He paused a moment as if to observe my attitude, to see if it were one of scoffing. I made no remark, but begged him to go on.
"After all," he continued, "even the books allow to hydrogen properties that are very amazing. It is the lightest of all things; it passes through and beyond all things; it is the nearest approach we know of to absolute nothing. Who can say that it is not related to the land of nothing, to—" He hesitated.
"You mean?" said I.
"I don't know what I mean. I only wonder. Take this case that happened at Ogdensburg, New York, during an ascension we made there. We had filled the balloon with hydrogen, and were just ready to start when the valve-cords that hang down inside the bag from the valve at the top became twisted and drew up out of reach from the basket. In vain I tried to get them free by poking at them with sticks and long-handled things; the cords would not come down, and of course no sane man would make an ascension with his balloon-valve beyond control. There was nothing for it but to get inside that great gas-bag and undo the tangle with my hands. So I called fifteen or twenty men to catch hold of the netting and pull the struggling balloon down over me until I could reach the cords. Then I—"
"Wait 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 aëronaut 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 displacing a foot of the air-ocean and to that extent adding buoyancy.