The colouring principle of cochineal is an acid, carminic acid, which was obtained by its discoverer, Warren de la Rue, in the following manner: 1 part of powdered cochineal is boiled with 40 parts of water for 20 minutes; after standing, the liquid is poured off from the sediment, and mixed with a solution of 6 parts of lead acetate, acidified by 1 part of acetic acid. The precipitate, which consists of impure lead carminate, is filtered from the colourless liquid and carefully washed; whilst still wet it is suspended in water, through which sulphuretted hydrogen is passed; the lead carminate is decomposed into lead sulphide and carminic acid, which dissolves in the water. The colouring matter is not yet quite pure; the treatment with lead acetate and sulphuretted hydrogen must be repeated, the solution then obtained is evaporated at a low temperature, the residue dissolved in boiling alcohol, phosphoric acid added to decompose traces of lead carminate still present, then ether, finally the clear liquid is separated from the precipitate and evaporated. By this process carminic acid is obtained in a state of complete purity; unfortunately the process is too complicated for practical application. Pure carminic acid is a purple red mass, which transmits red light at the edges and forms a pure scarlet red powder when ground. The aqueous solution gives, with alum and ammonia, a characteristic precipitate, the colour of which is the purest carmine-red; lead, zinc and copper salts produce purple red precipitates. The composition of carminic acid is expressed by the formula C₁₇H₁₈O₁₀. CHAPTER LI. THE MANUFACTURE OF CARMINE. The colouring matter of cochineal is used in the form of a lake under the name of carmine, the finest and most expensive colour used in painting. However simple the preparation of carmine may appear, it is not easy to obtain a product of great beauty; certain conditions, still unexplained, play an important part in the process. Until not long ago the manufacture of fine carmine was regarded as a secret; this it is no longer, and with proper care any one may produce an entirely satisfactory product. Many recipes have been given, some of the best will be mentioned. Whatever method is used to obtain carmine certain precautions must be taken, without which it is impossible to obtain a bright colour. Alkalis and alkaline earths in very small quantity affect the shade of carmine, so that spring water should never be used in its preparation. Pure rain water, or, still better, distilled water, should be employed.