es respecting its elementary nature
is altogether erroneous. It is not a simple element, but is composed of
two ingredients, oxygen and hydrogen, as was rigorously proved by
decomposing and forming it. By degrees, more correct views of the nature
of evaporation were introduced; gases and vapours were found to coexist
in the same space, not because of their mutual solvent power, but
because of their individual and independent elasticity. The
instantaneous formation of vapours in a vacuum showed that the
determining condition is heat, the weight of vapour capable of existing
in a given space being proportional to the temperature. More scientific
views of the nature of maximum density were obtained, and on these
principles was effected the essential improvement of the low pressure
steam-engine--the apparent paradox of condensing the steam without
cooling the cylinder.

In like manner much light was cast on the meteorological functions of
water. It was seen that the diurnal vaporization from the earth depends
on the amount of heat received, the vapour rising invisibly in the air
till it reaches a region where the temperature is sufficiently low.
There condensation into vesicles of perhaps 1/50000 of an inch in
diameter ensues, and of myriads of such globules a cloud is composed.
[Sidenote: Clouds and their nomenclature.] Of clouds, notwithstanding
their many forms and aspects, a classification was given--cirrus,
cumulus, stratus, etc. It was obvious why some dissolve away and
disappear when they encounter warmer or drier spaces, and why others
descend as rain. It was shown that the drops can not be pure, since they
come in contact with dust, soluble gases, and organic matter in the air.
[Sidenote: The return of water to the sea.] Sinking into the ground, the
water issues forth as springs, contaminated with whatever is in the
soil, and finds its way, through streamlets and rivers, back to the sea,
and thus the drainage of countries is accomplished. Through such a
returning path it comes to the receptacle from which it set out; the
heat of the sun raised it from the ocean, the attraction of the earth
returns it thereto; and, since the heat-supply is invariable from year
to year, the quantity set in motion must be the same. Collateral results
of no little importance attend these movements. Every drop of rain
falling on the earth disintegrates and disturbs portions of the soil;
every stream carries solid matter into the sea. It