Imágenes de páginas
PDF
EPUB

74 ADHESION FRICTION, LUTES AND CEMENTS. [47.

riences under similar circumstances a greater amount of friction

than if revolving in a brass socket; and the interposition of a

substance like plumbago or grease, the particles of which have

but very little cohesion, is a familar mode of reducing the amount

of friction in machinery.

Few substances admit of a greater variety of useful applications from their faculty of adhesion than caoutchouc; its perfect adhesion to glass adapts it admirably for stoppers, and enables the chemist to employ it for air-tight and flexible joints. This property of adhesion to the bodies whieh it touches, further fits it for bands for driving machinery, and for numberless other purposes.

(48) Cements.—The entire value of cements depends upon the operation of adhesion; and in the variety of cements rendered necessary by the variety of materials to be united, we have additional proof that adhesion is exerted between different kinds of matter with very varying degrees of intensity. Glue or gum may be used for joining pieces of pasteboard or wood, while it totally fails as a cement for glass or china, either of which needs some resinous material to unite its fragments; whilst for the union of marble, stone, or brickwork with each other, the use of mortar or some calcareous cement is required. The thinner the layer of cement, the more perfectly does it perform its task, as it more rapidly and completely adapts itself to changes of temperature, which, by causing it to expand unequally, would destroy the cohesion of its own particles if a thick mass were employed.

Cements of various kinds are in continual requisition in the laboratory. Wellboiled paste applied on thin paper forms an excellent covering for corks and other joints which are liable to be porous; it must be allowed to become nearly dry before it is used. Plaster of Paris made into a paste, not too stiff, may often be used; when dry it may be washed over with oil or melted paraffin to make it air-tight. Strips of well-soaked bladder may sometimes be employed advantageously; they form a firm joint when dry: but for most purposes where a temporary joint only is required, nothing is so convenient as a strip of sheet caoutchouc softened at the fire, and bound round the parts to be connected; when softened thus, it usually adheres perfectly without even requiring to be tied. When the joint is intended to be permanent, as, for example, when a brass cap is to be attached to the neck of an air-jar, a resinous cement, consisting of 5 parts of resin, 1 of yellow wax, and 1 of finely-powdered Venetian red, forms a convenient mixture: the resin and wax are melted together and incorporated with the Venetian red by stirring. Before applying it, both the glass and the metallic cap which are to be connected together must be warmed just sufficiently to melt the cement. When the joints are required to resist a considerable pressure without leaking, a mixture of equal parts of red and white lead ground into a paste with linseed oil, worked up with fibres of tow, and packed tightly into the joint, sets firmly, and is not liable to crack.

It not unfrequently happens that the adhesion between a cement and the bodies which it unites, surpasses the cohesion of the particles which compose the bodies themselves; from this cause we often see a film of wood split off, adhering to the surface of the glue, when a fracture occurs near one of these

[ocr errors][merged small][ocr errors]

joinings. The feat of splitting a bank-note into two laminar, which excited so much astonishment, was accomplished by cementing it firmly between two flat surfaces, and afterwards separating them; the cohesion of the paper being feebler than the adhesion to the cement, the paper was split through the middle. This method of splitting paper had, however, been long known to the buhl-cutter and inlayer.

(49) Capillarity.—The existence of adhesion between solids and liquids is so well known as to need no further illustration; but it produces many very important results, some of which must be noticed.

It is to the adjustment of adhesion and cohesion between solids and liquids under the simultaneous influence of gravity, that' the important phenomena of capillarity are due. If a perfectly clean glass tube, with a fine bore, and open at both ends, be plunged into water, or into any liquid capable of wetting it, the liquid will be found to rise in the tube considerably above the level of the surface in the vessel; and the finer the tube the higher does the liquid rise. The surface of the liquid will also be seen where

it approaches the out- Fig. 23.

side of the tube, or the side of the vessel containing it, to stand above the general level (fig. 23, A). The phenomenon may also be examined by placing vertically in a shallow vessel containing a little coloured liquid, two plates of glass with parallel faces, which are in contact by two of their vertical edges, and slightly separated at the opposite edges. The liquid will rise between the glass plates, the height of the column being inversely as its distance from the angle of contact between the plates. The upper boundary of the liquid will consequently describe a hyperbolic curve (fig. 23, B). The cause of the rise of the liquid is the adhesion between its particles and those of the glass; the limits to that rise are the action of gravity, and the cohesion amongst the liquid particles. As the action of gravity is equal under ordinary circumstances upon all the particles of the liquid, it reduces the liquid surface to a uniform level. When a tube is introduced, the uniformity of this action is interfered with, as the following considerations will show :—the particles in immediate contact with the side of the tube are partially supported by adhesion to its surface;

[graphic]
[ocr errors][merged small][ocr errors]

a longer column therefore becomes necessary in order to compensate for the diminution of downward pressure. Now let us conceive the particles of the elevated column of liquid to be arranged as a series of contiguous concentric cylinders: the particles of the outermost cylinder are sustained laterally by adhesion to the tube, those of the next cylinder are hung on to these, if the expression may be allowed, and supported solely by cohesion with their fellows; those of the third cylinder are hung on to the particles of the second, and so on, till we reach the central rod of particles. The surface of the liquid is in consequence necessarily curved;—the outer cylinder, or the portion of liquid in contact with the tube standing at the highest point. Now since adhesion is confined to the superficial layer, and, between the same substances, is ceteris paribus, constant in quantity for an equal extent of surface, the wider the tube the shorter will be the column sustained, as the contents of the column raised by cohesion increase more rapidly than the surface of the cylinder. The height of the column is found to be inversely as the diameter of the tube.

