Sal Prunella
was at one time in high esteem, as it was believed that by the process adopted for making it the nitre was specially purified. Purified nitre was melted in an iron pot and a little flowers of sulphur (1 oz. to 2 lb.) was sprinkled on it, a little at a time. The sulphur deflagrating was supposed to exercise the purifying influence on the nitre. The actual effect was to convert a small part of the nitrate of potash into sulphate. It was first called Sal Prunella in Germany from the belief that it was a specific against a certain plum-coloured quinsy of an epidemic character. Boerhaave advised the omission of the sulphur, but believed that melting the pure nitre and moulding it was of medicinal value by evaporating aqueous moisture.
Nitre and flowers of sulphur were deflagrated together before the Sal Prunella theory was invented, equal quantities being employed. The resulting combination, which was of course sulphate of potash, was known as Sal Polychrestum, the Salt of Many Virtues.