But it is time now that we return to the two brothers Montgolfier and their paper-bag of smoke. Their experiments proved at once that in smoke they had found something which was lighter than air, and which would, therefore, carry a light weight upwards. But of what this something was they had, at the time, but a confused idea. They imagined that the burning fuel they had used had given off some special light gas, with the exact nature of which they were unacquainted. The very word gas, be it here said, was in those days almost unknown, and of different gases, their nature and properties, most people had but the very vaguest notions. And so for some time the Montgolfiers and their followers supposed that the presence of this mysterious gas was necessary to the success of their experiments, and they were very careful about always using special kinds of fuel, which they supposed gave off this gas, to inflate their bags. Later experiments proved, however, what every one now knows, that the paper-bag rose, not because of the gases given off by the fire, but by reason of the hot air with which it became filled. Nearly all substances, no matter how solid, expand more or less under the influence of heat, and air expands very greatly indeed. By thus expanding heated air becomes lighter than the surrounding air, and, because it is lighter, rises upwards in the atmosphere, and continues to rise until it has once more regained the average temperature. [Illustration: MONTGOLFIER’S BALLOON.] Encouraged by the success of their first humble experiment, the Montgolfiers next tried their paper-bag in the open air, when to their delight it sailed upwards to a height of 70 feet. The next step was to make a much larger craft of 600 cubic feet capacity and spherical in shape, which they called a “Balloon,” because it was in appearance like a large, round, short-necked vessel used in chemistry which was technically known by that name. This great bag, after being inflated, became so powerful that it broke loose from its moorings, and floated proudly upwards 600 feet and more, and came down in an adjoining field. After a few more successful trials the brothers thought that the time had come to make known their new invention. Accordingly they constructed a great balloon of 35 feet in diameter, and issued invitations to the public to come and see the inflation. This was successfully made over a fire of chopped straw and wool, and the giant rose up into the sky amid the deafening applause of a huge multitude, and after attaining a height of 7000 feet, fell to the ground a mile and a half away.