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Usually, however, it is the weight of silver or copper deposited from their salts in a certain time which gives the current in amperes. Electro-plating is the principal application of this chemical process. In 1805 Brugnatelli took a silver medal and coated it with gold by making it the cathode in a solution of a salt of gold, and using a plate of gold for the anode.
In this way the necessary electrical resistance can very conveniently be inserted between the anode and cathode surfaces. The elimination of hydrogen from the cathode must be avoided, or at any rate must not accumulate. Moving the article being plated, while in the bath, taking care not to break the electrical contacts, is a good security against a streaky or foggy appearance in the deposit.
A better result will be obtained by using the following bath, and placing the pieces in contact with a strip of clean zinc, also immersed: The anode or feeding-plate used in this bath consists of pure Banca tin. A moderately strong battery is required, and the work is finished by scratch-brushing.
Since the hydrogen thus liberated is a reducing agent, the nitric acid in the solution is slowly reduced to ammonia, and it may happen that if the current is passed through for a long time, such a solution will become alkaline. Oxygen is liberated at the anode, but, since there is no oxidizable substance present around that electrode, it escapes as oxygen gas.
"Amalgamated copper strips or discs are placed in contact with the mercury and extended above it, to allow the gold and silver solution of cyanide to come in contact. "The electrodes are connected with the dynamo; the anode of lead being positive and the cathode of mercury being negative.
I apply this cathode to his skin in the dorsal region; under the back of the neck, and this anode in the lumbar region at the base of the spine just pieces of cotton soaked in salt solution and covering the metal electrodes, to give me a good contact with the body." I was fascinated. It was gruesome, and yet I could not take my eyes off it. Torreon stood blankly, in a daze.
In some cases, as for example, that of silver, solution of salts forming complex ions, like that of the double cyanide of silver and potassium, yield better metallic deposits. Most metals are deposited as such upon the cathode; a few, notably lead and manganese, separate in the form of dioxides upon the anode.
"As the gas is generated at the anode, a commotion is created in the liquid, which brings a fresh and saturated solution of electrolyte between the electrodes for electrolysis, and makes it continuous in its action.
The passage of the electric current, as we have seen on page 79, in the chapter on Electrolysis, is able to decompose the copper in the electrolyte and to precipitate chemically pure copper on the cathode, the copper of the solution being replenished from the raw material used as the anode by which the current is passed into the bath.
It should be noted that, in general, the changes occurring at the cathode are reductions, while those at the anode are oxidations. For analytical purposes, solutions of nitrates or sulphates of the metals are preferable to those of the chlorides, since liberated chlorine attacks the electrodes.
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