Page:The evolution of worlds - Lowell.djvu/269

Rh ice, and under atmospheric pressure, this speed proves to be a little over a mile a second—a speed, curiously enough, which is to that of light almost exactly as centimetres to miles. But some of the molecules are going

at speeds much above the mean; fewer and fewer as the speed gets higher. Just how many there are for any assigned speed, we can calculate by the same ingenious application of unknown quantities.

These speeds have been found for a temperature of freezing, and as the speed varies as the square root of the absolute temperature, we might suppose that when an adventurous or lucky molecule arrived at practically the limit of the atmosphere, where the cold is intense, it would become numbly sluggish. But let us consider