than it absorbs; its temperature is stationary when the quantities emitted and received are equal; and it grows warm when the absorption exceeds the radiation. A hot body surrounded by others colder than itself, affords an instance of the first case ; the second happens when all the substances within the sphere of each other’s radiation have the same temperature; and the third occurs when a body is introduced into a room which is warmer than itself. Of these theories the preference is very generally accorded to the latter. Most of the phenomena of radiation, indeed, admit of a satisfactory explanation by both ; but on the whole, the theory of Prevost is more generally appli- cable. A favourable example for tracing this preference is af- forded by the law of cooling in vacuo, established by Dulong and Petit. Another argument in its favour is deducible from the close analogy which subsists between the laws of heat and light. Luminous bodies certainly exchange rays with one ano- ther :—a feeble light sends rays to one of greater intensity ; and the quantity of light emitted by each, does not seem to be at all influenced by the vicinity of the ether. Since, therefore, the radiation of light is not prevented by the presence of other luminous bodies, it is probable that the radiation of heat is equally uninfluenced by the proximity of other radiating sub- stances. Adopting, for the reasons just stated, the theory of Prevost, it will be useful to examine a few instances of its application ;— and, first, in regard to the experiments with conjugate murrors. If a metallic ball in the focus of one mirror, and a thermometer in that of the other, be of the same temperature as the sur- rounding objects (say at 60°), the thermometer will remain stationary. It will indeed receive rays from the ball ; but as it emits an equal number in return, its temperature will be un- changed. Ifthe ball is above 60° the thermometer will rise,