Elements of chemistry, including the history of the imponderables and the inorganic chemistry of the late Edward Turner / [Edward Turner].

  • Edward Turner
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry, including the history of the imponderables and the inorganic chemistry of the late Edward Turner / [Edward Turner]. Source: Wellcome Collection.

    115/880 page 89
    reduced to a fine dry powder, is heated with care in a porous porcelain vessel, and the platinum placed in the middle of it so as to be completely surrounded by the peroxide. The external vessel containing the zinc plate is charged with acidulated or salt water. A single pair of this arrangement was found to have much greater activity in producing chemical effects than the same pair when excited by nitric and sulphuric acid as in Grove’s arrangement.] It must, be remembered that in a compound circle, the extreme plates at either end, not being in contact with the exciting fluid, are in reality superfluous, and serve only as conductors. Hence the current, instead of flowing from the zinc to the copper, seems to flow from the copper to the zinc. But if we abstract the extreme plates, which serve only as conductors, it is then seen that the direction of the current corresponds to that of the simple circle (see fig. 5.) During the action of a simple circle, as of zinc and copper, excited by dilute sulphuric acid, all the hydrogen developed in the voltaic process is evolved at the surface of the copper. This fact is not apparent when common zinc plates are used, owing to the numerous currents which form on the surface of the zinc (page 85); but when a plate of amalgamated zinc and another of platinum are introduced into dilute sulphuric acid of sp. gr. 1.068, no gas whatever appears until contact between the plates is made, and then hydrogen gas rises solely from the platinum, while zinc is tranquilly dissolved. On weighing the amal¬ gamated plate before and after the action has continued for half an hour or an hour, and collecting the hydrogen gas evolved during that interval, the weight of the hydrogen set free and of zinc dissolved will be as 1 to 32.3, being the ratio of their chemical equivalents. Faraday, who has proved this, has also shown that in a compound voltaic circle, say of 10 amalgamated zinc plates and 10 of platinum, each of the former during a given period of action loses exactly the same weight, and from each of the latter an equivalent quantity of hydrogen gas is evolved. This separation of one ingredient of the exciting solution at one plate, while the element previously combined with it unites with the other plate, seems essential to voltaic action. It is in some way connected with the passage of the current across the exciting liquid. Oxygen in a free state may by oxidizing zinc cause electric excitement; but the voltaic current is not established unless the oxygen formed part of a previous liquid compound in contact or communica¬ tion with both the plates. Among the different kinds of voltaic apparatus is usually placed the electric column of De Luc, which is formed of successive pairs of silver and zinc, or silver and Dutch metal leaf, separated by pieces of paper, arranged as in a voltaic pile. It is remarkable for its power of exhibiting attractions and repulsions like common electricity, but cannot produce chemical decomposition or any of the effects most characteristic of a voltaic current, and is rather an electrical than a voltaic instrument. It is quoted as a proof of electric developement by contact, since it will continue in action for years without being cleaned or taken to pieces. True it is that the more oxidable metal of the column is slowly corroded, and that no electricity is excited when the paper is quite or nearly free from hygro- metric moisture, the presence of which is necessary to the oxidation of the zinc and copper; but at the same time the quantity of electricity excited seems so disproportioned to the corrosion, that the one can scarcely be assigned as the cause of the other.
    digitised image 1
    digitised image 2
    digitised image 3