How do we know this?
When a bell is rung in a vacuum, no sound is heard.
Can sounds be distinctly heard when the air is rare?
No; the rarer the air the more poorly it conducts sound.
What proof have we of this?
On the top of a high mountain the human voice is heard only at a distance of a few yards; and a pistol when exploded sounds as faintly as the breaking of a stick.
Can sounds be heard better when the air is dense?
Yes; the denser the air, the better it conducts sound.
What proof have we of this?
The human voice sounds so distinctly in the dense air of deep mines, that conversation may be carried on in a whisper.