matter, then, depends on their size and distance. When they are small masses, such as two drops of water or globules of mercury, their mutual attraction will be inappreciable compared with that of the overpowering mass of the earth, unless, indeed, by bringing the drops sufficiently near we might atone for deficiency of matter. Such an attraction between small bodies does actually appear when we bring them very close together, and it is not impossible that it may be the same gravitating power in a different guise at the other extreme of nature.

Whether this may be proved or not, we shall adopt the ordinary distinction, which gives to attraction different names according to the distances at which it acts.

22. GRAVITATION is the attraction common to all matter when it acts at great or sensible distances, as between the moon and the waters of our earth, &c. It is incessant in its operation. No matter can be conceived to exist without it.

COHESION is the attraction between molecules of the same kind, binding them into masses, which must be overcome when we break or disintegrate a body.

Adhesion (including CAPILLARY ATTRACTION) is the attraction which exists in various degrees between dissimilar kinds of matter, as is illustrated in a marked degree by cements.

ATOMIC ATTRACTION is that which binds atoms together in groups or molecules, and is the ultimate cause to which we can as yet assign any phenomenon.

(There are other species of attraction called Magnetical and Electrical; but as they are not possessed by all bodies, nor at all times by the same body, they do not come under this group of common kindred attractions.)

Gravitation has been already considered sufficiently for the general purpose of this section. We shall now examine the part played by the other attractions in the production of the material appearances around us.

23. COHESION is the attracting quality by which a number of similar molecules are held together so as to form one

mass.

A porter, whose work is to lift and carry weights, has to fight with the power of gravity; but the woodman who fells a tree, the blacksmith who files a piece of iron, the turner who shapes legs of

Illustrations of Cohesive Attraction.

II

tables and chairs, all gain their livelihood by a constant struggle against cohesion.

24. As might be expected, the limits within which this molecular attraction appears are extremely minute. The gravitation between two metal balls of three inches diameter cannot be detected till the balls are approached almost to touching; and these are worlds in size compared with molecules. If, then, there is any correspondence between the attractions of gravitation and of cohesion, the appreciable range of the latter will be very much less than the breadth of a molecule, which we have seen is so small as almost to defy calculation.

From this argument, as well as from experiment, it appears that the cohesive influence of a particle will not extend beyond the breadth of a contiguous particle; and, consequently, that when two masses are made to cohere, it is merely the upper or surface molecules that are brought into play.

25. For cohesion of two masses we must, therefore, bring a sufficient number of molecules face to face, as well as press them very closely together. This done, we need no glue or paste to effect the junction. But this is not so easily done. For a surface that we deem smooth may be really so rough that, when we apply it to another, the two touch only at a few projecting points, whose cohesion will of course be insignificant.

Two pieces of common glass, though they appear quite smooth, will not usually cohere. Yet glass polishers are familiar with the fact that sheets of flat and highly polished plate glass laid on each other often stick together so that they cannot be separated without fracture.

26. The following are a few examples of the sufficiency of these conditions for perfect cohesion :

Similar flat-faced portions being cut off with a clean knife from two leaden bullets, and the fresh surfaces being well pressed together, the bullets will cohere, sometimes as firmly as if they had been cast in one piece. This is not owing to atmospheric pressure by the displacement of air between the two surfaces of lead, for the bullets cohere with equal firmness when placed in the vacuum of an air-pump.

Fresh-cut surfaces of caoutchouc (or india-rubber) may be united in a similar way. If a sheet of it be folded, and a strip cut off near the fold by one stroke of a pair of long scissors, the strip, by pressing together the two surfaces, will become a perfect tube with the seam scarcely visible,

:2

Restoration of Cohesion.

Clean sheets of lead or tin may be united by simply pressing them together between the powerful rollers of a flatting-mill.

The restoration of cohesion is beautifully seen in the gilding of china. A line or figure is drawn on the china with a mixture of oxide of gold and an essential oil. The article is then heated, whereby the essential oil and the oxygen of the gold are expelled, and a red brown pattern remains. This consists of pure gold in a finely divided state and without lustre. By rubbing it with a hard burnisher (hæmatite) the particles of gold are made to cohere and reflect the rich yellow colour of polished gold.

By sufficient pressure brick-dust may be formed into tiles, agatedust into buttons, broken ice into solid lumps, and slate-dust into rods for pencils.

Thus, too, sandy matter deposited at the bottom of the sea, and undergoing there the pressure of great depths for ages, is formed into the solid rocks which can be used for building.

27. Surfaces may, however, appear to the eye to be in contact when they are not actually so. Newton found, during some experiments on light, that a convex lens, or a watch-glass, laid on a flat piece of glass does not touch it, and cannot be made to touch it, even when compressed with a force of many pounds.

