[Illustration: FIG. 1.] Formerly chlorine was exclusively made in lead apparatus, because this metal is one of the least readily attacked. When such an apparatus is used for the first time a layer of lead chloride is formed, which, like a varnish, protects the metal beneath from further attack. Fig. 1 represents an apparatus formerly employed for the preparation of chlorine in chemical works. In the upper part of the pear-shaped vessel, K, there are four openings, two of which, D and C, are provided with water lutes. This means that the opening is surrounded by a moat containing water, into which the rim of the cover dips, thus making a joint. Through the middle opening goes the axle of the stirring apparatus, R; in the fourth is a lead safety funnel, J. Solid materials are introduced through D, liquid through J; the tube C carries away the chlorine formed; the tube A, furnished with a stop cock, can draw off the fluid contents of the apparatus. Since lead melts at low temperature, the apparatus cannot be heated over the fire without danger, therefore it is surrounded by an iron jacket, W, which is filled with water, or else the apparatus is heated by steam introduced into W. Larger quantities of chlorine are more conveniently prepared in an apparatus, of similar structure, made of stone or earthenware, which have the advantage over lead that they are not at all attacked by chlorine. [Illustration: FIG. 2.] Fig. 2 exhibits the construction of such an apparatus of medium size. It is constructed of sandstone or earthenware; the lid and some of the smaller parts can be made of either earthenware or lead. The pyrolusite is introduced at _G_ in large pieces; _H_ is the funnel for pouring in the acid; _E K D_, the steam pipe; _C_, the perforated false bottom upon which the pyrolusite lies; _F_, the delivery tube for the chlorine; _J_, the opening for running off the manganese chloride; _a_, the leaden cover. To prepare chlorine, 1 part (by weight) of common salt, 1 part of powdered pyrolusite, 2½ parts of vitriol and 1¼ part of water are used. The salt and the pyrolusite are introduced through _D_ into the apparatus (Fig. 1); the acid, diluted with the water, is poured in through the funnel, the materials are mixed by the stirrer and gently warmed until chlorine appears, when the application of heat must be considerably diminished or the chlorine will be violently evolved. Pyrolusite and hydrochloric acid are now generally used for the preparation of chlorine, because the solution of manganese chloride, left at the end of the operation, is valuable.