The cell in which the plates shown in Fig. 5 are used may contain a bi-chromate solution; and for experimental work, where electricity is required for a short time only, this will produce a stronger current. But remember that the solution eats the zinc rapidly, and the plates must be removed as soon as you have finished using them. The bi-chromate solution is made by slowly pouring four ounces of commercial sulphuric acid into a quart of cold water. This should be done in an earthen jar, since the heat generated by adding acid to water is enough to crack a glass bottle. Never pour the water into the acid. When the solution is about cold, add four ounces of bi-chromate of potash, and shake or mix it occasionally until dissolved; then place it in a bottle and label it: BI-CHROMATE BATTERY FLUID POISON Before the zincs are immersed in the bi-chromate solution they should be well amalgamated to prevent the acid from eating them too rapidly. The amalgamating is done by immersing the zincs in a diluted solution of sulphuric acid for a few seconds, and then rubbing mercury (quicksilver) on the surfaces. The mercury will adhere to the chemically cleaned surfaces of any metal except iron and steel, and so prevent the corroding action of the acid. Do not get on too much mercury, but only enough to give the zinc a thin coat, so that it will present a silvery or shiny surface. A two-fluid cell is made with an outer glass or porcelain jar and an inner porous cup through which the current can pass when the cup is wet. Fig. 7. [Illustration: FIG. 4] [Illustration: FIG. 7] [Illustration: FIG. 8] [Illustration: FIG. 6]