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THERMOSCOPES OF THE ITALIANS.

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he was in constant communication with Galileo, Descartes, Gassendi, Roberval, Hobbes, and others, sending them news of discoveries and inventions in exchange for similar favors. Mersenne was also an experimenter, repeating and verifying the labors of others and thus familiarizing himself with every branch of physical science, but he made no noteworthy discovery; he was the originator of the custom of propounding prize questions, a scheme for stimulating scientific work afterwards adopted by certain learned societies. His "Récréations des savans" was published in 1634.

There lived at that time in Southern France an obscure physician named Jean Rey, who did two things in his lifetime that ought to have brought him renown, but he was in advance of the age and his discoveries were not appreciated by his contemporaries. Rey applied his knowledge of chemistry to the solution of the much vexed problem "why do lead and tin increase in weight when calcined?" In a book published on this subject in 1638, he gave the correct explanation, recognizing that the metals combined with a constituent of the air, and anticipating the grand truths that made Lavoisier famous 150 years later.