Black and red were the symbols of darkness and light, of the evil and the good principle; and the union of these two principles represented the moral order. At a later time the idea helped to establish the alchemical doctrine that sulphur and mercury are the Principles of all things. _Experiment._--Various organic substances are analysed by heating in a distillation-apparatus; the products are, in each case, a solid residue, liquids which distil off, and certain spirits which are disengaged. The results supported the ancient theory which asserted that _earth_, _water_, _air_, and _fire_ are the four Elements of the world. The solid residue represented _earth_; the liquid products of the distillation, _water_; and the spirituous substances, _air_. _Fire_ was regarded sometimes as the means of purification, sometimes as the soul, or invisible part, of all substances. _Experiment_.-A strong acid is poured on to copper. The metal is attacked, and at last disappears, giving place to a green liquid, as transparent as water. A thin sheet of iron is plunged into the liquid; the copper re-appears, and the iron vanishes. What more simple than to conclude that the iron has been transformed into copper? Had lead, silver, or gold been used in place of copper, one would have said that the iron was transformed into lead, silver, or gold. In their search for "the pure and penetrating matter which applied to any substance exalts and perfects it after its own kind," the alchemists necessarily made many inventions, laid the foundation of many arts and manufactures, and discovered many facts of importance in the science of chemistry. The practitioners of the _Sacred Art_ of Egypt must have been acquainted with many operations which we now class as belonging to applied chemistry; witness, their jewellery, pottery, dyes and pigments, bleaching, glass-making, working in metals and alloys, and their use of spices, essential oils, and soda in embalming, and for other purposes. During the centuries when alchemy flourished, gunpowder was invented, the art of printing was established, the compass was brought into use, the art of painting and staining glass was begun and carried to perfection, paper was made from rags, practical metallurgy advanced by leaps and bounds, many new alloys of metals came into use, glass mirrors were manufactured, and considerable advances were made in practical medicine and sanitation. [Illustration: FIG. XI. _See p. 92._]