=Siderin Yellow.=—This not very handsome yellow consists of ferric chromate; it is obtained by adding a neutral solution of ferric chloride to a strong boiling solution of potassium bichromate so long as a precipitate is formed. Siderin yellow is said to be used both in oil and water, and to be particularly adapted for use in sodium silicate paints, since in the course of time it forms a stony mass with that substance. The low price of iron salts would make it desirable to employ chromates of iron, but it appears to be difficult to obtain a compound of constant composition. In experiments with this object the author did not succeed in obtaining products of the same shade. Others have probably been equally unsuccessful, for siderin yellow has never been used in quantity, as it would have been were there no difficulties in the way of its preparation. =Aureolin= is a double nitrite of cobalt and potassium, Co(NO₂)₂.3 KNO₂. This pigment is prepared by adding excess of potassium nitrite to a solution of cobalt nitrate acidified with acetic acid. As the liquid cools, a deep lemon yellow crystalline powder separates, which, when dry, is known as Indian yellow or aureolin. It is distinguished from other yellow pigments by being unaffected by sulphuretted hydrogen. The potassium nitrite required in the preparation of this pigment is most easily made by melting saltpetre in a thick iron vessel and stirring in fine iron filings in small quantities as soon as the saltpetre begins to decompose. The iron glows brightly and burns to oxide, the saltpetre changes to potassium nitrite. The mass is dissolved in a little hot water, the solution filtered and cooled, when most of the undecomposed saltpetre crystallises out, whilst the nitrite remains in solution. After further evaporation and separation of another crop of potassium nitrate crystals, the solution can be used to precipitate the aureolin. It is advisable to use strong solutions in the precipitation of aureolin; the finest precipitate is obtained in this way. If dilute solutions are used, the precipitate forms gradually; it is then coarse and has little covering power. According to the method of Hayes, aureolin is prepared by passing into a solution of cobalt nitrate the vapours produced by pouring nitric acid over copper and allowing air to enter. Caustic potash is added to the liquid from time to time. In this way all the cobalt can be obtained in the form of a yellow precipitate. =Tungsten Yellow.=—Finely powdered tungsten is introduced in small quantities into fused potassium carbonate so long as effervescence occurs. After boiling with water and filtering, calcium tungstate is precipitated from the filtrate by means of calcium chloride. The moist precipitate is added to hot dilute nitric acid until the liquid is only slightly acid, when it is boiled for half an hour and allowed to cool. The precipitate, after washing with a little water and drying, is a deep lemon yellow powder.