(50) Variations in Capillarity.—The elevation of the column of liquid in tubes of equal diameter varies with the nature of the liquid, the variation depending partly on the difference of cohesion between the particles of the liquid, partly upon the difference of adhesion between the liquid and the glass. In consequence of the decrease of both these forces by heat, the height of the column diminishes as the temperature rises-.

The following table from the experiments of Frankenheim shows the height at which the different liquids enumerated stand, at o° C. in a tube I millimetre in radius, (about 2'.r of an inch,) with the coefficient of correction for temperature, which multiplied by t, the number of degrees centigrade above 0° C, gives the amount to be deducted in millimetres from the number in column 3, in order to find the height of the capillary column at the temperature required:—

Capillary Elevation of Liquids in Glass Tube of 1 mm. Radius, at C.

[table]

52.] CAPILLARY DEPRESSION OF MERCURY. 77

(51) Capillary Depression of Mercury.—In liquids, such as mercury, where the cohesion preponderates over their tendency to adhere to the sides of the tube, the capillary action is reversed; the surface becomes convex instead of concave, and the height of the column within the tube is depressed below the general level. In a mass of liquid, each particle is maintained in its place by the mutual attraction of all the surrounding ones; but if a column be isolated from the mass of liquid by the interposition of the walls of the tube, the sides of which exert little or up equivalent adhesion, the cohesion of the mass below draws down the upper particles, and produces a depression of the column. This depression of mercury in glass renders a certain correction necessary in reading off the height of the mercurial column in the barometer, which always stands a little lower than the elevation due to the atmospheric pressure. The narrower the bore of the tube the greater is the depression. Experiment has shown that this capillary depression is nearly onehalf less in tubes that have had the mercury boiled within them, than in unboiled tubes, as the process of boiling expels the film of air, which adheres to the glass in unboiled tubes. By employing a tube of 16 or 20 millimetres in the bore, this correction becomes so trifling that it may be neglected. In a tube of 6mm" (1 inch) in diameter in which the mercury has been boiled, the depression is ri7imm', while with a similar tube of i3mm* in diameter it is only o*223mm- The capillary depression of mercury is slightly increased by elevation of temperature.

In reading off the level of mercury in a barometer, or in a graduated jar used for the measurement of gases, the height of the metal should be taken from the convexity of the curve; but „ '24

in estimating the volume of a liquid which wets the surface of the glass the determination should always be made from the bottom of the curve. The lines a a, b b, fig. 24, indicate the points in the two eases?

(52) Importance of Capillary Actions.—Capillarity plays an important part in the operations of nature, and in a variety of ways has been rendered subservient to the wants of man. A

[graphic]

• In accurate observations on the volume of gases confined over mercury, it becomes necessary to estimate the amount of error which is thus introduced.

78 CAPILLARY ACTIONS. [52.

familiar illustration of its employment is seen in the wicks of lamps and candles, which, being composed of a bundle of fibrous materials, furnish hair-like channels by which the oil or melted combustible is elevated to the flame, and supplied as fast as it is consumed. Capillarity influences the circulation of the liquids in the porous tissues of organized beings, and it is the principal mode in which water, with the various substances which it holds in solution, is supplied to the roots of growing plants. By its means, during the droughts of summer, fresh supplies of moisture are raised towards the surface, for the maintenance of vegetable life; and in the same way, when during winter the surface is hard bound bv a long dry frost, water is constantly finding its way from beneath, is solidified upon the surface, and remains stored up until a thaw ensues; when this occurs, the accumulated moisture mellows the soil and produces the well-known soft and plashy state of the ground which follows long-continued frosts, and which extends deeper, the longer the duration of the freezing temperature, although neither snow nor rain may have fallen. Few persons are aware of the immense action which may be developed by capillarity; if a plug of dried wood be fitted into a strong glass tube, and the end of the plug be immersed-in water, the wood becomes swelled by the imbibition of liquid owing to capillary action, and the tube is split. In some parts of Germany this action is turned to account in splitting millstones from the rock: holes are bored into its substance in the direction in which it is to be split, and into these holes wedges of dry wood are driven tightly; when exposed to moisture they swell, and large blocks of stone are thus detached with little labour or expense.

A curious illustration of the combined action of cohesion and adhesion, in overcoming gravity, is afforded by the following experiment:—Procure a small cylinder of fine copper-wire gauze, about 8 centimetres or 3 inches high, and 5 centimetres wide, closed also above and below with the same material, and furnished with a stout wire to serve as a handle; plunge it under water; considerable difficulty will be experienced in expelling the air, owing to the formation of a film of moisture over its surface, which, by the cohesion of the liquid particles composing it and by its adhesion to the wire gauze, prevents the escape of the air; when about half filled with water, lift the cylinder out of the liquid—the liquid will be securely retained: water may even be allowed to fall in a gentle stream upon the top of the gauze, when it will pass through and run out below, without, however, affecting the quantity of liquid within; but by giving the handle a slight jerk, the film of liquid which supported the pressure of the atmosphere will be broken, and the water will then immediately escape. (For a more complete treatise on Capillarity, the student is referred to Professor Clerk Maxwell's Theory of Heat, and to the article, Capillary Action, by the same author, in the ninth edition of the Encyclopedia Britannica, 1876.)

« AnteriorContinuar »