The history of the progress of the human mind shows, further, that the pure and simple acceptance of a scientific discovery is not enough to make it produce all the consequences we have a right to expect from it. It must, further, impregnating the mind with itself, pass, we might say, into the condition of an innate idea. Chemistry, in this very matter of the discovery of the weight of the air and of the gases, presents a striking example of the accuracy of our proposition. The ponderability of the air had been accepted by physicists for a long time, while chemists continued to take no account of it, although, as Mendeleef has remarked, no exact idea could be conceived, under such conditions, concerning most chemical phenomena. It is to the glory of Lavoisier that he first took account of this ponderability and of that of all the gases as well. When we reflect that it was not till about 1775, or a hundred and fifty years after Galileo, that this illustrious Frenchman began to set forth those ideas, it is not any wonder that the discovery of aërostats was not made till toward the end of the eighteenth century. Lalande was therefore much in the wrong when he said "it was so simple! why was it not done before?" It would not be just, however, to refer the discovery of aërostats solely to the efforts of the Montgolfiers. Like all inventors, like Lavoisier himself, these brothers, as Figuier has remarked, had the benefit of a long series of isolated labors, carried on often without special purpose, by which the elements of their invention had been gathered up. Père Lana, of Brescia, conceived a plan in 1670 for constructing a ship which should sustain itself in the air and move by the aid of sails. Four copper globes, in which a vacuum had been produced in order to render them lighter than the volume of air displaced, were to support the ship while the sails propelled it. The scientific conception of the empty globes was correct, but Père Lana did not think of the enormous collapsing force which the atmospheric pressure would exercise upon them. The idea of a sail which would give his aërial boat a resemblance to a vessel driven by the winds was wholly erroneous. Sixty-five years later, in 1735, Père Galien, of Avignon, gave a fairly clear expression to the theory of aërostats. Resting on the principle of Archimedes, he maintained that if he could fill a globe made of light cloth with a sufficiently rarefied air the globe would necessarily possess an ascensional force, which would permit it to lift itself up in the air with a ship and all its cargo. He proposed to draw this rarefied air from out of the upper regions of the atmosphere, down from the summits of high mountains, forgetting that the air, when brought down to the level of the ground, would contract in volume and assume the density of the ambient atmosphere.