30

Adhesion in Solids and Liquids.

likewise organic; on the other hand, we cannot join metals with any of these, but only with metallic solders. There is difficulty in cementing together bodies of opposite character, such as glass and metal, with any powerful degree of cohesion. The rate of increase and diminution caused by heat in glass and metals is so different that, if the two bodies cohere at a high temperature they are separated on cooling. The metal contracts so much more than the glass that cohesion is destroyed. The only metal which can be united to glass is platinum, and the cohesion of these two substances is applied to various useful purposes in chemistry. This exception is explained. by the fact that there is less difference in the rate of expansion and contraction of glass and platinum than of glass and other metals. We cannot lay down rules as universally true, of course; and it is possible, as with platinum and glass, that the similarity of character need only be one-sided, that is to say, one of expansibility under the action of heat. In soldering two metals of different kinds, for example, we use a solder intermediate in its degree of expansibility under heat, to the two metals: otherwise any change of temperature will destroy the joining. This must hold in every case of gluing or cementing; but there is most probably a further resemblance in nature, though we cannot as yet state it with much definiteness.

79. It is more difficult than may at first appear to determine the amount of adhesion between a liquid and a solid. A plate of glass balanced at the end of a weighing beam and then allowed to come in contact with water, adheres to the water, and with much greater force than the mere weight of the water remaining attached when again forcibly raised. But this does not show the amount of attraction between the glass and the water, because we cannot detach the plate clean and dry; we merely find the force required to overcome the cohesion of the liquid.

80. We find adhesion existing between gases and solids, and between gases and liquids. A tumbler of spring water, after standing a short time in a warm room, will be seen to have its inner surface studded with minute spherules of air which had been combined with the water, and which by change of temperature have been evolved from it and fixed by attraction on the glass. This is still more strikingly observed when the glass of water is submitted to a diminution of pressure in the receiver of an air-pump. Any solid introduced into the water, such as a sheet of tin plate, will equally attract them.

Adhesion in Liquids.-Solution.

31

Water has always more or less air mixed up with it and kept in it by adhesion; and it may be seen in the form of fine bubbles within the ice when the water is rapidly frozen. The water of certain deep lakes in colder countries than England, such as Canada, is frozen more gradually, and the air thus has time to escape. The ice thus obtained is as pellucid as the diamond, and quite free from air.

The difficulty of getting the air out of the pores in a solid is owing to adhesion. Thus, if we sink a cork in water it does not get thoroughly soaked for some time; gradually the water presses the air out, and by-and-by it appears clinging to the surface in small silvery beads. The air contained in the pores of a lump of sugar is seen shooting up from the surface after we drop it into a cup of hot tea. So it is due to this that it is next to impossible to get the air out of the minute cells of the lungs after it has been once introduced by breathing.

"Solution."

81. When a solid is dissolved in a liquid-as sugar or salt in water, sealing-wax in spirit of wine, gold in mercury-it is simply the power of adhesion overcoming the power of cohesion in the solid; and the limit to solution or the saturation point, when the liquid refuses to dissolve more of the solid, is when the attraction of adhesion just balances that of cohesion.

Whatever diminishes the cohesion increases the degree or power of solution; thus alum reduced to powder dissolves much more readily than in a single lump; and heat, because it increases the distances of the molecules and reduces the cohesion, favours solution proportionately.

No doubt there are exceptions to this last rule, for there are some cases where the solvent power of a liquid is reduced by heating, as, for example, lime in water; if we make a saturated solution of lime in ice-cold water, it will deposit one half of the lime when we raise the water to the boiling point. Another illustration of the same kind is seen in the solution of Glauber's salts; ice-cold water will hold about one-tenth of its weight of these salts in solution, but, as the water is heated up to boiling, hard gritty crystals are deposited to the extent of one-fifth of the original quantity dissolved.

The reason of this appears, however, to be that heat diminishes at once the adhesive and the cohesive attractions; and the latter being in most cases the more delicate or smaller in range is, so to speak, soon cut of the field in comparison with the adhesive; but in

32

Adhesion between Liquids.

these exceptional cases the adhesion diminishes more rapldiy than the cohesion with increase of heat, and hence the anomalous results. In solution a certain resemblance in the nature of the bodies adhering is necessary; thus, mercury dissolves the metals; oils dissolve fats; and alcohol resinous substances. And this is very important, because, along with the fact that the solution retains the properties of both the solid and the liquid, it serves to distinguish between this molecular adhesion and chemical attraction, which, as we shall consider presently, is strongest between particles of unlike nature.

"Adhesion between liquids."

82. When one liquid mixes with another, it is a case of solution and similar in character to the former; the mutual adhesion overcomes the individual cohesion; and when the different attractions balance each other the two are mutually saturated.

A drop of vitriol may be mixed with a pint or a quart of water with equal readiness, and every drop of the mixture will taste acid, showing that the adhesion of the water and vitriol is so great as to overcome the cohesion of the latter to any degree of minuteness.

