It is therefore a good plan to keep a rough profit and loss account of the gold in order to find the quantity in solution. Fifty dwts. per gallon (or 78 grms. per 4.5 litres) is recommended. A gallon of solution of this strength is worth about eleven pounds sterling in gold and cyanide, and a serviceable anode will be worth about 10 pounds. (Fine gold is worth nominally four pounds four shillings and eleven pence ha'penny per oz.) Gold may be easily obtained containing less impurity than one part in ten thousand. Sec. 137. Plating with Copper. Copper may be deposited from almost any of its salts in reguline form, the sulphate and nitrate being most usually employed. In the laboratory a nearly saturated solution of sulphate of copper with 1 or 2 per cent of sulphuric acid will answer most purposes. A current density of, at most, fifteen amperes per square foot may be used, either for obtaining solid deposits for constructional purposes or for calibrating current measuring instruments by electrolysis. A copper anode is of course employed. When coppering with a view to obtaining thick deposits it is a good plan to place the electrodes several inches apart, and, if possible, to keep the liquid stirred, as there is a considerable tendency on the part of copper deposits to grow out into mossy masses wherever the current density exceeds the limit mentioned. As the masses grow towards the anode the defect naturally tends to increase of itself, hence the necessity for care. The phenomenon is particularly marked at the edges and corners of the cathode. If the deposit becomes markedly irregular, the best plan is to stop the process and file the face of the deposit down to approximate smoothness. In coppering it is of the utmost importance that the cathode be clean and free from grease; it must never be touched (by the finger, for instance) from the time it is scratch-brushed till it is immersed in the plating bath. Any grease or oxidation tends to prevent the copper deposit adhering properly. A copper deposit oxidises very easily when exposed to the air. Consequently if the surface be required free from oxide, as, for instance, when it is to be silvered or gilt, it must be quickly washed when withdrawn from the coppering bath, scratch-brushed, and transferred immediately to the silvering or gilding bath. If the surface is to be dried, as in electrolysis calibrations, it must be rinsed quickly with boiling water and pressed between sheets of filter paper. Another method which has been recommended is to rinse the copper in--water slightly acidulated with sulphuric acid (which prevents oxidation), then in distilled water, and to dry by blotting paper and in front of a fire, taking care not to make the plate too hot. The wash water is sufficiently acidulated by the addition of two or three drops of acid per litre. So far as I know, the method of washing in acidulated water was first proposed by Mr. T. Gray.