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There you have the first cause of the origin of underground water. You may add the more general principle that all elements arise from all: air comes from water, water from air; fire from air, air from fire. So why should not earth be formed from water, and conversely water from earth? If the earth is capable of transmutation into other elements, water must be one of them, in fact, the most suitable of them. The two things are cognate; both are heavy, condensed, both driven by nature down to the very confines of the universe. Earth is formed from water; why not water from earth in like manner?
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But, you say, the rivers are too large to be accounted for in this way. Well, after you have considered the size of the rivers, just look at the size of the reservoir whence they issue. Are you surprised that a fresh supply of water is always forthcoming for them, since they flow on for ever, some even rushing down their channel with impetuous haste? Surely you might as well be surprised, when the winds drive hither and thither the whole atmosphere, that the supply of air does not fail, but flows on day and night unceasingly. And the wind, remember, is not confined to a definite channel, as rivers are, but goes with wide sweep over the broad expanse of heaven. You might well, too, be surprised that after so many breakers have spent their force, any succeeding wave is left. The truth is, nothing is ever exhausted that returns upon itself (i.e. is self-supported). All the four elements return alternately upon one another; what is lost in one is conserved by passing into another. Nature, too, weighs her parts as if with nice adjustment in the balance, lest their just proportion should be disturbed and the world topple over into ruin (= lose its equilibrium). All elements are in all. Air not only passes into fire, but it is never without fire. Deprive it of its heat and it will grow stiff, stagnant, hard. Air passes into moisture, but nevertheless contains moisture. Earth yields both air and water, and is never at any time devoid of water any more than it is of air. The mutual transition is the easier, because there is already an admixture of the element to which the transition is to be made. So (1)51, then, the earth contains moisture, which it forces out. (2)51 It contains air, which the darkness of its wintry cold condenses so as to form moisture. (3)51 By nature, too, it has itself the power of changing into moisture: this power it habitually exerts.