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A body wholly submerged, whether heavy or light, will be subjected to a similar pressure. It will be urged upwards by a force equal to the weight of a quantity of water sufficient to fill the space which it occupies; that is, of a quantity of water of the same bulk as itself. Hence a heavy body immersed in water will weigh less than it does out of the water, the difference being equal to the weight of the water it displaces. For example, a block of stone or other heavy substance is more easily raised at the bottom of the sea than it would be on land; because it is lighter than it would be on land by the weight of its own bulk of sea-water. Thus it is that persons, engaged in building piers and other subaqueous works, find themselves endowed with a kind of supernatural strength, so that they can easily lift, and adjust in their places, masses of stone which on land they would vainly endeavour to move.
The principle which has just been explained, affords a convenient method of finding what is called the specific gravity of different substances. By this term we mean simply the weight of a body compared with that of another body of the same size. Properly speaking, all weight is comparative; mahogany, for example, is a heavy substance compared with cork, but light when compared with stone. Even stones are light when compared with some of the metals. Thus our notions of light and heavy are vague and undefined, and FIG. 41.
some standard of comparison is required, to which the weight of all other substances may be referred. The substance which has usually been adopted for this purpose is water; and therefore, when we speak of the specific gravity of a body,
it is its weight, compared with the weight of an equal bulk of water, which is usually implied. Suppose now,
for example, that we wish to find the specific gravity of some kind of precious stone, of which we have only a small, irregularly shaped specimen. Let it first be weighed carefully, and then, having been suspended in water by a thread (as in fig. 41) attached to one of the scales, let it be weighed again. The loss of weight at the second trial will be the weight of a quantity of water equal in size to the stone; and, the weight of the stone itself being known, its specific gravity is easily calculated. The same method is also applicable, in a modified form, to solids specifically lighter
This mode of finding specific gravities is believed to have been discovered by Archimedes, a philosopher of Syracuse, about 200 years before Christ. The story goes that Hiero, king of Syracuse, had employed a goldsmith to make a crown for him, and had given him a mass of gold for that purpose. He suspected that the workman kept back part of the gold, and alloyed the crown with copper to make up the weight. But how was this to be proved? The question was referred to Archimedes, who long tried in vain to hit upon some mode of arriving at the truth. At last, being one day in the bath, he noticed how his body displaced the water and was buoyed up by it; and the thought suddenly struck him that if the crown were alloyed with a metal lighter than gold, it must be more bulky than a mass of gold of the same weight, so that he could detect the imposture by weighing it against such a mass in water. Elated beyond measure by his discovery, he rushed out of the bath, and ran home naked as he was, exclaiming in the Greek language, "Eureka! Eureka!" (I have found it! I have found it!)
Ir was explained in a former lesson that the free surface of a liquid at rest is everywhere at the same level, whatever be the size or shape of the vessel which contains it. But, though this is the general rule, there is one important and
remarkable class of cases, which may be said to form an exception. If we take a tube of glass with a very small bore, open at both ends, and dip one end into a vessel of water, the water will rise and remain in the tube at a considerable height above the level of the water outside; and the upper surface of the water which thus rises in the tube will be concave instead of horizontal. On examining more minutely, we shall find that a thin film of the water in contact with the outside of the tube is also a little higher than the rest of the water in the vessel. And further, round the whole edge of the water, where its surface meets the surface of the vessel containing it, the same thing may be observed; the liquid surface curves upwards as it approaches very near the solid. It is presumed, therefore, that there exists a certain attraction between the solid body and the liquid particles near it, which causes the latter to rise, in opposi tion to the force of gravity. This has been called capillary attraction, because its effects were first observed in tubes with very fine hair-like bores, though it is not by any means
confined to such tubes.
