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Capillary Phenomena.

57

placed on water; and to the same force is ascribed the difficulty met with in walking through thick mud. If we dip a glass rod into water, on withdrawing it a drop will be found to collect at the bottom, and remain suspended there. As the weight of the drop tends to detach it, there must necessarily be some force superior to this weight which maintains it there: this force is the force of adhesion.

The force of adhesion operates also between solids and gases. If a metal plate be immersed in water bubbles will be found to appear on the surface. As air cannot penetrate into the pores of the plate, the bubbles could not arise from air which had been expelled, but must be due to a layer of air which covered the plate and moistened it like a liquid.

64. Capillary phenomena.—When solid bodies are placed in contact with liquids, molecular attraction gives rise to a class of phenomena called capillary phenomena, because they are best seen in tubes whose diameters are comparable with the diameter of a hair. These phenomena are treated of in physics under the head of capillarity or capillary attraction: the latter expression is also applied to the force which produces the phenomena.

The phenomena of capillarity are very various, but may all be referred to the mutual attraction of the liquid molecules for each other, and to the attraction between these molecules and solid bodies. The following are some of these phenomena:—

i. When a glass rod is placed in a liquid which wets it, water for instance, the liquid, as if not subject to the laws of gravitation, is raised upwards against the sides of the solid, and its surface, instead of being horizontal, becomes slightly concave (fig. 54).

ii. If instead of a solid rod, a hollow tube be immersed in water (fig. 55), not merely is the liquid raised around the tube, but it rises in the inside to a height which is greater, the narrower the tube; and at the same time the surface of the liquid inside the tube assumes a concave form.

iii. If the tube is not moistened by the liquid, as is the case with mercury, the liquid is depressed instead of being raised, and the more so the narrower the tubes (fig. 56); and the surface, which was previously concave, now becomes convex. The surface of a liquid exhibits the same concavity or convexity against the sides of

a vessel in which it is contained, according as the sides are or are not moistened by the liquid.

Fig. 54.

Fig. 55.

Fig. 56.

65. Laws of capillarity.—Gay-Lussac has shown experimentally that the elevation and depression of liquids in capillary tubes, the internal diameter of which does not exceed two millimeters, are governed by the following laws :

I. When a capillary tube is placed in a liquid, the liquid is raised or depressed according as it does or does not moisten the tube, and the elevation varies inversely as the diameter of the tube, that is, it is two or three times as great when this diameter is two or three times as small.

II. The elevation varies with the nature of the liquid, and with the temperature, but is independent of the nature and thickness of the tube.

66. Effects due to capillarity. It is from capillarity that sap rises in plants, that oil rises in the wicks of lamps, and melted tallow in the wicks of candles. The interstices which exist between the fibres of the cotton of which the wicks are formed, act as capillary tubes in which the ascent takes place. In very porous bodies, the pores being in communication with each other form a series of capillary tubes, which produces the same effect. If a lump of sugar be placed in a cup in which a little coffee is left, the liquid is seen to rise rapidly and fill the entire piece; and it is even to be remarked that the sugar then dissolves more quickly than if it had been directly immersed in the coffee. This is due to the fact that in the latter case the air which fills the pores not being able to escape so rapidly as if the piece of sugar is only

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Absorption and Imbibition.

59

partially immersed, prevents the liquid from penetrating into the mass of the sugar, and thus retards the solution.

Insects can often move on the surface of water without sinking. This is a capillary phenomenon caused by the fact, that as their feet are not wetted by the water, a depression is produced which keeps them up in spite of their weight. Similarly a sewing needle gently placed on water does not sink, because its surface, being covered with an oily layer, does not become wetted. But if previously washed in alcohol, or in potash, it at once sinks to the bottom.

67. Absorption and imbibition.-The words absorption and imbibition are used almost promiscuously in physics; they indicate the penetration of a liquid or a gas into a porous body. Absorption is used both for liquids and gases, while imbibition is restricted to liquids.

Charcoal has a great absorbing power for gases. If a piece of recently heated charcoal be passed into a bell jar full of carbonic acid placed over a mercury trough, the volume of gas is seen to diminish rapidly, and it is found that the gas which has disappeared, in penetrating the charcoal represents a volume thirty-five times that of the solid. There are even gases, such as ammonia, of which charcoal can absorb ninety times its own volume.

