NATURAL HISTORY.

CENTURY I.

Experiments in consort touching the straining and passing of bodies one through another; which they call Percolation.

1. DIG a pit upon the sea-shore, somewhat above the high-water mark, and sink it as deep as the low-water mark; and as the tide cometh in, it will fill with water, fresh and potable. This is commonly practised upon the coast of Barbary, where other fresh water is wanting. And Cæsar knew this well when he was besieged in Alexandria: for by digging of pits in the seashore, he did frustrate the laborious works of the enemies, which had turned the sea-water upon the wells of Alexandria; and so saved his army, being then in desperation. But Cæsar mistook the cause, for he thought that all sea-sands had natural springs of fresh water. But it is plain that it is the sea-water; because the pit filleth according to the measure of the tide2; and the sea-water passing or straining through the sands leaveth the saltness.

2. I remember to have read that trial hath been made. of salt water passed through earth, through ten vessels one within another, and yet it hath not lost his saltness, as to become potable: but the same man saith, that (by the relation of another) salt water drained through twenty vessels hath become fresh. This experiment seemeth to cross that other of pits

Hirtius, De Bello Alexandrino, c 8. and 9. ; and see Aristot. Prob. sect. xxiii. 21. and 37.

2 Wells of fresh water close upon the sea shore sometimes ebb and flow with the tide. But this arises from the comparative levity of the fresh water, in consequence of which it is, so to speak, floated up when the tide comes in. Or it may arise from the presence of compressed air in the interstices of the soil which lies between the fresh and the salt water; an explanation which appears to be confirmed by recent experiments on the subject of drainage.

* This statement is taken from J. B. Porta. See his Natural Magic, xx. 1. Aristotle, in support of the opinion that fish are nourished by the fresh water present in the sea, states that a closed vessel of thin wax immersed for a certain time in the sea

made by the sea-side; and yet but in part, if it be true that twenty repetitions do the effect. But it is worth the note, how poor the imitations of nature are in common course of experiments, except they be led by great judgment, and some good light of axioms. For first, there is no small difference between a passage of water through twenty small vessels, and through such a distance as between the low-water and high-water mark. Secondly, there is a great difference between earth and sand; for all earth hath in it a kind of nitrous salt, from which sand is more free; and besides earth doth not strain the water so finely as sand doth. But there is a third point that I suspect as much or more than the other two; and that is, that in the experiment of transmission of the sea-water into the pits the water riseth; but in the experiment of transmission of the water through the vessels it falleth. Now, certain it is that the salter part of water (once salted throughout) goeth to the bottom. And therefore no marvel if the draining of water by descent doth not make it fresh. Besides, I do somewhat doubt that the very dashing of the water that cometh from the sea is more proper to strike off the salt part, than where the water slideth of her own motion.

3. It seemeth percolation, or transmission, (which is commonly called straining) is a good kind of separation; not only of thick from thin, and gross from fine, but of more subtile natures; and varieth according to the body through which the transmission is made as if through a woollen bag, the liquor leaveth the fatness; if through sand, the saltness, &c. They speak of severing wine from water, passing it through ivy wood, or through other the like porous body; but non constat.'

4. The gum of trees (which we see to be commonly shining and clear) is but a fine passage or straining of the juice of the tree through the wood and bark. And in like manner, Cornish diamonds and rock rubies (which are yet more resplendent than gums) are the fine exudations of stone.

5. Aristotle giveth the cause, vainly, why the feathers of birds are of more lively colours than the hairs of beasts; for no beast hath any fine azure, or carnation, or green hair. He saith, it is because birds are more in the beams of the sun than

is found to contain fresh water. If this is true, the explanation probably is, that the temperature of the sea at the depth to which the vessel is sunk happens to be below the dew point of the air at the surface.

1 Pliny, xvi. 63.; and Cato, De Re Rusticâ, cxi.

beasts; but that is manifestly untrue; for cattle are more in the sun than birds, that live commonly in the woods, or in some covert. The true cause is, that the excrementitious moisture of living creatures, which maketh as well the feathers in birds as the hair in beasts, passeth in birds through a finer and more delicate strainer than it doth in beasts: for feathers pass through quills; and hair through skin.

