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I.—In the so-called dry batteries the exciting substance is a paste instead of a fluid; moisture is necessary to cause the reaction. These pastes are generally secret preparations. One of the earlier “dry” batteries is that of Gassner. The apparatus consists of a containing vessel of zinc, which forms the positive element; the negative one is a cylinder of carbon, and the space between is filled with a paste, the recipe for which is:

Oxide of zinc	1 part
Sal ammoniac	1 part
Plaster	3 parts
Chloride of zinc	1 part
Water	2 parts

The usual form of chloride-of-silver battery consists of a sealed cell containing a zinc electrode, the two being generally separated by some form of porous septum. Around the platinum or silver electrode is cast a quantity of silver chloride. This is melted and generally poured into molds surrounding the metallic electrode. The exciting fluid is either a solution of ammonium chloride, caustic potassa, or soda, or zinc sulphate. As ordinarily constructed, these cells contain a paste of the electrolyte, and are sealed up hermetically in glass or hard-rubber receptacles.

II.—The following formula is said to yield a serviceable filling for dry batteries:

Charcoal	3 ounces
Graphite	1 ounce
Manganese dioxide	3 ounces
Calcium hydrate	1 ounce
Arsenic acid	1 ounce
Glucose mixed with dextrine or starch	1 ounce

Intimately mix, and then work into a paste of proper consistency with a saturated solution of sodium and ammonium chlorides containing one-tenth of its volume of a mercury-bichloride solution and an equal volume of hydrochloric acid. Add the fluid gradually, and well work up the mass.

III.—	Calcium chloride, crystallized	30 parts
Calcium chloride, granulated	30 parts
Ammonium sulphate	15 parts
Zinc sulphate	25 parts