oy above
the age of twelve knows that hot air rises upwards in the atmosphere,
and that it continues to rise until its temperature has become the same
as that of the surrounding air.

The next experiment was to try their bags in the open air. Choosing a
calm, fine day, they made a fire similar to that used in their first
experiments, and succeeded in making the bag rise nearly 100 feet. Later
on, a much larger craft was built, which was equally successful.

And now we must leave the experiments of the Montgolfiers for a
moment, and turn to the discovery of hydrogen gas by Henry Cavendish,
a well-known London chemist. In 1766 Cavendish proved conclusively that
hydrogen gas was not more than one-seventh the weight of ordinary air.
It at once occurred to Dr. Black, of Glasgow, that if a thin bag could
be filled with this light gas it would rise in the air; but for various
reasons his experiments did not yield results of a practical nature for
several years.

Some time afterwards, about a year before the Montgolfiers commenced
their experiments which we have already described, Tiberius Cavallo, an
Italian chemist, succeeded in making, with hydrogen gas, soap-bubbles
which rose in the air. Previous to this he had experimented with
bladders and paper bags; but the bladders he found too heavy, and the
paper too porous.

It must not be thought that the Montgolfiers experimented solely with
hot air in the inflation of their balloons. At one time they used steam,
and, later on, the newly-discovered hydrogen gas; but with both these
agents they were unsuccessful. It can easily be seen why steam was of no
use, when we consider that paper was employed; hydrogen, too, owed its
lack of success to the same cause for the porosity of the paper allowed
the gas to escape quickly.

It is said that the name "balloon" was given to these paper craft
because they resembled in shape a large spherical vessel used in
chemistry, which was known by that name. To the brothers Montgolfier
belongs the honour of having given the name to this type of aircraft,
which, in the two succeeding centuries, became so popular.

After numerous experiments the public were invited to witness the
inflation of a particularly huge balloon, over 30 feet in diameter.
This was accomplished over a fire made of wool and straw. The ascent was
successful, and the balloon, after rising to a height of some 7000 feet,
fell to earth about two miles away.

It may be imagi