Red precipitate was one of the first preparations of mercury known. It is traced to Geber, but when the works attributed to that chemist were written is doubtful. Avicenna in the tenth century was acquainted with it. In his writings he says of the metal mercury that “warmed in a closed vessel it loses its humidity, that is to say its liquid state, and is changed into the nature of fire and becomes vermilion.” Being obtained direct from mercury acted on by the air, it became known to the early chemical experimenters as “precipitatus per se.” Paracelsus obtained it by acting on mercury with aqua regia and heating the solution until he got the red precipitate. Then he reduced it to the necessary mildness for medicinal purposes by distilling spirit of wine from it six or seven times. Charas described a method of obtaining the precipitate by nitric acid but by a complicated process, and to the product he gave the name of arcana corallina. Boyle obtained the red oxide by boiling mercury in a bottle fitted with a stopper which was provided with a narrow tube by which air was admitted. The product was called Boyle’s Hell, because it was believed that it caused the metal to suffer extreme agonies. OTHER MERCURIAL PRECIPITATES. The multitude of experiments with mercury yielded many products, and often the same product by a different process which acquired a distinct name. Turbith mineral was a secret preparation with Oswald Crollius who gave it this name, probably, it is supposed, on account of its resemblance in colour to the Turbethum (Convolvulus) roots which were in his time much used in medicine. It is a subsulphate, made by treating mercury with oil of vitriol and precipitating with water. The precipitation of mercury by sal ammoniac was first described by Beguin in 1632. For a time it was given as a purgative and in venereal diseases. A double chloride of mercury and ammonium was also made by the alchemists and was highly esteemed by them, especially as it was soluble. It was called Sal Alembroth and also Sal Sapientiæ. The origin of the first name is unknown, but it has been alleged to be of Chaldean birth and to signify the key of knowledge. A green precipitate was obtained by dissolving mercury and copper in nitric acid, and precipitating by vinegar. This was also used in syphilis. Homberg put a little mercury into a bottle and attached it to the wheel of a mill. The metal was thereby transformed into a black powder (the protoxide.) By a careful and very gradual precipitation of a solution of nitrate of mercury by ammonia Hahnemann obtained what he called soluble mercury. Soubeiran proved that this precipitate was a mixture in variable proportions of sub-nitrate and ammonio-proto-nitrate of mercury. CALOMEL.