a minute," I interrupted. "Were you standing inside the balloon so
that you had to breathe hydrogen?"

The professor smiled. "I stood inside the balloon, but I breathed
nothing; I held my breath, which is one of the things I have practised.
Before I went inside I told my wife to note the time by her watch, and
if I did not come out before one hundred and twenty seconds had passed
to have the men drag me out. You see, I knew I could hold my breath one
hundred and twenty seconds, but no longer.

"Well, we carried out the plan, and I freed the cords in less than my
limit of time; then came the uncanny part of it--at least, it seemed so
to me. I had read that hydrogen will not transmit sound, but had never
tested it. It is true I had at various times taken hydrogen into my
lungs, but never had I tried to speak in hydrogen. Now was my chance,
and, with all my remaining breath I shouted as loud as I could inside
that balloon. Think of it; there were my wife and the men a few feet
distant, with only the thinnest tissue of silk between us, and a gas
that was like nothing. Yet my cry, that would have reached perhaps half
a mile in air, could not penetrate that little void. To those outside
the balloon it was as if I had not opened my lips. They heard nothing,
not even a whisper. I believe you might fire a cannon inside a bag of
hydrogen, and no faintest rustle of the discharge would reach your ears.
So, you see, a world of hydrogen would be a voiceless world."

"Did you say you have breathed hydrogen?" I asked.

"Yes; I have breathed it up to the danger-point. I know all the
sensations. There is first a mild exhilaration, then a sense of
sickening and head-throbbing, and finally a delicious languor that leads
into stupor. When you get there it is time to stop. In making
ascensions we have to be very careful not to breathe too much gas from
the balloon-neck which hangs open over the basket. More than one
aeronaut has been gradually overcome without realizing that he was in
danger."

The professor went on to tell of other singular things about this subtle
gas, notably that, speaking within limits, the higher you want a balloon
to rise, the _less_ hydrogen you must put in it. If you fill a balloon
full of hydrogen it will rise to no great height (and is very apt to
burst), since the gas has no space to expand in, and the way to keep a
balloon rising is to make it expand more and more as it goes up, each
foot of added volume d