43.]

COHESION.

69

It is obvious that a knowledge of the law of the decrease of density in the atmosphere furnishes the means of ascertaining the height of mountains by the employment of the barometer.

Young has calculated that if the air continued to diminish indefinitely in density, according to Boyle's law, I cubic inch (16:38 cub. centim.) of air of the mean density of that at the surface of the earth would, at a distance of 4000 miles from the earth's surface (or at a distance equal to the earth's radius), fill a sphere the diameter of which is equal to that of the orbit of Saturn; and, on the other hand, if a mine could be dug 46 miles deep into the earth, the air at the bottom would be as dense as quicksilver.

The observations of astronomers upon the amount of refraction experienced by the light of the heavenly bodies in traversing the atmosphere, however, led Wollaston to consider it to be probable that there is a limit to the upper surface of our atmosphere, as definite as that of the waters of the ocean, the repulsion of the particles being at length exactly balanced by their gravitation towards the earth.

§ II. COHESION.

(43) In the case of gases the predominance of elasticity due to repulsion is the leading characteristic; in the case of solids the opposite property of cohesion is that which first demands attention. Cohesion is the action which binds together the same kind of particles into one mass. It is this action which retains a bar of iron, a block of wood, or a lump of ice in a single piece.

It is obvious that the cohesion of different bodies varies greatly. Cohesion, however, appears to be uniform between particles of the same kind placed under circumstances similar as to temperature and structure. Owing to the difficulty of securing uniformity in texture and freedom from flaws, even in the most compact substances, such as the metals, it is difficult to estimate the coefficient of cohesion in any material with precision; although the general fact that iron is much tougher than copper, and copper than lead, is at once recognised. Two methods have been generally used to determine the cohesion of solids; the first consists in estimating the tension required to stretch rods of a given diameter of the substance under examination, until they give way; the second, in finding the amount of pressure required to crush a cube of the substance of given dimensions.

The strength of materials, all-important as it is to the engineer and to the architect, has little to do with chemistry, although variations in cohesion and aggregation of the same substance exercise a marked influence on the rapidity of many chemical actions. Gunpowder, for example, is reduced to grains in order that each portion may ignite quickly, and contribute its pressure to act upon the bullet; but the very same material, before it has been

sounding yet obtained was during the expedition of the Challenger, on March 23. 1875, in lat. 11° 24' N., long. 143° 16' E., where the depth was 4575 fathoms, 27,540 feet (8366 metres), or 5'199 miles.

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COHESION OF SOLIDS-HARDNESS.

[43.

granulated, and whilst in the form of hard compact masses, as it comes from the press, burns comparatively slowly, like a fuse, or a portfire.

(44) Reunion of Divided Solids by Cohesion.-Particles of a similar nature will, under the influence of cohesion, reunite, after complete separation, if brought sufficiently near to each other. This is shown on pressing together two clean, smooth, and freshlycut surfaces of lead; they will cohere, and a tension of some pounds will be required to separate them. In the same way, too, perfectly polished plates of glass cohere, sometimes so completely that they may be cut and worked as a single piece. This has happened in plate-glass manufactories.

This cohesion of divided solids is well exemplified by Whitworth's planes. These consist of two masses of cast iron with true surfaces, which when pressed together require the exertion of considerable force to separate them, so that the lower mass may be raised by lifting the upper one. Tyndall has shown that this phenomenon is observed in vacuo as well as in air.

According to the ratio that cohesion bears to other actions which, like heat and elasticity, tend to separate the particles of matter from each other, the body assumes the solid, the liquid, or the aëriform state. Considerable differences in physical properties are produced both in solids and in liquids by variations in the degree of cohesion existing among their particles.

(45) Cohesion of Solids.-In solids, these variations give rise to differences in hardness, elasticity, brittleness, malleability, and ductility.

The hardness of a body is measured by its power of scratching other substances, and it consists in the degree of resistance which the particles offer to the slightest change of relative position. To the mineralogist, the variations in the degree of hardness presented by different crystallized bodies, often furnish a valuable physical sign by which one mineral may be discriminated from others which resemble it. For the purpose of facilitating such comparisons, Mohs selected ten well-known minerals, which are enumerated in the following table, each succeeding one being harder than the one which precedes it: thus arranged, they constitute what he terms a Scale of Hardness, which has been generally adopted. In the examples selected, each mineral is scratched by the one that follows it, and the hardness of any mineral may be determined by reference to the types thus chosen. Suppose, for example, a body neither to scratch nor to be scratched by fluor spar-its hardness is said to be 4: if it should scratch fluor spar, but not apatite, its

45 a.]

VISCOSITY OF FLUIDS.

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hardness is between 4 and 5; the degrees of hardness being numbered from 1 to 10. The figures on the right indicate the number of minerals known of the same, or approximatively the same degree of hardness, as the substance opposite to which they stand:

The cause of the varieties of hardness observed in different bodies is not well understood. Even in the same substance, trifling variations in the external circumstances to which the body is subjected often produce extraordinary differences in the degree of hardness which it exhibits. A piece of steel cooled slowly from a red heat is comparatively soft; it may be cut with a file; and under strong pressure, it will even take impressions from a die: whilst the same piece of steel, if heated to redness, and suddenly cooled, becomes as brittle as glass, and nearly as hard as the diamond.

