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119. Capillary phenomena in everyday life. Capillary phenomena play a very important part in the processes of nature and of everyday life. Thus the rise of oil in wicks of lamps, the complete wetting of a towel when one end of it is allowed to stand in a basin of water, the rapid absorption of liquid by a lump of sugar when one corner of it only is immersed, the taking up of ink by blotting paper, are all illustrations of precisely the same phenomena which we observe in the capillary tubes of Fig. 99.
120. Floating of small objects on water. Let a needle be laid very carefully on the surface of a dish of water. In spite of the fact that it is nearly eight times as dense as water it will be found to float. If the needle has been previously magnetized, it may be made to move about in any direction over the surface in obedience to the pull of a magnet held, for example, underneath the table.
FIG. 108. Cross section of a
To discover the cause of this apparently floating needle impossible phenomenon, examine closely the surface of the water in the immediate neighborhood of the needle. It will be found to be depressed in the manner shown in Fig. 108. This furnishes at once the explanation. So long as the needle is so small that its own weight is no greater than the upward force exerted upon it by the tendency of the depressed (and therefore concave) liquid surface to straighten out into a flat surface, the needle cannot sink in the liquid, no matter how great its density. If the water had wet the needle, that is, if it had risen about the needle instead of being depressed, the tendency of the liquid surface to flatten out would have pulled it down into the liquid instead of forcing it upward. Any body about which a liquid is depressed will therefore float on the surface of the liquid if its mass is not too great. Even if the liquid
FIG. 109. Insect walking on the surface of water
tends to rise about a body when it is perfectly clean, an imperceptible film of oil upon the body will cause it to depress the liquid, and hence to float.
The above experiment explains the familiar phenomenon of insects walking and running on the surface of water (Fig. 109) in apparent contradiction to the law of Archimedes, in accordance with which they should sink until they displace their own weight of the liquid.
QUESTIONS AND PROBLEMS
1. Explain how capillary attraction comes usefully into play in the pen, camel's-hair brushes, lamp wicks, and sponges.
2. Candle grease may be removed from clothing by covering it with blotting paper and then passing a hot flatiron over the paper. Explain.
3. Why will a piece of sharp-cornered glass become rounded when heated to redness in a Bunsen flame?
4. The leads for pencils are made by subjecting powdered graphite to enormous pressures produced by hydraulic machines. Explain how the pressure changes the powder to a coherent mass.
5. Float two matches an inch apart. Touch the water between them with a hot wire. The matches will spring apart. What does this show about the effect of temperature on surface tension?
6. Repeat the experiment, touching the water with a wire moistened with alcohol. What do you infer as to the relative surface tensions of alcohol and water?
7. Fasten a bit of gum camphor to one end of half a toothpick and lay it upon the surface of a large vessel of clean still water. Explain the motion.
8. Shot are made by pouring molten lead through a sieve on top of a tall tower and catching it in water at the bottom. Why are the shot spherical?
9. Explain how capillary attraction makes an irrigation system successful.
10. In building a macadam road coarse stones are placed at the bottom, on top of them smaller stones, and finally little granules tightly rolled together by means of a steam roller. Explain how this arrangement of material keeps the road dry.
11. What force is mainly responsible for the return of the water that has gravitated into the soil? Would the looseness of the soil make any difference (dry farming)?
ABSORPTION OF GASES BY SOLIDS AND LIQUIDS
121. Absorption of gases by solids. Let a large test tube be filled with ammonia gas by heating aqua ammonia and causing the evolved gas to displace mercury in the tube, as in Fig. 110. Let a piece of charcoal an inch long and nearly
as wide as the tube be heated to redness and then plunged beneath the mercury. When it is cool, let it be slipped underneath the mouth of the test tube and allowed to rise into the gas. The mercury will be seen to rise in the tube, as in Fig. 111. Why?
FIG. 110. Filling tube with ammonia
This property of absorbing gases is possessed to a notable degree by porous substances, especially coconut and peach-pit charcoal. It is not improbable that all solids hold, closely adhering to their surfaces, thin layers of the gases with which they are in contact, and that the prominence of the phenomena of absorption in porous substances is due to the great extent of surface possessed by such substances.
That the same substance exerts widely different attractions upon the molecules of different gases is shown by the fact that charcoal will absorb 90 times its own volume of ammonia gas, 35 times its volume of carbon dioxide, and only 1.7 times its volume of hydrogen. The usefulness of charcoal as a deodorizer is due to its enormous ability to absorb certain kinds of gases. This property made it available for use in gas masks (see opposite p. 103) during the World War. If a little spongy platinum is suspended in a vessel above wood alcohol, it will glow brightly because of the absorption into the platinum of both vapor of alcohol and oxygen. The rapid
One of the greatest of mathematical physicists; born in Edinburgh, Scotland; professor of natural philosophy at Marischal College, Aberdeen, in 1856, of physics and astronomy in Kings College, London, in 1860, and of experimental physics in Cambridge University from 1871 to 1879; one of the most prominent figures in the development of the kinetic theory of gases and the mechanical theory of heat; author of the electromagnetic theory of light-a theory which has become the basis of nearly all modern theoretical work in electricity and optics (see p. 426)
A GAS MASK
A great variety of poisonous gases having a density greater than air were set free and carried by the wind against the Allied armies in the World War, and others were fired in explosive shells. Until gas masks were devised these gases, settling into the trenches, wrought frightful havoc among the troops. The absorptive power of charcoal, especially when impregnated with certain chemicals, proved an effective barrier against the deadly fumes, since all of the air entering the lungs of the soldiers had to be inhaled through the charcoal within a canister carried in the bag designed to hold the gas mask. The illustration shows an American gas mask adjusted to the head of an American soldier