The accompanying diagram, Fig. 29, is given as an example of a plant that may be employed with good results. _a_ is the sulphur melting pan with its steam jacket _b_, and steam pipe _c_; _d_ is a cast-iron still, arranged within a gas furnace _e_; _f_ is a crucible for melting and volatilising zinc, the said crucible being contained in a gas furnace _g_; _h_ is an automatic apparatus for freeing the mouth of the zinc vessel from deposited sulphide of zinc; _i_ is the collecting apparatus; and _k_, the condensing apparatus. (3) _Mixtures._--There are a number of compound pigments commonly known as zinc whites, which only deserve the name in so far as they contain a proportion, greater or smaller, of some zinc salt. At the same time it must be admitted that some of these combinations possess very good qualities, and that the foreign ingredients largely correct the weak points of the zinc compounds. Freeman’s.--This pigment, when ground with oil in the customary way, forms a paint equal in body and covering power to the best white lead, while it is superior in colour, permanence, and density, and is free from odour and noxious qualities. It is produced by grinding together “zinc white” (either oxide or sulphide), lead sulphate and barium sulphate, in certain proportions, in the dry state, in an edge-runner mill. By thus grinding the several pigments together, their particles become intimately incorporated and undergo changes in character. The barium sulphate not only cheapens the product by reason of its low cost, but also imparts a distinct feature in rendering the paint more free working. The proportions generally adopted, calculated by weight, are 5 parts lead sulphate, 2 of zinc white, and 1 of barium sulphate. The duration of the grinding will necessarily vary in accordance with various governing conditions, but it should be continued until the mixture has a density of about 200 lb. per cubic foot. Orr’s.--The pigment known as Orr’s enamel or Charlton white, is a compound of oxide and sulphide of zinc and sulphate of strontia, or of baryta. It is prepared in two ways. (_a_) Barytes is calcined for some hours at white heat with charcoal, and the calcined mass is lixiviated with water to wash out the barium sulphide; to one-half of this solution is added zinc chloride, which produces a precipitate of zinc sulphide, leaving barium chloride in solution. To this mixture of zinc sulphide and barium chloride is added the remaining half of the barium sulphide and some zinc sulphate, the result of which is a double precipitate of zinc sulphide and barium sulphate. This is water-washed, filter-pressed, dried, calcined at red heat, thrown immediately into cold water, ground very fine, and finally dried. (_b_) The second process closely resembles the first, but celestine takes the place of the barytes. Either form of Orr’s enamel is a good useful white pigment, very permanent, mixing well, of excellent covering power, and pure in colour.