It was heavy, this vapour, heavier than the densest smoke, so that,
after the first tumultuous uprush and outflow of its impact, it sank
down through the air and poured over the ground in a manner rather
liquid than gaseous, abandoning the hills, and streaming into the
valleys and ditches and watercourses even as I have heard the
carbonic-acid gas that pours from volcanic clefts is wont to do.  And
where it came upon water some chemical action occurred, and the
surface would be instantly covered with a powdery scum that sank
slowly and made way for more.  The scum was absolutely insoluble, and
it is a strange thing, seeing the instant effect of the gas, that one
could drink without hurt the water from which it had been strained.
The vapour did not diffuse as a true gas would do.  It hung together
in banks, flowing sluggishly down the slope of the land and driving
reluctantly before the wind, and very slowly it combined with the mist
and moisture of the air, and sank to the earth in the form of dust.
Save that an unknown element giving a group of four lines in the blue
of the spectrum is concerned, we are still entirely ignorant of the
nature of this substance.