this purpose some substance in a soft or liquid state is interposed, which is capable itself of solidifying and drawing the surfaces together by this species of attraction. In this way wood may be attached by gelatine, or glue; porcelain, by albumen, or white of egg, and lime; and bricks, by mortar; so that the adhesion of the surfaces of junction may offer even greater resistance to disruptive force than the cohesion of any other part.

§ 81. The heterogeneous adhesion of solids to each other is also of the greatest practical importance, when it assumes the form of friction, that is, of superficial opposition to mechanical force, or the motion of two bodies in contact (§ 58). All other things being equal, friction is less between surfaces of dissimilar matter than between those of the same matter: thus the resistance of iron, moving upon iron, is expressed in terms of the pressure by the fraction, while that of iron upon copper is only. The interposition of substances whose force of cohesion is small, such as plumbago, or grease, greatly decreases friction: thus the resistance of iron upon iron is diminished to 1th by the application of tallow; and of iron upon copper to

th. The complete investigation of this opposition of forces belongs to the science of mechanics; it would, however, have been a defect in this preparatory view, not to have indicated an antagonism of forces on which we are dependent in almost every action of our lives. It is friction which prevents our feet slipping back every time we attempt to advance a step; and those who have attempted to walk upon polished ice well know how to appreciate the passive resistance of heterogeneous adhesion.

§ 82. A beautiful example of the adjustments which take place of the two antagonist forces of homogeneous cohesion and heterogeneous adhesion, is afforded by the phenomena of, what is termed, capillary attraction. The simplest case of this action arises when we dip a glass tube of very fine bore (hence termed a capillary tube) into a liquid which is capable of wetting it; upon attentively examining the appearances presented by such an arrangement, we shall find that the liquid rises in the tube to a certain height above the usual level, and assumes a concave surface at its upper extremity. We shall also observe, than on the exterior surface of the tube, as well as where it is in contact with the containing vessel, the liquid will stand above the general surface.

Now we have here three forces concerned; the force of gravitation, by the action of which upon the moveable particles of the liquid the general level of the mass is maintained; the interior force of homogeneous attraction, drawing the similar particles together, and resisting any force which may tend to separate them; and the exterior force of heterogeneous attraction, between the solid and the liquid. The latter has power to draw up the included liquid (which is cut off by the glass from the action of the homogeneous attraction of the surrounding mass) against the action of the two former forces to a point at which they are all in equilibrio; and this power increases in some high ratio inversely as the distance; so that the nearer the approximation of the active particles, the greater the elevation. This is not only shown by the gradual upward curve of the liquid, as its distance from the solid decreases, but may be demonstrated by varying the size of the tube; when the rise of the liquid will be found to increase rapidly as the diameter decreases.

The concave surface of the liquid thus evidences the superior force of the attraction of adhesion, which must exist throughout its contact with the tube below the surface; and the particles are drawn inwards and downwards by their own attraction with less force than when a part of their weight is not thus supported. The consequent diminution of pressure from above downwards, is equivalent to a diminution of gravity, and is hydrostatically balanced with the exterior liquid by an addition to the length of the interior column.

The same action may be strikingly illustrated by dipping the ends of two plates of glass in coloured water, at a very small and gradually decreasing distance from each other; the water will rise between them, and the force increasing as the distance diminishes, will be shown by the hyperbolic curve of the upper surface, the heights of the different points of the surface of the water being inversely proportional to the distances from the angle, a property of the curve which is thus called by mathematicians,

§ 83. If, on the other hand, the liquid into which the capillary tube is plunged, be incapable of wetting it, that is to say, if the homogeneous attractive force should be superior to the heterogeneous attractive force, a different order of phenomena will arise, but illustrative of the same principles. A fine

glass tube, thus immersed into mercury, will exhibit a depression of the internal column of liquid below the general level; the upper surface will be convex instead of concave; and at the point of contact, both of the exterior of the tube and of the surface of the containing vessel, the liquid will be depressed. The forces concerned in the phenomena are the same as before, but now the homogeneous attraction has the superiority. The mercury within the tube is cut off by the inactive surface of the glass from the homogeneous attraction of the surrounding mass; its own self-attractive force, therefore, acts as from its centre, drawing its particles together, and causing them to assume more or less of a globular aggregation, according to the same law which determines the spheroidal form of a suspended drop of liquid. The increase of attraction from without inwards which is manifested by the convex surface must, of course, exist to an equal degree below the surface, and is equivalent to an increase of gravity in the column which is hydrostatically balanced with the exterior liquid by a diminution of its length. The amount of this action is also inversely proportionate to the diameter of the tube (22).

