readily dissolved in the soil and appropriated as nourishment by the
roots of plants.

It will be observed that all these beneficial effects of dust are due to
its presence in such quantities as are produced by natural causes, since
both gentle showers as well as ample rains and deep blue skies are
present throughout the vast equatorial forest districts, where
dust-forming agencies seem to be at a minimum. But in all
densely-populated countries there is an enormous artificial production
of dust--from our ploughed fields, from our roads and streets, where
dust is continually formed by the iron-shod hoofs of innumerable horses,
but chiefly from our enormous combustion of fuel pouring into the air
volumes of smoke charged with unconsumed particles of carbon. This
superabundance of dust, probably many times greater than that which
would be produced under the more natural conditions which prevailed when
our country was more thinly populated, must almost certainly produce
some effect on our climate; and the particular effect it seems
calculated to produce is the increase of cloud and fog, but not
necessarily any increase of rain. Rain depends on the supply of aqueous
vapour by evaporation; on temperature, which determines the dew point;
and on changes in barometric pressure, which determine the winds. There
is probably always and everywhere enough atmospheric dust to serve as
centres of condensation at considerable altitudes, and thus to initiate
rainfall when the other conditions are favourable; but the presence of
increased quantities of dust at the lower levels must lead to the
formation of denser clouds, although the minute water-vesicles cannot
descend as rain, because, as they pass down into warmer and dryer strata
of air, they are again evaporated.

Now, there is much evidence to show that there has been a considerable