During the five years until 1650 the department of acoustics came under his consideration, so that in that year we have from his pen a treatise called "Musurgia Universalis," with the subtitle, "The Art of Harmony and Discord; a treatise on the whole doctrine of sound with the philosophy of music treated from the standpoint of practical as well as theoretic science." During the next five years astronomy was his special hobby, and the result was in 1656 a treatise on astronomy called "Iter Celeste." This contained a description of the earth and the heavens and discussed the nature of the fixed and moving stars, with various considerations as to the composition and structure of these bodies. A second volume on this subject appeared in 1660. The variety of Father Kircher's interests in science was not yet exhausted, however. Five years after the completion of his two volumes on astronomy there came one on "Mundus Subterraneus." This treated of the modern subjects of geology, metallurgy, and mineralogy, as well as the chemistry of minerals. It also contained a treatise on animals that live under the ground, and on insects. This was considered one of the author's greatest books, and the whole of it was translated into French, whilst abstracts from it, especially the chapters on poisons, appeared in most of the other languages of Europe. Part of it was translated even into English, though seventeenth-century Englishmen were loath to draw their inspiration from Jesuit writers. {123} Jesuits were, however, at this time generally acknowledged on the Continent to be leaders in every department of thought, sympathetic coadjutors in every step in scientific progress. Strange as it may appear to those who will not understand the Jesuit spirit of love for learning, two of the most distinguished scientists whose names are immortal in the history of physical sciences in different departments during this century, Kepler and Harvey, were on intimate terms of friendship with the Jesuits of Germany. Harvey, on the occasion of a visit to the Continent, stopped for a prolonged visit with the Jesuits at Cologne, so that some of his English friends joked him about the possibility of his making converts of the Jesuits. These witticisms, however, did not seem to distract Harvey very much, for he returned on a subsequent occasion to spend some further days with his Jesuit scientific friends along the Rhine.