among physicians, the same with exanthema. See Exanthema.

in chemistry, denotes the formation of a kind of mealy powder on the surface of certain bodies. Efflorescence is occasioned either by decomposition or drying. The efflorescence which happens to cobalt and martial pyrites is of the first; and that observed on the crystals of marine alkali, Glauber's salt, &c. of the latter kind. An efflorescence is sometimes also a species of crystallization, the nature of which is not well understood; as, the beautiful vegetations which shoot up from vitriolated tartar acidulated either with the vitriolic or nitrous acids, the saline spicula which are observed to shoot from salt butter, &c.

Besides the common crystallization of salts, all of them have the property of appearing in the form of an efflorescence, or small saline spicula, when mixed with any thick substance, particularly lime. Whatever salt happens to be made use of, there is little or no difference in the efflorescence. Thus, in butter very much salted, the sea-salt shoots in the form of long spiculae, though the sea-salt itself never shoots but in the form of cubical crystals. In like manner, Glauber's salt will appear in the form of an efflorescence, as well as the soda alkali, &c. nor will the form of the crystals of the efflorescence be perceptibly different from those of sea-salt. The efflorescences which we see very commonly upon walls are in general Glauber's salt. In some cases (but seldom in such efflorescences as we have examined) they are composed of soda alkali. The reason of these differences is not known. In almost all cases of this kind there seems to be a real growth of salt. On one spot of a plaster wall about two feet square, which we observed particularly, this growth was very evident. The produce was a true Glauber's salt; and by frequently taking off the efflorescence, eight ounces were procured; nor did the prolific virtue of the wall seem to be in the least impaired by the waste.