A solution of yellow prussiate is added to the iron solution in such quantity that a very small excess of iron salt is left; there results a white or rather, since the ferrous sulphate always contains small quantities of ferric oxide, a pale blue precipitate. This is allowed to settle, the liquid drawn off as completely as possible from the precipitate, and nitric acid then added, or a solution of bleaching powder, followed by the amount of sulphuric acid required to decompose it. The nitric acid or the chlorine liberated from the bleaching powder speedily effects the oxidation, changing the pale blue colour to the deep blue of Chinese blue. The precipitate should be left for several days in contact with the liquid in order to complete the oxidation; it is then well washed and dried. Pieces of definite shape and size bearing the trade mark of the firm are often pressed from the blue when it has acquired a stiff consistency. There are many formulæ for the preparation of Chinese blue, differing in regard to the quantities of green vitriol, yellow prussiate, and nitric acid or bleaching powder to be used. It is, however, clear that only _one_ formula can be correct, that in which the quantities of materials are equivalent, since the reactions take place between equivalent quantities. In practice the equivalent quantities would not be weighed to the tenth of a gramme—there would be no object in such a proceeding, since in works chemically pure substances are not used; but manufacturers should work according to equivalent quantities in order to use up their materials as completely as possible. No increase in labour is involved, the same labour weighs one or another quantity. The proportion of materials in which the least loss is involved is here given. Nine kilogrammes of green vitriol are dissolved in 100 litres of water, 15 kilogrammes of strong sulphuric acid are added, and then a solution of 15 kilogrammes of yellow prussiate in 100 litres of water. The solutions are kept in constant motion during the mixing. As soon as the decomposition is finished, without waiting for the precipitate to settle, steam is led in and 20 kilogrammes of nitric acid of 1·3298 specific gravity added in small portions. The heating by direct steam is continued until red vapours are no longer evolved from the liquid, which is a sign that the oxidation is finished.