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.