that they may multiply and propagate their like." So they may be transmitted on and on. In some diseases the transmission takes place only through contact; in other diseases the fomites, or sparks of infection, may be transmitted by intermediate agents, garments for example; in still others the fomites may be carried from a distance by the air.

Paracelsus and Cardanus too had some idea of the - seeds of disease. But Fracastoro stated clearly, with rational arguments, the explanation of the transmission of disease which has been demonstrated in our own generation. His works were widely read, and a hundred years after him Italian physicians, taking up his theory, demonstrated more adequately the nature of infection.13

As already remarked, the progress of one or another of the physical sciences in the sixteenth century seems to have hung upon the employment of some valid method of experimental observation which might lead to the grasp of a central fact or principle. This in turn served as a basis for further discoveries. Vesalius had used such a method; two generations after him Harvey grasped such a basic fact in his demonstration of the circulation of the blood. Without the foundation of Vesalius's anatomy, his contemporary Fracastoro presented a theory of infection which might have afforded a basic fact for medicine had the natural knowledge of the time and the means of biological investigation been adequate to demonstrate its validity. - Vesalius had demonstrated the value of his method; Harvey also succeeded in demonstrating his great fact. But the nature and action of the seminaria contagionum were not demonstrated for three centuries. Until then Fracastoro's theory could not become the basis of new measures for the prevention and cure of disease.

Paracelsus, with his keen intuitions and experiments and his Will-o-the-wisp speculations, was a natural product of sixteenth century medicine, so conspicuously wanting

13 The best account of Fracastoro is that of Charles and D. Singer: "The scientific position of Girolamo Fracastoro:" Annals of Medical History, Vol. I, pp. 1-34 (1917). See also A. C. Klebs in Johns Hopkins Bulletin, Vol. XXVI, No. 297 (1915), and a paper by Sir William Osler in the Proceedings of the Charaka Club of New York,

in valid conceptions of disease. Not in medicine alone was there crying need to winnow the chaff from the common acceptances even of the learned and intelligent. This might not come to pass until habits of testing facts had become more prevalent, and general conceptions of material probabilities and the regular action of natural agencies had had time to enter men's blood. Girolamo Cardan of Pavia and Milan (1501-1576) is another telling illustration of the opinions, sound and baseless, obtaining among the intelligent.

Of his difficult childhood and early struggles against the hindrances of illegitimate birth and the merited antipathy of professional brethren, of his slowly won repute as a physician, of his professional visits extending to the ultima thule of Scotland, of his great fame as an author and astrologer, and the calamities of his later years, he has left the detailed and picturesque story in his De vita propria.14 His father, Fazio Cardan, a jurist and mathematician, lived at Milan and was a familiar of Leonardo da Vinci, and the son, Girolamo, Hieronimo, or Jerome, from his youth was devoted to mathematics, physical studies and gambling. Having studied at Pavia, he graduated at Padua in medicine. He relied on portents and dreams and his interpretation of their forecastings, which assured him among other things, that he was destined to strive for immortality. There was no more careful and scientific astrologer in his time. He won his first repute as a physician by cures effected in Milan; where with boldness and acerbity he attacked the current errors of practice in his first published work, De malo recentiorum medicorum medendi usu. (Venice 1536). He had always written voluminously,' even before he could get a printer.

Devoured with ardor and ambition, he published a notable work on Arithmetic and another on Algebra, which, with all allowance made for the writer's use of others' formulae, was a substantial contribution to the

14 Henry Morley has followed this work and given its substance delightfully, along with further data, im his Jerome Cardan (2 Vols., London,

progress of that science. He worked and wrote prodigiously; no current topic of intellectual curiosity escaped him-dreams, palmistry, ghosts, portents in earth and heaven, and of course astrology. All these matters were accepted by intelligent men, and verified by their observations, and by none more carefully and scientifically than by Cardan. He also wrote tremendous commentaries on Hippocrates and volumes of admirable moral aphorisms. He was a man of versatility and manifold intellectual aptitude. The two books which gave him widest and most lasting fame were his De Subtilitate Rerum, worked upon for years and published in 1551, and his supplemental De Varietate Rerum, published six years later. They were encyclopaedic treatises upon the contents of the universe, including the spiritual beings in it malignant and benign-Demons, Angels and Intelligences. The sciences were included, the arts, and the curiosities of well-vouched for fact!

"Sub

The De Subtilitate was written from a certain point of view, and with an underlying (rather deeply buried!) thought: it should set forth the principle of the subtle nature and effects of things as apprehended, not without difficulty, by the senses and the intellect. tilitas," says Cardan, is a "certain ratio, by which sensible things are comprehended by the senses and intelligible things by the mind difficile, with difficulty." 15 With difficulty; so is it still and so will it be. With all imaginable increase of knowledge, the natures, origins, and effects of things are not likely to become obvious.

Before leaving this natural philosopher we return to the apparently innocent fact, that Cardan's father Fazio was one of Leonardo's familiars in Milan. The son was not a man gratuitously to acknowledge indebtedness to any modern; in his book on Algebra he had used an algebraic formula of one Tartaglia, though he had sworn not to publish or divulge it; and of course, with no reference to its parentage. Likewise the De Subtilitate seems mutely indebted to the writings of Leonardo upon paint15 Quoted by Morley, o. c. II, p. 60.

ing and physics.16 Cardan probably had access to these writings, and would have been the last man to mention it; although he refers to the celebrated painter more than once. He had passed his boyhood and youth literally in the service of his father Fazio, who died in 1524 when the son was twenty-three years old. If Leonardo had, as he says himself, borrowed books from Fazio, why should not Fazio have borrowed something in return; and his son have used the material?

16 This seems proved by Duhem, Études sur Léonardo da Vinci, I, pp. 223-245.

CHAPTER XXXIII

THE REVOLUTION IN ASTRONOMY AND PHYSICS

WE gathered in the last chapter that a number of opinions and beliefs which the enlightenment of our own time regards as superstitions made part of the scientific or quasi-scientific data of the sixteenth century, and might deflect or impede the progress of more valid knowledge or more consistent theorizing. Melanchthon in Germany was a clear thinker and a prodigy as a scholar, though of course not an investigator in physical science. He selected from what he found in books. But doubtless his firm belief in the influence of the stars on human fates would affect his presentation of the subject matter of his book on Physics. It is hard for us to discover a scientific mind or method in a man like Cardanus who was infatuated with dreams and portents, and an accomplished astrologer. Paracelsus rejected astrology, and yet involved his thinking in occultism, and accepted many things to us absurd. An independent but somewhat mystical genius like Paracelsus, and such a wonder of a Cabalistic Platonist as Pico della Mirandola, made use of conceptions which have since lost their validity; but they bent them to quasi-rational meanings.1

As is well known, the fifteenth and sixteenth centuries brought a marked revival of those beliefs and reasonings . concerning the influence of the stars upon human affairs, with the casting of horoscopes, all making up the agelong pseudo-science of astrology. These beliefs permeated the opinions and judgments of mankind, and engaged the constant study of scholars, scientists and statesmen. Astrology was held the noblest of sciences, and its enlightenment and guidance sought in the most important affairs

1 See ante p. 319 and p. 277 sqq. as to their handling of the conception of magic taken from the Cabbala.