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.
In the condition of ignorance of the properties of gases that existed in that age, it did not occur, and could not have occurred, to Père Galien to use other gases than air; no more could he have thought of employing heat to rarefy the air, for the first not very precise notions on the decrease in densities of gases by heat only date from Priestley. But when Cavendish, in 1765, had fully studied hydrogen gas, and shown that as it was prepared then it was seven times lighter than air. Black was enabled to suggest that by filling a light bag with hydrogen the bag would be able to raise a certain weight in the air. The labors of Cavendish, Black, and the discoveries of oxygen, nitrogen, and other gases by Priestley, were described by Priestley a few years afterward in the celebrated book on The Different Kinds of Air—a book which Stephen and Joseph Montgolfier had in their possession.
