gases
was not understood. But he had followed the physical investigations of
the seventeenth century, and was well acquainted with Torricelli's
demonstration of the weight of the atmosphere. The only practical way
for him to make a vessel lighter than air was to empty it of the air
within it, and Torricelli's invention of the barometer seemed to bring
such a device within reach. The common pump begat the barometer; the
barometer begat the balloon. But the enormous pressure of the atmosphere
on a vessel encasing a vacuum, though Lana had triumphed over it in
argument, could not be so easily dealt with in practice. The success of
the balloon was delayed until, by the discovery and production of a gas
lighter than air, a frail and thin envelope could be supported against
the pressure from without by an equal pressure from within.

For ballooning what was chiefly necessary was a thorough knowledge of
gases and of the means of producing them. The older chemistry, or
alchemy, devoted all its attention, for centuries, to the precious
metals, and knew nothing of gas. Medical chemistry, which succeeded it,
was concerned chiefly with the curative properties of various chemical
preparations. When Robert Boyle, and the investigators who came after
him, put aside this age-long preoccupation with wealth and healing, and
set themselves to determine, by observation and experiment, the nature
of common substances, and the possibility of resolving them into simpler
elements, modern chemistry began. Four states of matter, namely, earth,
air, fire, and water, were recognized by the older chemists, and were by
them called elements; it was the work of the eighteenth century to
investigate these, and especially to separate the constituents of air
and of water. In 1774 Joseph Priestley discovered oxygen. In 1782 Henry
Cavendish showed that hydrogen, when burnt, produces water. At a much
earlier date hydrogen had been produced by the action of acid on metals,
and had been found to be many times lighter than air. Dr. Joseph Black,
professor of chemistry in the University of Edinburgh, was the first to
suggest, in 1767, that a balloon inflated with hydrogen would rise in
the air; and the experiment was successfully tried with soap-bubbles by
Tiberius Cavallo, in the year 1782.

Nevertheless, the famous first balloon, which ascended in 1783, was not
filled with hydrogen, and was invented by what may be called a happy
accident. The brothers Josep