human
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.
gpt
Did you say you have breathed hydrogen?
human
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.