by tin and lead when they are calcined. "Now," he said, "that I have made my preparations, that is, have laid the foundations of my answer to his question" (as to the source of the seven ounces which the two pounds and six ounces of tin gained when heated for six hours; the difficulty of the problem was enhanced by its being necessary also to find the other matter required to compensate for the loss which the tin sustained through expansion in heating): "to this question, then, resting on the formulations already laid, I answer and maintain gloriously, that the increase in weight comes from the air, which has been thickened in the vessel, made heavy, and in no way adhesive, by the vehement and long-continued heat of the furnace; which air mingles with the earth (the frequent stirring aiding this) and attaches itself to the smallest particles; not otherwise than water makes sand heavy when sand is wet and the mass is stirred, by moistening and adhering to its smallest grains."
Several authors had already spoken of the increase of weight in metals on calcination. Cardan, in his Traité de la Subtilité, tried to explain the increase of the weight of lead in the formation of white lead by saying it was because the lead died and lost the celestial heat which was its soul and made it lighter; and added that an animal is always heavier dead than living. Rey remarked, in answer to this, that lead is void of life and can not be compared with the body of an animal, and showed that it was easy, by a known process, to recover the lead from its earth. Further, "nothing increases in weight except by the addition of matter or by contraction of volume," and this can not take place in the present case, even under Cardan's hypothesis, for the celestial heat in disappearing takes away matter, while on the other hand the volume increases perceptibly through the whole duration of the experiment. It will be noticed that Rey shared in the prejudice of his time in regard to the weight of animals increasing after their death, and with him many of the learned men of the period. Père Mersenne was the first to refute this error. He ascertained by experiment that a dog and a hen weigh more, though very little more, alive than dead, and wrote to Jean Rey, September 1, 1631, "You can yourself try the experiment without losing any of the blood, or a hair, or a feather, of the animals, by smothering them, as we have done."
Scaliger had undertaken to refute Cardan's assertions, and said that the increase in the weight of the calcined lead was caused by the fire consuming its aerated particles, comparing lead with the tile, "which is heavier baked than unburned." Nothing could be more simple than Rey's answer: "If the lead loses airy particles, would it not diminish in volume? On the contrary, it increases. And then, if this reason is correct, why do not
