The U. S. Pharmacopoeia directs this to he made by double decomposition between sulphate of zinc and carbonate of soda, mixed in boiling hot solution. It has been introduced among the officinal preparations as a substitute for calamine, or impure carbonate of zinc, which found in the shops, is often a wholly surreptitious substance, containing no zinc whatever, and therefore not to be relied on. The officinal carbonate of zinc is really a subcarbonate; the oxide of zinc being in considerable excess, in consequence of the escape of carbonic acid during the reaction of the two salts used in its preparation.

Precipitated carbonate of zinc is a soft, light, white powder, insoluble in water, and without smell or taste. It is, however, soluble in most acids; and, when applied locally to a secreting surface, may be considered as undergoing solution in very small proportion in the extravasated liquid, through the instrumentality of an acid contained in it, or some other chemical reagency.

Therapeutic Application

This is exclusively topical and external. Probably in consequence of the slight solution, just referred to, which it may be supposed to undergo in the moisture of the surface to which it is applied, it may acquire a very moderate degree of the excitant and astringent properties which characterize the soluble preparations of zinc, and thus produce a positive impression, such as it could not produce in a perfectly insoluble state. But it probably also acts, when in the form of powder, by absorbing the irritating secretions of the diseased surface, and thus in some degree correcting their influence; and, whether in powder or ointment, it has some effect by the exclusion of atmospheric air. It is used in excoriations, whether from the chafing of opposed surfaces, as in fat persons and particularly children; from acrid secretions, as of the upper lip in coryza, or from superficial injuries; also in chapped hands and sore nipples, and sometimes in scalds and blisters. It is applied in the form of powder dusted on the part, or in that of a cerate (Ceratum Zinci Carbonatis, U. S.), made by incorporating two parts of the powder with ten of simple ointment. This has been substituted for the old Turner's cerate, which was prepared from calamine.

* In the case of a girl of fourteen, troubled with an almost incessant barking cough, no doubt of an hysterical character, a cure was effected, under the care of Dr. Harley. by the use of valerianate of zinc, assafetida, and camphor, aided by a cold douche, with frictions to the spine, night and morning; but it is impossible to say what share the valerianate had in the cure. (Med. T. and Gaz.. Aug. 1863, p. 116.) - Note to the third edition.