Absorption takes place in all parts of plants, but more especially in the rootlets and by the leaves. These organs absorb, in the form of water, carbonic acid, and ammonia, the oxygen, hydrogen, carbon, and nitrogen necessary for the growth of the plants.

Absorption also plays an important part both in the nutrition and respiration of animals. Animal tissues can even absorb solid substances. For instance, in those processes of the arts where the workmen have to handle salts of mercury or of lead, these metals are gradually absorbed into the system, and produce serious evils.

68. Effects due to imbibition.-Imbibition has been defined as being the penetration of a liquid into the pores of a solid body. It is a capillary effect, for the pores being in intercommunication act like small tubes; thus it is that water rises in wood, sponge, bibulous paper, sugar, sand, and in all bodies which possess pores of a perceptible size.

Owing to imbibition, tobacco soon dries if kept in a wooden box, while it remains fresh if kept in a metal one, for then its moisture is not absorbed by the metal as by the wood.

When water is absorbed by animal or vegetable matters their

volume increases. Thus if a tolerably large sheet of dry paper be measured and be then moistened, it will be found to have appreciably increased by this process. This property is made use of in stretching paper on drawing boards; the paper is moistened and is then glued or fastened with pins round the edge of the board. In drying the paper contracts, and is tightly stretched. For the same reason, too, wall papers which have been fastened on cloth along the walls, are frequently liable to be torn.

In bending wood, the side to be bent is heated, and the other side moistened. This being lengthened owing to the water it absorbs, while the other is contracted in consequence of the dryness, a curvature ensues on the heated side.

It is often observed that, owing to the changes of volume which they undergo under the influence of moisture and dryness, the furniture of our rooms is frequently heard to crack when the weather changes.

By the absorption of moisture ropes become shorter; and lengthen when they dry. This may seem opposed to what has been stated about moistened paper, but the explanation is not difficult. Ropes are formed of fibres twisted together, and as these fibres swell owing to the water they absorb, the rope becomes larger, and hence each fibre should make in coiling a longer circuit; and the rope will become more shortened the more it is moistened. For this reason, too, new cloths shrink considerably when they are moistened for the first time.

It is related that Pope Sixtus, wishing to raise in a place in Rome, an obelisk brought from Heliopolis to Rome under Caligula, for fear of disturbing the operation, ordered the spectators to preserve profound silence under pain of death. The obelisk was on the point of being placed on its pedestal, when the ropes began to stretch, owing to the great traction to which they were exposed, and the operation was in great danger. A voice from the crowd— that of the architect Zapaglia-cried out, 'Wet the ropes,' which was done, and the operation successfully performed.

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Tenacity.

61

CHAPTER VII.

PROPERTIES SPECIAL TO SOLIDS.

69. Tenacity. Besides the general properties which we have hitherto been considering, and which are met with in solids, liquids, and gases, there are some special to solids which deserve mention, on account of the numerous applications which they present. They are-tenacity, hardness, ductility, and malleability. Tenacity is the resistance which bodies oppose to being broken, when subjected to a greater or less traction. The tenacity of any particular body is determined by giving to it the form of a cylindrical or prismatic rod, one end of which is then firmly fixed in a vertical position to a support. To the lower end is fixed a scalepan, in which weights are successively added until the rod breaks. The breaking weight represents the limit of the tenacity of a rod for a given section.

Of all substances iron has the greatest tenacity. A cylindrical iron rod with a section of a square centimeter, only breaks with a weight of 13,200 pounds. A rod of boxwood of the same dimensions, breaks with a weight of 2,640, and one of oak with 1,540 pounds; a steel wire supports a load of 39,000 times its own length; laths constructed of fine iron wire, the to th of an inch in diameter, can support a load of 60 tons for each square inch of section.

30

Tenacity is directly proportional to the breaking weight, and inversely proportional to the area of a transverse section of the wire.

Tenacity diminishes with the duration of the traction. A small force continuously applied for a long time will often break a wire, which would not at once be broken by a larger weight.

Not only does tenacity vary with different substances, but it also varies with the form of the body. Thus, with the same sectional area, a cylinder has greater tenacity than a prism. The quantity of matter being the same, a hollow cylinder has greater tenacity than a solid one.

The shape has also the same influence on the resistance to crushing, as it has on the resistance to traction. A hollow cylinder with the same mass, and the same weight, offers a greater