The most famous of these successors of Galileo was a man of the name of Torricelli, who lived in Rome. He had studied the works of Galileo as a boy of sixteen, and had himself written a book on Mechanics. Galileo saw the book and invited the young man to stay with him in Florence. It is said that the two became great friends, and that when Galileo was very old and blind his declining days were cheered by the conversation of young Torricelli. When Galileo died, his patron, the Grand Duke of Tuscany, made Torricelli Professor of Mathematics at the Academy in the place of Galileo. It was not long before Torricelli thought of a new and striking experiment concerning the vacuum. He decided to fill a glass tube with the heavy liquid quicksilver or mercury instead of water, and expected that the so-called resistance of the vacuum would be about fourteen times greater in the case of quicksilver than in the case of water, so that quicksilver could only be sucked up something between two and three feet instead of thirty-three feet. He found great difficulty in getting a suitable glass tube, since the glass-blowers of that time had not yet learned to make strong glass tubes, although they were very clever at making all kinds of bottles. Torricelli himself never carried out this experiment, but it was performed by a friend of his in the year 1643. It was described by Torricelli in some letters which he sent in the next year to a friend in Rome. That friend straightway wrote to some of his friends in Paris, and the news he conveyed to them created a great sensation. This was the experiment: A glass tube three feet long, and closed at one end, was filled with mercury. It was then stopped at the open end with the finger, and that end was carefully brought under the surface of a dish of mercury. On removing the finger it was found that some mercury flowed out of the tube, leaving a space of about six inches vacant at the top. This space was a vacuum. When Pascal, the great French man of science, heard of this experiment, he said, “It appears that the vacuum is not impossible in Nature, and that she does not shun it with so great a horror as some imagine.” [Illustration: Fig. 33.--Early form of Barometer.]