In general use we find very few artificial white pigments; these are lead, zinc and barium compounds. Circumstances may arise which make it expedient for the colour maker to manufacture other white pigments, for example, a demand for them, or favourable opportunities for obtaining the requisite raw materials. The white lead pigments, of which there is a large number, as we have indicated, all have the great disadvantage that they are not permanent, that is, are changed by atmospheric influences. It is well known that lead is a very delicate reagent for sulphuretted hydrogen, with the sulphur of which it forms a black compound. Now the air, especially in towns, contains sulphur in the form of sulphuretted hydrogen or ammonium sulphide; though the quantity is very small, the fate of every white or coloured lead pigment is decided by it; after a longer or shorter time it will be discoloured, will gradually darken, and finally be turned black. In spite of this great changeableness of lead pigments they are used by artists and painters, although the majority could be replaced by more permanent pigments entirely unaltered by the atmosphere. WHITE LEAD. This pigment, which in addition to other good properties has remarkable covering power, was amongst the earliest known artificial pigments. Already in the fourth century before Christ, Dioscorides described the preparation of white lead, which was obtained by exposing lead to the action of the vapours of vinegar, removing the white layer and treating it with water. The Roman writers describe a similar method; they use the name _cerussa_, under which white lead is known to-day in commerce. Although white lead has been so long known, it was left to Bergmann, in 1774, to show that it contained carbonic acid; before that it was believed to be lead acetate. The development of analytical chemistry was followed by a knowledge of its constitution and the use of more rational methods of manufacture. Whilst in the middle ages the manufacture of white lead was almost exclusively in the hands of the Dutch and Venetians, in later years it gradually spread, and now many works are concerned in the manufacture of this pigment. That adulterations of white lead were not rare in former times appears from the writings of Basil Valentine, an alchemist of the fifteenth century. Commercially white lead is known under most varied titles, of which the following are the principal: White lead, Venetian white, Dutch white, Krems white, Kremnitz white, flake white, etc. According to its chemical composition, white lead is a compound of lead carbonate and lead hydroxide, that is, a basic lead carbonate. Commercial white lead, apart from intentional admixtures of other white substances, contains lead carbonate and lead hydroxide in varying proportions, as is shown by the following analyses by Mulder, who found that all the samples examined by him were composed according to one or other of the following formulæ:—