The following table exhibits the amount of depression in tubes of different diameters. In one column is included the result of experiments when the mercury has been boiled in the tube; in another, the results after boiling. In the first case a thin layer of air coats the tube; in the second, this is expelled.

(22) CD represents a capillary tube, immersed in a liquid capable of wetting it, in the vessel e f g h. The liquid stands at a considerable height above the general level of the liquid, and is also drawn up by the sides of the containing vessel, and the exterior surface of the

§ 84. Capillary action is of great importance in many natural and artificial arrangements; and as familiar instances of it, we may refer to the absorption of water by a piece of sponge, and to the beautiful manner in which the flame of a lamp or a candle is supplied with the liquid combustible by its ascent in the wick. The parallel interstices of a bundle of small wires will perform this office as well as the fibres of cotton, which are commonly employed, and being incombustible, may be perpetually used, provided they be kept clean. The natural pores of vegetable bodies are also well adapted to the spontaneous ascent of liquids, and it is probable that the motion of the sap of plants is considerably dependent upon this power. The fact may be pleasingly exemplified by dipping one end of a piece of cane into spirits of turpentine, which will rise to the other, and may be ignited.

It is by this force, again, that water is imbibed by the porous materials of the earth's surface, and moisture distributed in due proportions to the roots of vegetables. The amelioration of soils by culture is brought about by changes effected in them in this respect; and the influence of differences, even in their original constitution, may be traced in the superior verdure of chalk hills over those of sand or gravel.

The forces of heterogeneous adhesion and homogeneous cohesion are contrasted in an interesting manner, by closing the upper aperture of an air jar with fine wire gauze. If the jar be now immersed in a water bath with the mouth downwards, the air will of course be expelled and the water will enter, and upon raising it again the air will re-enter through the gauze and displace the water. But if the wire gauze be first wetted, the adhesion of the film of water to the metal will be so great, that the jar upon being raised from the bath will support a considerable column of the liquid against the pressure of the

tube, so as to form a curve between them. The form of the surface of the liquid within the tube is shown at i k l m. A B represents a capillary tube immersed in a liquid which is incapable of wetting it. The liquid is here seen depressed below the general level, and its surface in the vessel, a b c d, is curved from the depression which also takes place at the points of contact at its surface, and the exterior surface of the tube. The form of the surface within the tube is shown at n o p q.

atmosphere; water, however, may be poured into the jar without the penetration of air or disturbing the film of moisture.

A curious engine has lately been constructed for raising water upon the principle of heterogeneous adhesion. An endless hempen band of 6 inches width is made to revolve 1000 times per minute over a pulley. The lower part dips into a well of water which is drawn up by it from a depth of 135 feet, and delivered into a cistern at its point of suspension, at the rate of 83 gallons per minute.

§ 85. The action of filtration, to which the chemist is so frequently obliged to recur, is also dependant upon capillary action. The pores of the filter become wetted by the liquid, and transmit it to the under surface by a force of adhesion which is independent of the pressure. When once it has penetrated it collects into drops, by the force of cohesion, which fall from the force of gravity. Not only may solids be thus separated from liquids, but liquids which imperfectly mix, from each other. For this purpose the filter must be previously wetted with the liquid which it is intended to transmit, when that alone will pass, and the other be rejected. In this manner spirits of turpentine and water may be parted from each other.

§ 86. Few persons, perhaps, are aware of the prodigious amount of the force which may thus be called into action, and which may be measured by the opposition which it is capable of overcoming. If a dry plug of wood be tightly fitted into one end of a stout tube of glass, or porcelain, and a projecting portion be allowed just to dip into water, the wood will swell with such force, by the intrusion of the liquid into its pores, as to burst the tube, though capable of resisting a pressure of more than 700lbs. on the square inch. This power is turned to an economical account in parts of Germany and France, for separating blocks from the solid rock, for mill-stones. Holes are bored in its substance, into which wedges of dry wood are tightly driven, and when these are exposed to moisture they swell, from the capillary action, and effectually split it in the direction in which they have been placed.

§ 87. The same kind of attraction and superficial action takes place between solids and aëriform bodies, although not so open to common observation: but an easy experiment will