The fourth stage begins with Boyle, and closes with the eighteenth century. Still under the dominion of theoretical alchemy, practical alchemy was rejected by it, and its interest was concentrated on the collection of facts. It led up to modern chemistry, which begins with Lavoisier, and the introduction of the balance in the study of chemical change. Chemical theory, then, in our author's time stood somewhat thus. Metals as regarded their elemental composition were considered to partake of the nature of earth, water, and air, in various proportions. Fossils, or those things generated in the earth which were not metals, were again subdivided into two classes--those which liquefy on being heated, as sulphur, nitre, etc., and those which do not. The metals were considered to be composed of sulphur and mercury. These substances are themselves compounds, but they act as elements in the composition of metals. Sulphur represented their combustible aspect, and also that which gave them their solid form; while mercury was that to which their weight and powers of becoming fluid were due. This theory was due to two main facts. Most ores of metals, especially of copper and lead, contain much sulphur, which can be either obtained pure from them, or be recognised by its smell when burning. This gave rise to the sulphur theory, while the presence of mercury was inferred doubtless from the resemblance of the more commonly molten metals, silver, tin, and lead, to quicksilver. The properties of each metal were then put down to the presence of these substances. The list of seven metals is that of the most ancient times--gold, electrum, silver, copper, tin, lead, iron; but it is clearly recognised that electrum is an alloy of gold and silver. Most of the facts in this book are derived from Pliny through Isidore, but, that the theory is Arab in origin, one fact alone would convince us. A consideration of the composition of the metals shows us that tin is nearest in properties of all metals to the precious ones, but tin is precisely the metal chosen by Arab alchemists as a starting-point in the Chrysopoeia. Beside their scientific interest these passages have supplied many analogies. When Troilus is piling up his lover's oaths to Cressida, his final words are: "As iron to adamant, as earth to centre;" our chapter on the adamant supplies the origin of this allusion in part, astronomy gives the other. Diamonds are still, unfortunately, the precious stones of reconciliation and of love our author bespeaks them. The editor has not lengthened the chapter by extracts giving the occult properties of gems, and has contented himself by quoting from the chapter on glass a new simile and an old story.