Albertus Magnus' other great scientific pupil, Roger Bacon, the English Franciscan friar, followed more closely in the scientific ways of his great master, devoting himself almost entirely to the physical sciences. Altogether he wrote some eighteen treatises on chemical subjects. For a long time it was considered that he was the inventor of gunpowder, though this is now known to have been introduced into Europe by the Arabs. Roger Bacon studied gunpowder and various other explosive combinations in considerable detail, and it is for this reason that he obtained the undeserved reputation of being an original discoverer in this line. How well he realized how much might be accomplished by means of the energy stored up in explosives, can, perhaps, be best appreciated from the fact that he suggested that boats would go along the rivers and across seas without either sails or oars, and that carriages would go along the streets without horse or man power. He considered that man would eventually invent a method of harnessing these explosive mixtures, and of utilizing their energies for his purposes without danger. It is curiously interesting to find, as we begin the twentieth century, and gasolene is so commonly used for the driving of automobiles and motor boats, and is being introduced even into heavier transportation as the most available source of energy for suburban traffic, at least, that this generation should only be fulfilling the idea of the old Franciscan friar of the thirteenth century, who prophesied that in explosives there was the secret of eventually manageable energy for transportation purposes. Succeeding centuries were not as fruitful in great scientists as the thirteenth, and yet, in the second half of the thirteenth, there was a Pope, John XXI, who had been a physician and professor of medicine before his election to the Papacy, three of whose scientific treatises--one on the transmutation of metals, which he considers an impossibility, at least as far as the manufacture of gold and silver was concerned; a treatise on diseases of the eyes, to which good authorities have not hesitated to give lavish praise for its practical value, considering the conditions in which it was written; and, finally, his treatise on the preservation of the health, written when he was himself over eighty years of age--are all considered by good authorities as worthy of the best scientific spirit of the time.