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Show 228 EARTH AND WATER. they be in solid masses or in fragments, in powder or melted, and whether they belong to the animal, the vegetable, or the mineral kingdom,-and under the term "water," all of that substance, whether solid or liquid, or whether pure, or where it forms so much the prevailing ingredient in any compound as to give its own character decidedly to that compound, as in the case of sea-water, or of mineral springs; there is no knowing how much of these, as thus distinguished, may have existed at any period of the globe's history; and there i~ no knowing how they may have changed and shifted from time to time. Water may, however, be decomposed and again reproduced in so endless a variety of ways, and both the oxygen and the hydrogen which, in the present state of our chymical knowledge, we consider as its elements, are so active, and enter into combinations, as mixtures, with so many substances, that we have every reason to believe that the relative quantities of land and water, according to the sense in which the terms have been explained, are not for any two successive moments exactly the same. Very many of the metals exist in the earth in the state of oxides, or combinations of the metal with oxygen; and not a few of them ha'\te a third ingredient, or are triple salts. The alkalis, and many of the earths, have been proved by experiment to be hvdrates of metals, or compounds of those · metals with the other ingredient of water; and it is probable that, when more powerful means of chymical decomposition shall have been discovered, all the earths will be found to contain hydrogen, as well as all the alkalis and most of the salts. No man could, therefore, though he could gauge all the seas and lakes, measure all the rivers and streams, and weigh all the clouds, venture to give even an approximate estimate of the quantity of water and its elements, even for one time. · The seasonal changes of it are also considerable. CHANGES OF QUANTITY. 229 ~n England it is in fogs and fens in the winter, and m the ?rops on the fields, and the leaves, flowers, ~nd frmts of other annual and deciduous vegetables, ~n the. sum~er. I~ climat~s farther to the north, it IS dunng wmter piled up m snow and 1ce : and in summe~ it is ~ither at work in t~e more scanty vegetatiOn, or It has ebbed away to the ocean in the spring "freshes" and flooJs. The action of gravit~tio.n di~tributes it equally in the ocean; and ~~en xt nses m vapour, the action of heat disperses 1t m the atmosphere. What these causes may from time to time produc. e we cannot calculate ; but within a very long penod of duration-one as long at least as we have any thing like authentic information, it does not appear that the great collected quantity has varied much. Now, according to the estimates which on such a subject must be vague, if the solid parts of the e.arth.-~ho~e parts that we have denominated land m dtstmctwn from water-were in the form ·of a regula~ spheroid, the form which gravity and the revolutwn and rotation of the earth (the only ex_ter!lal causes that we know that could act upon pnnc~ples that we understand in moulding the earth mto It~ form),-the water would cover it with a crust (t! the term may b~ use~l), or. shell of water, somethu;tg about two miles m thiCkness. That would give a. pressure of more than two tuns on every square mch of the solid nucleus, exclusive of the pressu~e of the atmosphere,-or a pressure eqtial to the wetght of the whole navy of Great Britain on a surface equal to the floor of an ordinary-sized room. When we reflect upon that immense resistance of pressure which the internal powers of the earth that elevated the mountains had to overcome we may cease to wonder at the results that have been produced ; nor need we be in the least astonished when we go to mountainous countries and fmd lJ , |