Mellite, or honey-stone, is mentioned in mineralogical treatises in the sixteenth century. That it consisted of the alumina salt of a special acid (_mellic acid_) was shown by Klaproth in 1799. _Prussian blue_ was accidentally discovered in 1710 by a dyer named Diesbach. Its mode of manufacture was first made known by Woodward in 1724. The peculiar reaction by which it was obtained was made the subject of investigation by many chemists of the period without any decisive result. Scheele observed that, when the salt which occasioned the blue colour with vitriol was distilled with sulphuric acid, a volatile acid, inflammable and soluble in water, was obtained. This acid received from Bergmann the name of _acidum cœrulei berolinensis_, or “Berlin-blue acid,” subsequently shortened by Guyton de Morveau to _prussic acid_. Scheele also prepared the cyanides of silver and ammonium. That prussic acid was free from oxygen was established by Berthollet. Anhydrous prussic acid was first obtained by Von Ittner, who first established its highly poisonous nature. Bolim, in 1802, had previously observed the presence of prussic acid in oil of bitter almonds, the poisonous character of which was known to Dioscorides. Porret first definitely isolated _potassium ferrocyanide_, and subsequently discovered the _thiocyanates_, the quantitative composition of which was ascertained by Berzelius in 1820. That prussic acid was a compound of hydrogen and cyanogen was established by Gay Lussac in 1815. _Cyanic acid_ was discovered by Wöhler in 1822, in which year also L. Gmelin discovered the _ferricyanides_. _Fulminating mercury_ was first prepared by Howard in 1800, and _fulminating silver_ by Brugnatelli in 1802. These were recognised by Liebig, in 1822, to contain a peculiar acid, which he termed _fulminic acid_, and which he showed to have the same composition as the cyanic acid discovered by Wöhler. _Uric acid_, so named by Fourcroy, was discovered in gall stones by Scheele in 1776. _Urea_ was first definitely isolated by Fourcroy and Vauquelin in 1799, and was synthetically prepared by Wöhler in 1828. The bitter principles of plants and their medicinal virtues early attracted attention, but the first attempt to isolate them was made by Fourcroy and Vauquelin in the case of the Peruvian bark, long known for its power as a febrifuge. In 1806 Vauquelin obtained _quinic acid_. _Cinchonine_ was first isolated by Gomes in 1811. The chemical nature of opium was the subject of numerous inquiries in the early years of the nineteenth century. In 1805 Sertürner detected the existence of _meconic acid_, and in 1817 that of _morphine_, which he recognised as an alkaloid. _Narcotine_ was discovered by Robiquet in 1835. The investigation of other bitter substances was undertaken by Pelletier and Caventou, who in 1818 discovered _strychnine_, _brucine_ (1819), and _veratrine_ (1820).