Brittleness is exhibited by bodies the particles of which resist displacement with regard to each other, except within extremely narrow limits. It is generally observed in hard and

elastic substances.

Malleability and ductility, or the property of extending under the hammer, and of fitness for drawing into wire, are the opposite of brittleness, the molecules of the solid admitting of very considerable relative displacement without losing their cohesion. These modifications of cohesion are exhibited only by the metals, and by a few only of them.

(45a) Viscosity of Fluids.-Amongst liquids, considerable differences are observed in the degree in which the cohesion is exhibited. Limpid liquids are those which, like ether or spirit of wine, display great mobility of their particles; bubbles produced in such liquids by agitation, rise quickly to the surface, break and disappear. In oil, syrup, and gum-water, the particies move sluggishly; such liquids are termed viscous. The viscosity of liquids presents a certain analogy with the malleability of solids. In a few instances, whilst the solid is melting under the influence of heat, a viscous state is observed intermediate between the hardness of solids and the perfect mobility of liquids. Melted sugar, or barley-sugar, is a case in point. The occur

72

INFLUENCE OF HEAT ON COHESION.

[45 a. rence of viscosity, as an intermediate state, is rare, except in the case of a mixture of two substances, one of which melts at a temperature a little higher than the other. Glass, which is a mixture of several silicates of different degrees of fusibility, offers a striking example of this kind; indeed to this condition it owes the plastic properties by which it is rendered capable of adaptation to the multifarious purposes to which it is now applied. Other bodies, such as the different varieties of wax and fat, soften, without being actually viscous, before they finally melt. A true chemical compound usually passes at once from the solid to the liquid form, as when ice, for example, by fusion, becomes water. A few of the simple bodies, however, present some remarkable cases of the occurrence of viscosity preceding fusion; such, for instance, as phosphorus, and those metals which, like iron and potassium, admit of being welded,' a process in which two pieces of the metal are united into one mass by hammering or pressing them together whilst they are in the soft condition which is observed before fusion.

(46) Influence of Heat on Cohesion.-All analogy leads to the conclusion that cohesion would be entirely destroyed in every elementary body by a sufficient elevation of its temperature; though there are some bodies which have not as yet been liquefied, and many which have not been converted into vapour. The three conditions in which the same chemical compound may exist, exemplified by ice, water, and steam, according to the temperature to which it is exposed, are shown by a vast number of other bodies. Gold itself has been first melted and then volatilized by the intense heat of the sun's rays, concentrated by a burning lens. On the other hand, by a sufficient reduction of temperature, united with a certain degree of pressure, a number of gases have been reduced, first to the liquid, and several even to the solid condition. The action of cohesion, like that of heat, is therefore universal. If the repulsion exerted by heat could be carried sufficiently far, there is reason to believe, that every known substance, not actually decomposable by heat, might be made to appear as a gas; and, by a reduction of temperature sufficient to allow cohesion to exert its sway, every known gaseous substance would probably exist in the solid state.

is

In gases, cohesion appears to be entirely overcome, and it does not exert itself sensibly, except in cases where the gas approaching the point at which, by increase of pressure, or reduction of temperature, it assumes the liquid form (Note, § 27 and 197).

47.]

ADHESION-FRICTION.

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§ III. ADHESION DIFFUSION OF LIQUIDS AND GASES.

(47) Adhesion.-Analogous to cohesion, or the action which holds similar particles together, is that of adhesion, which is exerted between the particles of dissimilar kinds of matter. It not unfrequently rises high enough to destroy cohesion, as when sugar or salt becomes dissolved in water. A rod of glass or of wood dipped into water or oil comes out wetted in consequence of this action. It is exerted between different bodies with very different degrees of intensity, as may be illustrated by the following experiment:

Take two glass dishes, sift over the bottom of one a layer of lycopodium or of finely-powdered resin, and over the other a layer of powdered glass: if a little water be sprinkled upon each, the drops of water in the dish of resin will be covered by a thin film of the powder, and when the dish is inclined will roll about like shot, the cohesion of the particles of the liquid predominating over their adhesion to those of the solid: whilst on the powdered glass, from the superior adhesion of glass to water, the drops sink in and are absorbed.

If the solid becomes wetted, a certain preponderance of the adhesion over the cohesion of the particles is obviously necessary; for if the cohesion exceeds the adhesion, as when glass or iron is plunged into mercury, the solid is not wetted. Extraneous circumstances, however, greatly modify the exertion of this action. If a film of air, of oil, or of any foreign matter, be diffused over the surface of the solid, it is no longer the surface of the solid and the liquid which are concerned, but the liquid and the surface of air or of oil with which the solid is covered. A clean glass is immediately wetted with water, but if the slightest film of grease exist upon its surface, the water runs off almost entirely.

Adhesion gives rise to a variety of important phenomena; it is mainly concerned in the production of capillary action, of solution, and of the diffusion of liquids; it is also exerted in osmosis, and less directly in the process of the intermixture and diffusion of gases. In this chapter some remarks will therefore be made upon each of these subjects in succession. Adhesion is the more especially worthy of attentive study by the chemist, because in its manifestations it is more nearly allied than any other force to chemical attraction.

Adhesion is exerted between bodies of all kinds, and when it occurs between solids, it is the principal cause of that resistance to motion which is termed friction. As a general rule, friction is greater between similar kinds of matter, less between those which differ in nature. An iron axle moving in an iron socket expe