The reason is that the molecules are not brought near enough to each other for cohesion to act. In the case of certain bodies, however, the parts may be brought within the range of molecular attraction, by pressure, by partial melting, or by simple contact. Two pieces of lead with smooth, freshly cut surfaces will cohere strongly if pressed firmly together. Several pieces of iron may be formed into one coherent mass by the process of welding, in which the parts are softened by intense heat, and then hammered together. If a piece of pure india-rubber be cut in two, and the parts brought together again, they will unite and cohere strongly. The force of adhesion gives value to mortar, glue, and all kinds of cements. SOLUTION is due to adhesion. Thus, when sugar dissolves in water, it is because the adhesion between the molecules of sugar and water is stronger than the cohesion between the molecules of sugar. When a liquid tends to spread over the surface of a solid it is said to wet it, as water upon glass. If it gathers in globules it does not wet it, as quicksilver upon glass. In the first case the force of adhesion between the water and the glass overcomes the force of cohesion which would tend to collect the water in globules. In the second case the formation of the globules shows that the force of cohesion in the mercury is greater than that of adhesion between the glass and the mercury. Fig. 6. 23. Capillarity. Fig. 7. Fig. 8. When a body is plunged into a liquid which is capable of wetting it, as when a glass rod is plunged into water, it is observed that the liquid is slightly elevated about the body, taking a concave form, as shown in Fig. 6. If a hollow tube is used instead of a rod, the liquid will also rise in the tube, as shown in Fig. 7. The smaller the bore of the tube, the higher will the liquid rise, and the more concave will be its upper surface. A tube one hundredth of an inch in diameter will support a column of water four inches high. Two Instead of a tube two plates of glass brought very near together may be placed in water, and the water will rise in the space between them. The nearer the plates, the higher the liquid will rise. plates one hundredth of an inch apart will support a column of water two inches in height. If the plates are in contact at the edges on one side, and slightly separated at the other, as shown in Fig. 9, the water takes the shape of a curve called the hyperbola. When a tube is plunged into a liquid which is not capable of wetting it, as when glass is plunged into quicksilver, the liquid is depressed both ou Fig. 9. the outside and on the inside, taking a convex surface, as shown in Fig. 8. The smaller the tube, the greater will be the depression, and the more convex will be the upper surface. 24. Applications of Capillarity. It is in consequence of capillary action that oil is raised through the wicks of lamps, to supply the flame with combustible matter. The fibres of the wicks leave between them a species of capillary tubes, through which the oil rises. If a piece of sugar have its lower end dipped in water, the water will rise through the capillary interstices of the sugar and fill them. This drives out the air and renders the sugar more soluble than when plunged dry into water, in which case the contained air resists the absorption of water, and retards solution. If a bar of lead be bent into the form of a siphon, and the short arm be dipped into a vessel of mercury, the mercury will rise into the lead by capillary action, and flowing over the edge of the vessel will descend along the lower branch and escape from the lower extremity. In this way the vessel may be slowly emptied of the quicksilver. Many fluids may be drawn over the edges of the containing vessels by a siphon of candle-wicking or other capillary substance. 25. Absorption is the penetration into a porous body, of any foreign body, whether solid, liquid, or gaseous. Carbon, in the form of charcoal, has a great capacity for absorbing gases. If a burning coal be introduced into a bellglass filled with carbonic acid collected over mercury, the volume of the gas is diminished by being absorbed by the coal. It is found that the charcoal absorbs in this way thirtyfive times its own volume of the gas. Charcoal also absorbs other gases in even still greater quantities. Spongy platinum absorbs hydrogen so rapidly as to heat the platinum red-hot. In vegetables and animals we have many examples of absorption. The roots of plants absorb from the earth the material necessary to the growth of the stem and branches. In the animal world, absorption plays an important part in the process of nutrition and growth. Animal tissues also absorb solid substances. For example, workmen engaged in handling lead absorb through the skin and lungs more or less of this substance, which often gives rise to very serious diseases. When vegetable and animal substances absorb water, they generally augment in volume. This fact explains many phenomena of daily observation. If a large sheet of paper be moistened, it increases in size, and again contracts when dried. This property is employed by draughtsmen to stretch paper on boards. The paper is moistened, and after being allowed to expand, its edges are glued to a drawing-board ; on drying it is stretched, forming a smooth surface for drawing upon. The same property causes the paper to peel from the walls of a room when exposed to moisture. When a workman would bend a piece of wood, he dries one side and moistens the other. The side which is dried contracts, and the opposite side expands, so that the piece is curved. It is the absorp tion of moisture that causes the wood-work of houses, furniture, etc., to swell and shrink with atmospheric changes, and which necessitates their being painted and varnished. Paints and varnishes, by filling the pores, prevent absorption. 26. Osmose. - If two liquids of different kinds are separated by a porous or membranous partition, each liquid will begin to pass through the membrane and mix with the other, and after a time there will be a mixture of both liquids on each side of the partition. This movement of the liquids is called osmose. The currents are generally unequal, so that there is an actual gain of substance on one side, and a corresponding loss on the other. The current that acts to produce an increase on one side is called endosmose, and the opposite current is called osmose. To illustrate this process, let a bladder filled with strong syrup be tied to the end of a glass tube, and the whole placed in a vessel of water, as shown in the figure. The syrup soon becomes diluted by the flowing in of water, and the mixture rises in the tube; at the same time a portion of the syrup flows out and mixes with the water. The flowing in of the water is endosmose, and the flowing out of the syrup is exosmose. Similar results are obtained by using other liquids. The principle of osmose is of very great importance in animal and vegetable physiology. The circulation of fluids through the tissues and vessels of the animal body, the absorption of water by the roots of plants, the circulation Fig. 10. of the sap, and many other vital phenomena depend upon this principle. out 27. Dialysis. The practical application of the principle of osmose in separating the constituents of a liquid is called dialysis. Substances which are capable of forming crystals will, when in solution, readily pass through membranes or porous partitions. Pure sugar and various kinds of salts are substances of this kind. On the other hand, substances which do not crystallize, like gelatine, gum arabic, etc., do not so readily pass through septa. Hence pure crystallizable sugar may, by this process, be separated from the syrup of sorghum, or that of the beet-root, which contains gummy substances that would otherwise prevent crystallization. 28. Tenacity is the resistance which a body offers to rupture when subjected to a force of traction, that is, a force which tends to tear the particles asunder. The tenacity of a body may be determined in pounds. For this purpose it is wrought into a cylindrical form, having a given crosssection; its upper end is then made fast, and a scale-pan is attached to the lower end; weights are then placed in the pan until rupture takes place. These weights measure the tenacity of the body. Metals are the most tenacious of bodies, but they differ greatly from each other in this respect. The following table exhibits the weights required to break wires of 1855 of an inch in diameter, formed of the metals indicated: Iron. Platinum Silver 10000 549 lb. 302 " 274" 187 " 150 " 27 " Gold Lead It has been shown by theory and confirmed by experiment, that of two cylinders of equal length and containing the same amount of material, ouc being solid and the other hollow, the latter is the stronger. This latter principle is also true of cylinders required to support weights; the hollow cylinder is better adapted to resist a crushing force than the solid one of the same weight, and hence it is that columns and pillars for the support of buildings are made hollow. This principle also indicates that the bones and quills of birds, the |