Potassium sulphate, K2S04, a salt known early in the 14th century, and studied by Glauber, Boyle and Tachenius, was styled in the 17th century arcanum or sal duplicatum, being regarded as a combination of an acid salt with an alkaline salt.
The acquisition of this arcanum was regarded as the most essential element in the catechetical discipline, and marked off its possessors from the rest of the world.
Cappel was also the author of Annotationes et Commentarii in Vetus Testamentum, Chronologia Sacra, and other biblical works, as well as of several other treatises on Hebrew, among which are the Arcanum Punctuationis revelatum (1624) and the Diatriba de veris et antiquis Ebraeorum literis (1645).
Kunkel to have been known as an arcanum or secret medicine to the electoral house of Saxony in the middle of the 16th century, but it was first described by J.
Other encyclicals, such as those on Christian marriage (Arcanum divinae sapientiae, 10th February 1880), on the Rosary (Supremi apostolatus oficii, 1st September 1883, and Superiore anno, 5th September 1898), and on Freemasonry (Humanism genus, 20th April 1884), dealt with subjects on which his predecessor had been accustomed to pronounce allocutions, and were on similar lines.