If such be the case with a smooth hard substance like glass, which does not readily tarnish, we can easily see how much more difficult it will be to bring within cohering distance two metallic surfaces, which speedily acquire a spongy oxidised film on their surface when exposed to the atmosphere.

This is the reason that the metals require, almost all of them, to be heated or melted before they will cohere in a solid mass. The oxidised portion then rises as dross to the surface and allows free contact of the pure metals; the intense heat also expels the air, which would serve to prevent contact in the cold state. In the art of soldering, by which metals are made to cohere, borax, sal ammoniac, and other fluxes are used for the purpose of dissolving the oxides of the metals produced by heat, and thus bringing into contact perfectly clean metallic surfaces. They then cohere firmly.

28. The degree of cohesion among the particles of a body will depend on their mutual distance; and this, as we shall consider more particularly afterwards, will vary with the amount of heat or heat-motion present in the mass.

Meantime we must anticipate one of the subjects to be treated Lereafter, so far as to explain that heat is to be regarded as a minute

Cohesion in Solids.

13

vibration of the particles of a body, which is never wholly absent, but which may be increased or diminished within wide limits. According to this theory, when we add to the heat of a body by any means, we merely increase the agitation amongst its molecules. These will therefore elbow each other more widely apart, and the result will be an increase in the apparent bulk of the body. Hence it is that, as a general rule, increase of heat expands bodies and lessens their internal cohesion.

29. Cohesion and heat or heat-motion, then, are the two antagonists in nature, on whose relations the physical condition of all bodies depends, and whose relative changes determine the most obvious distinction of substances-the distinction, namely, into Solids, Liquids, and Gases.

A SOLID is a collection of particles cohering firmly together so is to keep their positions with respect to each other practically unchanged. We are not to suppose that the particles are absolutely fixed together and devoid of all motion, but only that the heat or quivering motion is of small range, and the cohesion is the resistance offered to the particles being pulled altogether out of place, or the limit of their motion being exceeded. In solids we may say, then, that the cohesion entirely overpowers the opposing heat-vibration.

A LIQUID is a body whose particles are all balanced more or less exactly between these two opponents, and being, as it were, inclined to neither party, are perfectly free to move amongst each other. The particles are just on the border of the territory of cohesive attraction.

A GAS, lastly, is a body whose particles have been separated beyond this limit by heat-motion imparted to them, and, being practically freed from all restraint, are moving to and fro amongst each other in ceaseless confusion. Motion is here predominant, and the power of cohesion altogether in abeyance.

"Cohesion in solids."

30. As cohesion plays the most important part in solids, so its modifications produce in them the most noteworthy varieties of character. Thus differences of cohesive power, coinciding with differences of molecular structure, and probably of the shapes of molecules, occasion the various physical properties in solids known as porous, dense, crystalline, hard, brittle, elastic, pliant, malleable,

14

Physical Properties of Bodies.

ductile, tenacious. We shall consider in detail the peculiarities on which these several physical qualities depend, as a knowledge of them as well as of the materials, where each of the properties is specially developed, has already proved of the utmost value to the arts and to civilisation.

"Porous."

31. There is no body, even the most dense, whose molecules are everywhere in absolute contact; and there will thus be intermolecular spaces or pores, which may be filled with air or any fluid whose molecules are fine enough to enter the spaces.

A considerable quantity of water may pass into a bit of chalk, a lump of sugar, or even a piece of stone, and be hidden away among the pores without increasing the bulk. Chalk is said to absorb as much as a third of its own bulk of water.

A kind of agate stone, called hydrophane, is opaque until wetted, when its pores get filled with water (even to one-sixth of its weight), and under these circumstances the stone becomes translucent, giving a passage to light.

32. Pores exist in the densest metals, for by strong compression they may be squeezed so as to occupy less bulk, and this can only be by bringing the molecules nearer and lessening the intermediate spaces.

Pressure will drive water through solid gold or copper, as was seen in the famous Florentine experiment, where a hollow gold ball, being filled with water and strongly pressed by a screw to test the compressibility of water, was found to perspire all over. We have by a similar experiment seen water thus forced through a copper ball and deposited on the outside as a fine dew.

Filters are bodies, suitably shaped, whose pores are sufficiently large to allow the passage of water, or other liquids, but small enough to detain any solid impurities.

33. Animal and vegetable bodies are the most porous, for internally they are a multitude of interlacing channels whereby, during life, the nourishing fluids may circulate.

Bone is a tissue of cells and partitions, as little solid as a heap of empty packing-boxes. Wood is a congeries of parallel tubes or fibres like bundles of organ pipes.

Condensed wood is now prepared for various purposes-as for