Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/281

Rh w.i'rEi>..] Section II.—Water. Of all the terrestrial agents by which the surface of the earth is geologically modified, by far the most important is water. When following hypogene changes in a foregoing part of this article, we found how large a share is taken by water in the phenomena of volcanoes and other subterranean nioveinents. When we returned to the surface of the earth and began to watch the operations of the atmosphere, we saw how impossible it is to consider these apart from the action of the aqueous vapour by which the atmosphere is pervaded. We must now study in detail the working of this wonderful geological agent itself. The substance which we term water exists on the earth in three wcll—knowii forms — (1) gaseous, as invisible vapour; liquid, as water; and (3) solid, as ice. The gaseous form has already been noticed in our inquiry into the geological characteristics of the air. It is in the air that this condition of the water—siibstance prevails. By the sun’s heat vast quantities of vapour are con- tinually raised froin the surface of the seas, rivers, lakes, snow-fields, and glaciers of the world. This vapour remains invisible until the air containing it is cooled down to below its dew—point, or point of saturation,—a result which follows upon the union or collision of two aerial currents of differ- ent temperatures, or the rise of the air into the upper cold regions of the atmosphere, where it is chilled by expansion, by radiation, and by contact with cold mountains. At ﬁrst minute particles 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 considerabl_e spaces of the sky, they give rise to the pheno- mena 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 con- siderable 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 down 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 the springs, gather into broader and yet broader rivers, whereby the drainage of the land is carried out to sea. Thence once more the vapour rises to reappear in clouds, and feed the innumer- able watcr—cliannels by which the land is furrowed from mountain—top to sea-shore. Here then is a vast system of circulation, ceaselessly re- newed. And in that system there is not a drop of water which is not busy with its allotted task of changing the face of the earth. When the vapour ascends into the air it is almost chemically pure. But when, after being con- densed into visible form, and working its way over or under the surface of the land, it 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 inust have worked marvels upon the surface of our planet in past time, and that it may effect vast transforma- tions 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. The subject of the geological operation of water upon GEOLOGY 267 the globe may be conveniently studied under the following subdivisions :— A. TERRESTRIAL WA'i‘ERs.——Under this head are to be considered—(l) the liquid state, including rain, under- ground water, brooks, rivers, and lakes; and the solid state—frost, river—ice, snow, hail, glaciers. B. OCEANIC W.-iTF.1is.—Inc1uding the inﬂuence of marine currents, tides, and waves, and the part taken by the sea in the general geological 1-égime of the earth. A. TERRESTRIAL WA'r1-ms. I. IN THE LIQUID sr.-vra. § 1. Iluin. Rain effects two kinds of changes upon the surface of the land. It acts cliemically upon soils and stones, and sinking under ground continues, as we shall ﬁnd, a great series of similar reactions there. It acts meclzanically, by washing away loose materials, and thus powerfully affecting the contours of the land. I. CHEMICAL Ac'rIoN.—This depends mainly upon the nature and proportion of the substances abstracted by rain from the air in its descent to the earth. Rain always absorbs a little air, and as we have already seen (p. 220) air always contains carbonic acid as well as other ingredi- ents, in addition to its nitrogen and oxygen. If rain be regarded as an agent washing the air and taking impurities out of it, we may the better realize how by means of these it is enabled to work many chemical changes which, were it to reach the earth as pure water, it could not accomplish. Composition of It’ai7i-ll’ater.—Numerous analyses of rain-water show that it contains in solution about 25 cubic centimetres of gases per litre. An average pro- portional percentage is by measure—nitrogen, 664; oxygen, 3l'2 ; carbonic acid, 2'4,—the oxygen being in greater proportion than in air, owing to its greater solu- bility in water. Common salt, ammonia, sulphates, nitric acid, inorganic dust, and organic matter are usually present in minute quantities in rain water. So far as we know at present, the three ingredients which are chieﬂy effective in the chemical reactions due to rain are the oxygen, carbonic acid, and organic matter.‘ Pe7‘mcabilit_7/ of all Ifoclcs by ll'ater.—Though minerals and rocks differ vastly in their degree of porousness, there is none known which is not in some degree permeable by water. Even such hard and apparently impenetrable sub- stances as ﬂint and agate are found to be permeable. For, in fact, rocks and minerals when examined with the micro- scope are seen to be made up of variously-shaped grains, crystals, or particles, and it is in the minute channels and interstices between these particles, or even through the particles themselves, that the water works its way. Evi- dently, the smaller the interstices the less easily will the water force a passage into or through the stone. This per- meability, though well marked upon the surface of the land, becomes still more so underground, where the rocks are sometimes quite satura.ted with water. Lialzilily of all Rocks to alteration by Water.—There is probably no known substance which is not, under some condition, soluble in water containing carbonic acid or other natural reagent. Rain-water, descending with the gases, acids, and organic matter it has abstracted from the air and soil, effects a chemical disintegration of the rocks. .Tl)lS action was referred to in the description of the air as 1 The organic matter is revealed by the putrid smell which 10118‘ kept rain water gives out. The reader who VlSlle-S to pursue tléls Til?- ject may consult the elaborate tables of analyses in lit Angus I111 3 Air and Rain. See also the section on air, ante, p. -20.