Copy 1, Volume 1

Hand-book of chemistry / Translated by Henry Watts.

  • Gmelin, Leopold, 1788-1853
Date:
1848-1872
Catalogue details

Licence: Public Domain Mark

Credit: Hand-book of chemistry / Translated by Henry Watts. Source: Wellcome Collection.

    519/562 (page 495)
    of the combined heat either of the zinc or of the water.—A similar evo-: lution of heat likewise takes place when zine is placed in contact with copper, &c. Hence, in simple galvanic circles, a rise of temperature . generally takes place, both in the metals and in the liquid. The evolu- tion of heat must be the same for a given quantity of zinc oxidated, whether the oxidation takes place by ordinary or by electro-chemical action. In the latter case, however, it must be observed that the hydro- gen receives its negative electricity, not from the zine but from the cop- per, the latent electricity of which—«. e. its caloric—is decomposed, and. thereby diminished in quantity,—and that the liberated positive elec- tricity goes from the copper through the connecting wire to meet the negative electricity of the zinc, and combines with it in the zinc to form heat. The greater therefore the quantity of heat gencrated in the zine, the smaller will be the evolution of heat in the cell. The quantity of heat evolved in the cell would therefore increase as the conducting. power of the wire diminished, if the oxidation of the zine could then go on with equal rapidity; but since the oxidation of common zinc is retarded under such circumstances, and that of amalgamated zinc almost wholly arrested, the contrary result is obtained. | If an amalgamated zine plate and a silver plate platinized by Smee’s method (p. 419), are immersed in 2]b. of dilute sulphuric acid, and pro- duce a current in a thick conducting wire of sufficient force to decompose 9 grains of water in an hour, the temperature of the 2 lb. (= 18432 grains) of liquid rises 4°7° Fah. in an hour. If from this we deduct the heat developed by the combination of the sulphuric acid with the oxide of zinc produced, there remains 2°1° Fah. for the rise of temperature due to the current. (Joule.)—[18°482 x 2°1 = 38707°2. Consequently, one grain of water would be heated 38707°2° Fah. or 21504 C°. At the same time, since the current acting for an hour decomposes 9 grains of water, 8 grains of oxygen are transferred to the zine : 21594 = 2688; while, therefore, 1 part of oxygen has been transferred from the hydrogen to the zine, a quantity of heat has been evolved in the trough sufficient to raise the temperature of 1 part of water by 2688 C° or 4838° Fah. Since the two plates were united by a thick wire, which allowed the negative electricity to pass without hindrance from the zine to the copper, the heat eyolyed in the wire may be reckoned as nothing. The difference in the quantities of heat evolved in the combination of 1 part of oxygen with zine and with hydrogen respectively is (5290 — 3000) = 2290. The ex- periment just described gave 2688: this excess arises from the cireum- stance mentioned by Joule himself, that a small quantity of zinc was oxidated by ordinary chemical action.—In a second experiment, in which the two plates were placed only half an inch instead of an inch apart, the rise of temperature in the liquid was 4:4°,—or, after deducting the heat developed by the combination of oxide of zine with sulphuric acid, 1:85° Fah. [The greater proximity of the plates probably diminished the ordinary chemical action. Calculation as above gives for every 1 part of oxygen which combines with zinc, a rise of temperature in 1 part of water amounting to 2368 C°., which agrees very nearly with the quantity of heat developed by the oxidation of the zinc. | ab a [Joule explains the development of heat on totally different prin- ciples, but appears not to be aware that in his calculations he has taken the quantity of the current for its intensity—a mistake of frequent occurrence. | , et Grove’s battery gives similar results. When the circuit is closed by: