THE ESSAYS OF JEAN REY. 249 



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 Key'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 1643. 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 Re- 

 sistance 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 Mate- 

 rial 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 oc- 

 cupies 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 up- 

 ward 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 



