AUREOLIN YELLOW.--This colour is prepared by precipitating carbonate of cobalt from a solution of a cobalt salt with carbonate of potash; this precipitate is dissolved in acetic acid, and to it is added nitrite of sodium; nitrite of potassium and cobalt are thrown down as a yellow powder. Experiments made on the freshly-precipitated colour prepared in this way, have proved it to be invariably destroyed by caustic potash; but aureolin yellow prepared in another way, put into a flask with some caustic soda, has remained unchanged for several days, and proved to be perfectly stable under this treatment. Samples of the same yellow, exposed to the action of ammonia, soda, and potash for a considerable length of time, have not changed. This is really a most important fact, because aureolin yellow is so beautiful a pigment that one wishes it would stand for ever. It is, in fresco and silicious painting, a great desideratum to have colours that will stand the action of lime and caustic alkalies. Our colours for fresco painting are very limited, and if aureolin will stand the action of caustic alkalies, it may be safely used both for fresco and silicious painting. CADMIUM YELLOW.--The pigment known as cadmium yellow is important on account of its permanence and brilliance. It is prepared by passing a stream of sulphuretted hydrogen gas through a slightly acid solution of a salt of cobalt, usually the nitrate or sulphate, whereby the sulphide of cadmium is precipitated as an impalpable powder. It is filtered off, well washed with water, and finally dried at a low temperature. By this method there is considerable difficulty in producing any precise, shade of yellow, which is dependent on the proportion of precipitant used, and can only be controlled after long practical experience. Therefore some modifications have been introduced with the object of securing definite tints. The ordinary colour is a pure chrome yellow. For a lemon-yellow shade, a solution of yellow sulphide of ammonium is added to one of sulphate of cadmium. For an orange tint, the cadmium solution (chloride or sulphate) is made very acid by addition of hydrochloric acid before passing the sulphuretted hydrogen; or the cadmium solution, as for lemon-yellow, is boiled, and receives the ammonium sulphide solution while still boiling. But none of these modifications results in a really durable pigment, on account of the presence of traces of free sulphur or acid. The best brands of this pigment being somewhat expensive, there is great inducement for adulteration, which usually takes the form of orpiment or of chrome. The former can be detected by the ordinary tests for arsenic, and the latter by the process described on p. 266. CHROME YELLOWS.--A very important family of yellow pigments are the “chromes,” consisting mainly of chromic acid in combination with lead, iron, or zinc.