1. Its electromotive force should be high and constant; 2. Its internal resistance should be small; 3. It should be perfectly quiescent when the circuit is open; 4. It should give a constant current, and therefore must be free from polarization, and not liable to rapid exhaustion; 5. It should be easily cared for, and if possible, should not emit corrosive fumes; 6. It should be cheap and of durable materials. =Single and Two Fluid Cells.=--The distinction between a single and a two fluid cell has already been given. The single fluid cell of Volta with its zinc and copper plates represents the simplest form of primary cell. In the _two fluid cell_, the positive (zinc) plate is immersed in the exciting liquid (usually dilute sulphuric acid) and is decomposed by the action upon it, while the negative plate is placed in the liquid depolarizer which is decomposed by the hydrogen arrested by it, thus preventing polarization. In some forms of cell, the two liquids are separated by a porous partition of unglazed earthenware, which, while it prevents the liquids mixing except very slowly, does not prevent the passage of hydrogen and electricity. [Illustration: FIGS. 44 and 45.--Leclanche cell and porous cup. This very common form of cell is an example of the single fluid type, with a solid depolarizer surrounding the negative element; the latter is generally carbon, the positive element being zinc. The liquid used is a strong solution of ammonium chloride, commonly known as sal-ammoniac, and which resembles table salt. In the porous cup type of cell, a carbon slab is placed in the porous cup, and is surrounded by a mixture of small pieces of carbon and manganese dioxide, the top being covered by means of pitch, leaving one or two small holes for air and gas to pass through. The depolarizer will take care of a limited amount of the hydrogen produced when the cell is on closed circuit, but if the circuit be closed for any length of time polarization occurs. The cell is thus of the open circuit class, and will furnish a good current where it is required only intermittently. Zinc is dissolved only when the cell is being used. This type of cell, or its modification, is used for gas lighting and bell work. The cell requires very little attention. Water must be added as the solution evaporates, and the zinc rod replenished when necessary. The electromotive force is about 1.48 volts and the internal resistance about 4 ohms.] Complete depolarization is usually obtained also in single fluid cells, having in addition a depolarizing solid body, such as oxide of manganese, oxide of copper, or peroxide of lead, in contact with the carbon pole. Such cells really do not belong to the single fluid cells, and are considered in the two fluid class. A few examples of single and double fluid primary cells will now be described.