Some nitrate of potash dissolved in water and coloured with litmus placed in the glass trough, changes red on one side of the cardboard by the liberation of acid, and is not affected on the other.
In these experiments the oxygen, iodine, chlorine, and nitric acid are liberated at the electro-positive pole, and are hence termed electro-negative bodies, whilst hydrogen and the alkalies are set free at the electro-negative pole, and are therefore called electro-positive bodies. Faraday has modified these terms, and calls the two classes "anions" and "cathions," and the two poles "anodes" and "cathodes."
Anode, from ανα, up, and ὁδος, a way: the way which the sun rises. Anions, from ανα, up, and ειμι, to go: that which goes up; a substance which passes to the anode during the passage of a current of electricity. Cathode, from κατα, down, and ὁδος, a way: the way which the sun sets. Cathion, from κατα, down, and ειμι, to go: that which goes down; a substance which passes to the cathode during the passage of electricity from the anode to the cathode.
Ninth Experiment.
In the process of the electrotype is presented a valuable application of the chemical power of the voltaic circle or battery, and it may be conducted either as a single cell operation or by distinct batteries. In the former case the most simple arrangement will suffice; the only articles necessary are—a large mug or tumbler; some brown paper and a ruler; a bit of amalgamated zinc, four inches long and half an inch wide; a short length of copper wire; some black lead, blue vitriol, and oil of vitriol.
The mould from which the electrotype is to be taken can be made of common sealing wax, plaster of Paris, white wax, gutta percha, or fusible alloy. Supposing the first to be selected—viz., a common seal, it is first thoroughly black-leaded,[D] then one end of the copper wire is bent round the top of the amalgamated zinc, and the other is gently warmed and melted into the side of the seal, leaving a small portion uncovered by the wax, which is then well black-leaded. A few ounces of blue vitriol are dissolved in boiling water, and when cold the solution is poured into the tumbler, and the porous cell to contain the mixture of eight parts water to one of sulphuric acid is made by rolling the brown paper three or four times round the ruler and closing the end, and fixing the side with a little sealing wax. The porous cell of brown paper is now filled with the dilute acid, and placed in the tumbler containing the solution of blue vitriol, the amalgamated zinc being arranged in the paper cell, and the attached seal in the copper solution; in about twelve hours a good deposit of copper is produced, and a perfect cast in metal of the seal obtained. (Fig. 186.)
[D] The application of plumbago, or black lead, for electrotype purposes, was first made by the late lamented Mr. Robert Murray.
Fig. 186.
a a. The tumbler containing the solution of sulphate of copper. b b. The brown paper cell containing the dilute sulphuric acid, inside which is the amalgamated zinc with wire attached to the seal d.