THE ESSAYS OF JEAN REY. 255 



The final objection to his theory was one of Rey's own sugges- 

 tion : Why does not earth go on increasing in weight indefinitely ? 

 Because " the thickened air attaches itself to it and continues to 

 adhere to the most minute of its particles, so that its weight goes 

 on increasing from the beginning to the end ; but when it is all 

 enveloped in air it can not take any more." He concludes then, 

 and terminates his treatise by declaring with pride that he has 

 found the real way of the truth, breaking the road for his success- 

 ors, and advising them not to go astray from it. 



This is the summary of the works of Jean Rey. A skillful 

 experimenter, he knew how to use the balance, and it was the 

 balance that suggested to him the result of his experiments. His 

 book is a brief one. A single principal experiment is described, a 

 single object is pursued in it. But he made two great advances 

 in science. He discovered the weight of the air, being the first 

 to publish that hypothesis, and verified it by experiments in 

 chemistry and physics. The increase of the weight of lead and 

 tin on calcination had been noticed for a long time by the al- 

 chemists, and even Galen knew of it. But nobody before Rey 

 found that the cause of that increase in weight came from the air 

 from that thickened and heavy air. It was certainly a remark- 

 able achievement to announce such a fact at a time when chemis- 

 try had made so little advance. No gas was yet known ; and it 

 was not till about 1719 that a misunderstood man of science, 

 Mortrel d'Eldment, found means to decant air through water into 

 bottles, and taught in a public lecture in Paris " how to make air 

 visible and perceptible enough to measure it in pints or in what- 

 ever quantity you will." It was not his fault, therefore, that he 

 did not advance further. Translaied for Tlie Popular Science 

 Monthly from the Revue Scientifique. 



As a possible solution, or working hypothesis, of the reason of the migration 

 of tiirds, Canon Tristram suggests: " Instinct in mammals and birds attracts them 

 to the place of their nativity. Wben the increasing cold of the northern regions, 

 in which they all had their origin, drove the mammals southward, they could not 

 retrace their steps, because the increasing polar sea, as the arctic continent sank, 

 barred their way. The birds reluctantly left their homes as winter came on and 

 followed the supply of food. But as the season in their new residence became 

 hotter in summer, they instinctively returned to their birthplaces, and there 

 reared their young, retiring with them when the recurring winter impelled them 

 to seek a warmer climate. Tliose species which, unfitted for a greater amount of 

 heat by their more proti'acted sojourn in the northern regions, persisted in revis- 

 iting their ancestral homes, or getting as near to them as they could, retained a 

 capacity for enjoying a temperate climate, which, very gradually, was lost by the 

 species which settled down more permanently in their new quarters, and thus a 

 law of migration became established on the one side, and sedentary habits on the 

 other." 