That there really is an attraction, at least in some cases, between solids and liquids in contact, may be easily shown in another and very simple way. If the hand, for example, be plunged into water, and drawn out again, it will be quite wet; that is, its surface will be coated over with a thin film of water, which will continue to adhere to it, notwithstanding the tendency of gravity to make it fall off. There must, therefore, be an attraction between the surface of the hand and the molecules of the water, sufficient to overcome the weight of these molecules. If, however, the hand be dipped in quicksilver, it will be found, on withdrawing it, that it is perfectly dry. Not a particle of the quicksilver will adhere to it. There may, notwithstanding, be some attraction between the hand and the quicksilver, but, if there is, it is not so powerful as in the case of water. Suppose now that we dip a piece of glass in water, and then in quicksilver. The result will be the same as before; the water will wet the glass, the quicksilver will not. But we must not suppose,
as we are very apt to do, that water will thus wet any solid body which may be immersed in it, for there are some substances from which water runs off, leaving them dry, exactly in the same way as quicksilver runs off a piece of glass. If the hand itself be rubbed over with a little grease, water will not readily adhere to it. On the other hand, if a sovereign, or any other piece of gold, be dipped in quicksilver, it will be found, when taken out again, to be covered with a thin white coating of that liquid. This, then, is the general result at which we arrive, that certain liquids are attracted so powerfully by certain solids, that they will adhere to these solids in spite of gravity; while, in other cases, if there be any attraction, it is not strong enough to produce the same effect.
Now this distinction between a solid which is, and one which is not, wetted by any given liquid is of great importance in connection with capillary attraction. For it is not in all cases true that a liquid rises in a capillary tube higher. than its level outside. It has been already stated that water does so in a capillary tube of glass; but then glass is one of the substances which water will wet. If the same tube be dipped into a vessel of quicksilver, or any liquid which does not wet glass, the result will be quite different; the quicksilver will stand lower in the tube than it does outside, and will have its upper surface convex, instead of concave. Round the outside of the tube, too, the liquid surface will curve downwards as it approaches very near the solid. And the same thing is true generally. If the solid with which a liquid is in contact is one which that liquid will wet, capillary attraction will cause the liquid to rise; but if otherwise, the liquid will not be raised, but depressed.
The height to which a liquid rises in a capillary tube depends on the diameter of the bore. The finer the bore, the farther will the liquid ascend. But capillary attraction, as already remarked, is not confined to tubes. If we take two plates of glass, and hold them in water very near each other, the water will rise between them, just as it would rise in a tube; and it will rise higher and higher in propor
tion as the space between the plates is diminished. This may be beautifully shown by holding the plates in such a way as to touch each other at one side, with a very small space between them at the other side. The water will rise highest at the side where they touch, and gradually lower and lower as they diverge, its surface thus forming a curve somewhat like that represented in fig. 42.
There are many illustrations of capillary attraction in phenomena with which we are
all familiar. Why, for instance, does the whole of a lump of sugar become gradually wet, when the least part of it is dipped in water? Simply because its pores act like so many capillary tubes, through which the water ascends. It is for a similar reason that the ground
floor of a house is so often damp, unless the site is naturally very dry, or thoroughly drained. Look at the wick of a lamp, always wet with oil, though the store of oil is considerably below it. The oil must be constantly oozing upwards through its porous substance by capillary attraction. The same principle partly accounts for the ascent of the sap in trees and other vegetables; and also for the diffusion through the soil of that genial moisture, by which the hardest clod is penetrated, softened, and fertilized.
QUESTIONS FOR EXAMINATION.
What is a fluid? Which fluids are most elastic? What is the fundamental principle regulating the pressure of fluids? Explain the principle of the Bramah press. Give instances of its power. State the "hydrostatic paradox." On what does the pressure of a quantity of water on any surface depend? Show how a little water may produce dreadful natural convulsions. What was the use of aqueducts? Why are they no longer necessary? What is the condition that a solid may float in a liquid? Show that it must displace its own weight of the liquid. How may the weight of a ship be found on this principle? In what position may a man float safely? Why does an unskilful person sink? How do fishes rise and sink at will? What is specific gravity? How may the specific gravity of a heavy solid be found? Who discovered this method of doing it? Relate the story. To what rule are capillary phenomena an exception? What are these phenomena, and why so called? State instances of solids which are