due, not to the rarefaction of the air within it--which was its
true cause--but to the evolution of some light gas disengaged by
the nature of the fuel used. It followed, therefore, almost as
a matter of course, that chemists, who, as stated in the last
chapter, were already acquainted with so-called "inflammable
air," or hydrogen gas, grasped the fact that this gas would
serve better than any other for the purposes of a balloon. And
no sooner had the news of the Montgolfiers' success reached
Paris than a subscription was raised, and M. Charles, Professor
of Experimental Philosophy, was appointed, with the assistance
of M. Roberts, to superintend the construction of a suitable
balloon and its inflation by the proposed new method.

The task was one of considerable difficulty, owing partly to
the necessity of procuring some material which would prevent
the escape of the lightest and most subtle gas known, and no
less by reason of the difficulty of preparing under pressure a
sufficient quantity of gas itself. The experiment, sound
enough in theory, was eventually carried through after several
instructive failures. A suitable material was found in
"lustring," a glossy silk cloth varnished with a solution of
caoutchouc, and this being formed into a balloon only thirteen
feet in diameter and fitted without other aperture than a
stopcock, was after several attempts filled with hydrogen gas
prepared in the usual way by the action of dilute sulphuric
acid on scrap iron.

The preparations completed, one last and all-important mistake
was made by closing the stop-cock before the balloon was
dismissed, the disastrous and unavoidable result of this being