as events proved, they were not far wrong. For a time they
imagined that the fire they had used generated some special gas,
and if they could find out the nature of this gas, and the means
of making it in large quantities, they would be able to add to
their success.

Of course, in the light of modern knowledge, it seems strange
that the brothers did not know that the reason the bags rose, was
not because of any special gas being used, but owing to the
expansion of air under the influence of heat, whereby hot air
tends to rise. Every schoolboy 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.