6. The clarifying of liquors by adhesion is an inward percolation; and is effected when some cleaving body is mixed and agitated with the liquors; whereby the grosser part of the liquor sticks to that cleaving body, and so the finer parts are freed from the grosser. So the apothecaries clarify their syrups by whites of eggs, beaten with the juices which they would clarify; which whites of eggs gather all the dregs and grosser parts of the juice to them; and after the syrup being set on the fire, the whites of eggs themselves harden, and are taken forth. So hippocras is clarified by mixing with milk, and stirring it about, and then passing it through a woollen bag, which they call Hippocrates Sleeve; and the cleaving nature of the milk draweth the powder of the spices and grosser parts of the liquor to it; and in the passage they stick upon the woollen bag.

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7. The clarifying of water is an experiment tending health; besides the pleasure of the eye, when water is crystalline. It is effected by casting in and placing pebbles at the head of a current, that the water may strain through them.

8. It may be, percolation doth not only cause clearness and splendour, but sweetness of savour; for that also followeth as well as clearness, when the finer parts are severed from the grosser. So it is found, that the sweats of men that have much heat, and exercise much, and have clean bodies and fine skins, do smell sweet; as was said of Alexander3; and we see commonly that gums have sweet odours.

Rather that hair is not susceptible of the action of the sun's rays as feathers are. See Aristot. De Coloribus, 6. It is remarkable that almost, if not absolutely, the only case in which hair exhibits something of the iridescent lustre of which the feathers of birds and the scales of fishes offer so many examples, is that of an animal of burrowing habits; the chrysochloris or cape mole, of which several species are known. 2 Hippocras was made by boiling together red wine and spice. Its name is of course derived from its being strained in the manner described in the text. See Strutt, Manners and Customs, iii. 74. and compare Hippocrates, De Affectionibus, ii. p. 420. of Kuhn's edition.

3 Plutarch in Alexandr. p. 666. Lord Herbert of Cherbury affirms that his personal attendants could testify that he possessed this advantage.

Experiments in consort touching motion of bodies upon their

pressure.

9. Take a glass, and put water into it, and wet your finger, and draw it round about the lip of the glass, pressing it somewhat hard; and after you have drawn it some few times about, it will make the water frisk and sprinkle up in a fine dew. This instance doth excellently demonstrate the force of compression in a solid body. For whensoever a solid body (as wood, stone, metal, &c.) is pressed, there is an inward tumult in the parts thereof, seeking to deliver themselves from the compression. And this is the cause of all violent motion. Wherein it is strange in the highest degree, that this motion hath never been observed nor inquired; it being of all motions the most common, and the chief root of all mechanical operations. This motion worketh in round at first, by way of proof and search which way to deliver itself; and then worketh in progress, where it findeth the deliverance easiest. In liquors this motion is visible; for all liquors strucken make round circles, and withal dash; but in solids (which break not) it is so subtile, as it is invisible; but nevertheless bewrayeth itself by many effects; as in this instance whereof we speak.' For the pressure of the finger, furthered by the wetting (because it sticketh so much the better unto the lip of the glass) after some continuance, putteth all the small parts of the glass into work, that they strike the water sharply; from which percussion that sprinkling cometh.

10. If you strike or pierce a solid body that is brittle, as glass or sugar, it breaketh not only where the immediate force is; but breaketh all about into shivers and fitters; the motion, upon the pressure, searching all ways, and breaking where it findeth the body weakest.

11. The powder in shot, being dilated into such a flame as endureth not compression, moveth likewise in round, (the flame being in the nature of a liquid body) sometimes recoiling, sometimes breaking the piece, but generally discharging the bullet, because there it findeth easiest deliverance.

12. This motion upon pressure, and the reciprocal thereof, which is motion upon tensure, we use to call (by one common

The solution of continuity of the earth's surface, observed in violent earthquakes, has been referred by Humboldt to this class of phenomena; the earthquake being in fact, as schylus might have called it, a kûμа xeрσaîov.