Rosaniline or roseine, a white substance, is the base of aniline. It is a powerful alkali, readily combining with acids to form highly coloured salts, many of which have a tendency to crystallize, like magenta. This base is most easily extracted from the acetate of aniline. The boiling solution of that salt decomposed by a large excess of ammonia, yields a crystalline precipitate of a reddish colour, and when the colourless liquid is separated by filtration from the precipitate, it deposits on cooling perfectly white needles and tablets of pure rosaniline. This substance unites to acids in three different proportions forming three kinds of salts. The salts that contain one equivalent of acid are extremely stable compounds; for the most part they have a green metallic reflection like some insects’ wings; by transmitted light they are red, and their solutions in alcohol have the magnificent crimson colour of magenta. A bright purple dye is furnished by mixing equal weights of magenta and aniline. When this mixture is kept at the temperature of 329° for some hours and then mixed with water and hydrochloric acid to remove any excess of magenta or aniline, the result is an insoluble purple residuum or precipitate, but which when well washed with water becomes soluble in alcohol and boiling water slightly acidulated with acetic acid. When the insoluble purple residue is boiled several times with dilute hydrochloric acid, a fine blue dye is formed; azuline, the most beautiful of the blue dyes, which resists the action of the strongest acids, and which is produced by oxidizing aniline under high pressure. It was first prepared at Lyons from phenic acid, a product of the distillation of coal; when pure it appears under the form of copper bronze-coloured crystals soluble in alcohol, to which they communicate a magnificent blue colour tinged with red; but most of the blue dyes are derived from carbolic acid and from creosote. A blood-red colour is the direct result of mixing the muriatic and phenic acids. Aniline, the great source of the coal tar colours, yields also a fine yellow. A vast deal of talent has been employed in the research of colouring dyes both at home and abroad, in which the manufacturers themselves have shown great scientific knowledge. Attempts have been unsuccessfully made to obtain a green dye from chlorophyll, the green colouring matter of plants. The want was for a short time supplied by Lo-hao, a Chinese dye, but being unstable it was given up. However the very same substance has been procured from the Rhamnus cathartica (Buckthorn), one of the commonest European trees. M. Charwin of Lyons, who made the discovery, has utilized a waste substance, and rendered it permanent as a dye. It is the only known substance which with proper reagents is capable of producing all the seven colours of the spectrum.[14]