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Rh question—one of "the most difficult that philosophy has ever brought forth." It was not without emotion that he took up the pen: "Believing I have reached the end of the matter, I produce these my essays, not without well foreseeing that I shall be called rash, for in them I shall disturb some of the maxims that have been approved for ages by most philosophers. But what can there be rash in exposing the truth to the light after having found it?" The book of twenty-eight chapters or essays is divided into distinct parts. In the first part the author, as it were, prepares his reader for the ideas he is about to set forth in the second. He first demonstrates the weight of the air—an entirely new fact in science then—and next he applies the ideas he has just enunciated to the explanation of the weight of lead and tin when calcined in the air.

The great physicists of the seventeenth century had as yet produced nothing when Jean Rey's essays appeared. Otto von Guericke was only twenty-one years old; Torricelli was still studying mathematics at Rome, and his celebrated experiment was not performed by Viviani till 1 643. Galileo was the only one who might at that moment have had established ideas on the weight of the air, and his Dialogues on the Motion and the Resistance of Fluids was not published at Leyden till 1638.

Rey was therefore the first person who declared that the air has weight, and he alone has the right to all the honor for this important discovery. His first essay is entitled Everything Material under the enclosure of the Sky has Weight. He supposed that the earth occupied the center of the world. "Matter, filling at every point the space inclosed under the curvature of the sky, is continually urged by its own weight toward the center of the world. True it is that earth, being the heaviest, promptly occupies that place, and, forcing its contraries into retreat, makes water, second in weight, also second in place; so that air, driven from the lowest and the second place, is confined to the third, leaving to fire, the least ponderous of all, to abide in the highest region." Thus Jean Rey showed very precisely that all bodies have weight, that there is nothing light in Nature, and "no upward movement that is natural." Let us give his own words: "I say, if there was a channel from the center of the earth up into the region of fire, open at both ends and full of the four elements, everything in its usual place, that, on drawing the earth down, water would descend to occupy its place, leaving its own to the air, and the air leaving its place to fire. Then, withdrawing the water from that place, the air would come down to fill it; and this, too, being taken away, the fire would take possession of the vacant space and fill the whole channel, descending to the center, just by that being removed which prevented its doing so. Those