On the other hand what chemists call the essential oils are but slightly soluble in water; if we shake, say oil of peppermint with water, only a very small quantity of the oil will adhere to and permeate the water, and the two will separate again almost entirely, the adhesion not being sufficient to overcome the individual cohesions.

A very interesting illustration of this contest between adhesion and cohesion is seen when we let fall on the surface of water a drop of any oil which is but sparingly soluble in it. Naphtha, paraffin, creasote, or turpentine may be used to show the effect. Adhesion tends to draw the drop to the liquid so as to mix thoroughly, and its cohesion tends to prevent this; so that the extent to which the drop will spread, and its behaviour in the act will depend on the mutual relations of the two attractions, and will consequently vary for every substance. Thus each oil has its own cohesion figure, as it is termed, and may be distinguished by this from all others; an important fact to the chemist, enabling him to detect differences and mixtures of oils with great readiness.

83. Under the power of cohesion alone, any mass of liquid assumes the spherical form, as is seen in the case of rain-drops in the air; or more beautifully and permanently with a little oil dropped gently into a mixture of alcohol and water of about the same density

Capillary Attraction.

33

But if the cohesive attraction be interfered with, as by adhesion, for instance, the form is no longer spherical; as we see in the shape of a drop on a window-pane or of a soap-bubble before it is detached from the pipe by which it is blown.

Of course in large masses of liquid, cohesion ceases to influence the shape, being quite overpowered by the earth's attraction, which, tending to draw all the particles as near its centre as possible, makes the surface what we called level; that is, everywhere at the same distance from the earth's centre.

"Capillary attraction."

84. The adhesion of solids and liquids is well exhibited in an important class of phenomena, where it is usually designated by the special name of capillary attraction.

When a plate of glass is dipped in water, the liquid clings to the glass, and is consequently raised up to a small height above the surrounding level.

If we dip two plates, and bring their faces very near together, we shall see the water creep up between the plates—to a greater or less height according as they are more or less approached. There will also be a slight drawing together of the plates by the cohesive action of the liquid between; just as, when a gymnast raises himself up between two ropes by grasping one in each hand, there is an inclination of the ropes at the lower end. This effect of adhesion, in raising liquids between near surfaces of solids, can be best seen and studied by using glass-tubes of various degrees of fineness, because in these we have the surfaces approached on all sides round about. Capillary is the name usually given to this attraction from its having been first noticed in small glass tubes scarcely larger than hairs (capillus, a hair).

85. If one end of an open glass tube of fine bore is dipped in water, the liquid will rise up to a considerable height above its level, and higher and higher as we use finer bores. A bore of one half the width of another will raise the liquid to twice the height. With the same tube water will be drawn up fully twice as high as spirit of wine, because the cohesive force of the former is much greater than that of the latter.

But a difference in the material of the tube has no effect on the height provided only the liquid can wet the tube. A tube of wood and one of glass will raise water to the same height if they have the same size of bore.

34

Capillary Attraction.

86. The following examples illustrate the operation of this prin ciple :

Water, ink, or oil coming in contact with the edges of a book is rapidly absorbed far inwards among the leaves.

A picce of sponge or a lump of sugar touching water by its lowest corner soon becomes moistened throughout. A heap of sand on a wet surface becomes speedily wet to the summit.

The walls of the upper rooms of houses built on a damp soil are often rendered damp by the moisture traversing by capillary attraction the porous bricks from the foundation upwards.

The wick of a lamp lifts the oil to supply the flame from two or three inches below it.

Ink passes to the point of a pen between the two edges of the slit.

A mass of cotton thread hanging over the edge of a glass from the water within it will empty it drop by drop. The corner of a towel dipping into a basin of water will empty it in the same way.

Dry wedges of wood driven into a groove formed round a pillar of stone near one end will, on being moistened, swell with sufficient force to rive off the portion from the block. In some French quarries, millstones are thus cut from the rock.

A great weight suspended by a dry rope may be raised a little way by merely wetting the rope; the moisture imbibed by capillary attraction causes the rope to swell laterally, and so to get shorter.

87. Capillary attraction plays a most important part in the world of animal and vegetable life, on the one hand influencing the fluid circulation and the passage of the various secretions through the porous tissues of all organized beings; and on the other, conveying the life-giving moisture through the soil to the roots of vegetation. We cannot indeed say that it is the sole cause in maintaining the circulation of the sap in living plants, for the evaporation, &c., constantly going on through the leaves and skin is most probably, like the breathing of animals, the real source of activity, and capillarity in itself would be incapable of producing a continuous flow.

Capillary depression."

88. When a tube of small bore is dipped in a liquid incapable of wetting it, as a glass tube in mercury, or an oiled glass tube in water, there is a lowering of the level of the liquid within the tube, its surface being convex or rounded in place of concave or