While thus engaged, still working at physiology, Stensen turned his versatile mind to other problems, as well as to those of comparative anatomy, and especially to those of the infant, indeed hardly as yet born, science of geology. His work "De solido intra solidum" is thought by geologists to be a brilliant effort toward the beginning of their science In 1672 he returned for a while to his native city of {149} Copenhagen, but within two years he was back again at Florence; and then there came to him, while as yet a young man of some thirty-six summers, a sudden and profound change in his life. In his early days he had heard much, too much perhaps, of the doctrines of Luther. Probably he had been repelled by the austere devotion which ruled the paternal roof. And, as his answer to Bossuet shows, his university life and studies, his intercourse with the active intellects of many lands, and his passion for inquiry into natural knowledge, had freed him from passive obedience to dogma. He doubtless, as did many others of his time, looked upon himself as one of the enlightened, as one raised above the barren theological questions which were moving the minds of lesser men Yet it was out of this sceptical state of mind, that life in Italy and intimate contact with ecclesiastics and religious, so often said to be likely not to have any such effect, brought this acute scientific mind into the Catholic Church. Nor did he become merely a formal adherent, but an ardent believer, and then an enthusiastic proselytizer. One American writer of a history of medicine, in his utter failure to comprehend or sympathize with the change that came over Stensen, speaks of him as having become at the end of his life a mere "peripatetic converter of heretics." This phase of Stensen's life has, however, as ample significance as any that preceded it. Steno's expectations of the professorship of anatomy at Copenhagen were disappointed, but the appointment went to Jacobson, whose work indeed is scarcely less distinguished than that of {150} his unsuccessful rival. The next few years Stensen passed in Paris, where he was assiduous in making dissections, and where he attracted much attention; and then, somewhat later, in Italy; in 1665 and 1666 he was in Rome. Thence he went to Florence, in order to perfect himself in Italian. The next few years he spent in this city, having received the appointment of body physician to the Grand Duke, as well as an appointment of visiting physician, as we would call it now, to the Hospital of Santa Maria Nuova.