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Rh subject came up for discussion at one of the Grand Duke's scientific parties, and Galileo took up the cudgels against the opinion of Aristotle. The company was divided, and among those who supported Galileo's side was Cardinal Barberini, who afterwards became Pope Urban VIII. The chief point alleged by the Aristotelian side was that a thin slab of ebony will float while a ball of ebony sinks at once. Galileo pointed out that the thin slab would also sink if it was wetted all over, and that similarly a flat piece of lighter wood would not remain at the bottom but would rise in spite of its shape. He performed many ingenious experiments with bodies compounded of wax and lead so as to be nearly of the same specific gravity as water, but apparently failed to point out that the floating of heavy bodies involves not only the hydrostatic pressure but also capillary action which is a distinct phenomenon. His treatise contained many experiments and clever arguments but met with violent opposition. Galileo himself always seems to have arrived very quickly at his conclusions, but the great difficulty of convincing others led him to multiply his experiments far beyond what he considered necessary in his own case; and this naturally strengthened his convictions. In this way he justified the aphorism that Ignorance had been his best teacher.

Many attacks on his principles of hydrostatics as set forth in the "Discourse on Floating Bodies" were published, and produced scathing replies either from Galileo himself or from his loyal pupil Castelli. The most elaborate, attacks, by, Colombe and Grazia, called forth the most detailed refutation, an essay printed at Florence in 1615, ostensibly by Castelli but really written by Galileo himself. It is not to his credit that in this essay, after completely crushing the arguments of Colombe and Grazia, he suggests that as he is only the pupil, they would