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.

    470/562 (page 446)
    Influence of the Surface of the Electrodes upon Decomposition. Electrodes of small surface evolve as much detonating gas from acidu- lated water in a given time as those of a larger surface. (Faraday.) When, however, the current is strong, the decomposition diminishes if the electrode be too small to allow free passage to the current; thus, a Daniell’s battery of ten pairs yields 2°7 cubic inches of detonating gas, whether the electrodes are formed of broad plates of platinum, or of wires; but if the latter are covered with resin to within a quarter of an inch of their points, the quantity of gas per minute is reduced to 2°83 cub. in.; and when they are coated up to their points, it is only 0°8 cub. in,: in this last case, the gas is driven into the liquid with some force. (Daniell.) —With currents of small quantity, on the contrary, the decomposition is increased by diminishing the size of the electrodes, and thereby concen- trating the current. (Wollaston, Matteucci.) The deflection is likewise stronger when the positive electricity enters by a narrower, and the nega- tive electricity by a wider electrode, than in the contrary case [because a greater quantity of gas is evolved at the negative electrode]. (Matteucci.) Decomposition of Individual Compounds. Nicholson and Carlisle discovered the decomposition of water, Hi- singer & Berzelius that of salts, Sir H. Davy that of the alkalis. Water. The electric current acts more readily on water contained in a bundle of asbestus fibres than on a column of that liquid of equal length and thickness in a U-tube. (H. Davy.) . Many substances which are soluble in water fayour its decomposition (p. 443); but with a powerful battery, the same quantity of detonating gas 1s obtained in a given time, whether the water is mixed with a large or a small quantity of sulphuric acid, or holds in solution, potash, soda, carbonate of potash, sulphate of ammonia with excess of ammonia, or sul- phate of soda. Dilute sulphuric acid of sp. gr. from 1:25 to 1°386 is best adapted for the purpose. Acid of 1:495 sp. gr. yields a somewhat greater quantity of gas, because it retains less oxygen gas in solution, With acid of still greater strength, e. g. 2 measures of oil of vitriol to 1 mea- sure of water, only 0°57 volume of oxygen gas is obtained for every 2 volumes of hydrogen,—possibly because the predisposing affinity of the sulphuric acid favours the production of peroxide of hydrogen. (Faraday.) Connell likewise obtained with these different solutions, and also with that of boracic acid, constantly the same quantity of detonating gas. When water is divided by a membrane into two portions, and one of the electrodes of a powerful battery immersed in each of them, the water rises in the negative and sinks in the positive division. (Porret.)—This effect is exhibited by distilled water and rain-water, which are bad con- ductors, but not by saline solutions,—and is due to the resistance which the water opposes to the passage of positive electricity. (De la Rive.)— On the contrary: This effect is seen only in water which holds in solu- tion some salt, by the decomposition of which, alkali collects in the nega- tive and acid in the positive division; and the rise of the water is not a