De la Rue's chloride of silver cell (fig. 18) is, from its constancy and small size, well adapted for medical and testing purposes. The "plates" are a little rod or pencil of zinc Z, and a strip or wire of silver S, coated with chloride of silver and sheathed in parchment paper. They are plunged in a solution of ammonium chloride A, contained in a glass phial or beaker, which is closed to suppress evaporation. A tray form of the cell is also made by laying a sheet of silver foil on the bottom of the shallow jar, and strewing it with dry chloride of silver, on which is laid a jelly to support the zinc plate. The jelly is prepared by mixing a solution of chloride of ammonium with "agar-agar," or Ceylon moss. This type permits the use of larger plates, and adapts the battery for lighting small electric lamps. Skrivanoff has modified the De la Rue cell by substituting a solution of caustic potash for the ammonium chloride, and his battery has been used for "star" lights, that is to say, the tiny electric lamps of the ballet. The Schanschieff battery, consisting of zinc and carbon plates in a solution of basic sulphate of mercury, is suitable for reading, mining, and other portable lamps. The Latimer Clark "standard" cell is used by electricians in testing, as a constant electromotive force. It consists of a pure zinc plate separated from a pool of mercury by a paste of mercurous proto-sulphate and saturated solution of sulphate of zinc. Platinum wires connect with the zinc and mercury and form the poles of the battery, and the mouth of the glass cell is plugged with solid paraffin. As it is apt to polarise, the cell must not be employed to yield a current, and otherwise much care should be taken of it. Dry cells are more cleanly and portable than wet, they require little or no attention, and are well suited for household or medical purposes. The zinc plate forms the vessel containing the carbon plate and chemical reagents. Figure 19 represents a section of the "E. C. C." variety, where Z is the zinc standing on an insulating sole I, and fitted with a connecting wire or terminal T (-), which is the negative pole. The carbon C is embedded in black paste M, chiefly composed of manganese dioxide, and has a binding screw or terminal T (+), which is the positive pole. The black paste is surrounded by a white paste Z, consisting mainly of lime and sal-ammoniac. There is a layer of silicate cotton S C above the paste, and the mouth is sealed with black pitch P, through which a waste-tube W T allows the gas to escape. The Hellesen dry cell is like the "E. C. C.," but contains a hollow carbon, and is packed with sawdust in a millboard case. The Leclanche-Barbier dry cell is a modification of the Leclanche wet cell, having a paste of sal-ammoniac instead of a solution.