
~ 
Parr Il. Secor. ii. § 1.) WATER CIRCULATION. 329 
regions of the atmosphere, where it is chilled by expansion, by 
radiation, and by contact with cold mountains. According to 
recent researches, condensation appears only to take place on free 
surfaces, and the formation of cloud and mist is explained by con- 
densation upon the fine microscopic dust of which the atmosphere is 
full’ At first minute particles of water vapour appear, which 
either remain in the liquid condition, or, if the temperature is 
sufficiently low, are at once frozen into ice. As these changes 
take place over considerable spaces of the sky, they give rise to 
the phenomena of clouds. Further condensation augments the 
size of the cloud-particles, and at last they fall to the surface 
of the earth, if still liquid, as rain; if solid, as snow or hail; 
and if partly solid and partly liquid, as sleet. As the vapour is 
largely raised from the ocean surface, so in great measure it falls 
back again directly into the ocean. <A considerable proportion, 
however, descends upon the land, and it is this part of the condensed 
vapour which we have now to follow. Upon the higher elevations 
it falls as snow, and gathers there into snow-fields, which, by means 
of glaciers, send their drainage towards the valleys and plains. 
Elsewhere it falls chiefly as rain, some of which sinks underground 
to gush forth again in springs, while the rest pours down the slopes 
of the land, feeding brooks and torrents, which, swollen further by 
springs, gather into broader and yet broader rivers, whereby the 
accumulated drainage of the land is carried out to sea. Thence 
once more the vapour rises, to reappear in clouds and rain and to feed 
the innumerable water-channels by which the land is furrowed from 
mountain-top to sea-shore. 
In this vast system of circulation, ceaselessly renewed, there is 
not a drop of water that is not busy with its allotted task of 
changing the face of the earth. When the vapour ascends into the 
air it is comparatively speaking chemically pure. But when, after 
being condensed into visible form, and working its way over or 
under the surface of the land,1t once more enters the sea, it is 
no longer pure, but more or less loaded with material taken by it 
out of the air, rocks, or soils through which it has travelled. Day 
by day the process is advancing. So far as we can tell, it has 
never ceased since the first shower of rain fell upon the earth. 
We may well believe, therefore, that it must have worked marvels 
upon the surface of our planet in past time, and that it may effect 
yast transformations in the future. As a foundation for such a 
belief let us now inquire what it can be proved to be doing at the 
present time. 
§ I.-Rain. 
Rain effects two kinds of changes upon the surface of the land. 
(1.) It acts chemically upon soils and stones, and sinking under 
ground continues, as we shall find, a great series of similar reactions 
* Coulier and Mascart, Naturforscher 1875, p. 400. Aitken, Proc. Roy. Soc. Edin: 
December 1880. 
