CHAPTER II.

MOLECULAR ATTRACTIONS.

81. Minute Constitution of Matter.

24. Its Interior Structure.-From the force which acts between masses at all distances, we now pass to the study of a class of forces which only come into play when bodies are in contact. They seem to pertain to the interior structure of substances, and hence, before treating of them, it becomes important to refer to that interior structure, or how matter is believed to be constituted.

25. Porosity of Matter.-If we place a little water upon chalk or cloth, it disappears; in a certain sense it penetrates them, but it only passes into vacant places termed pores. Not only loosely-composed substances, as soil and flesh, but wood, rocks, stones, and even dense metals, have the same porous texture. Liquid mercury passes through lead, and water has been also forced through the pores of gold. Matter is, therefore, held to be universally porous.

26. Motions of Internal Parts.-If a closed India-rubber bag, filled with air, be squeezed, it will be compressed into less bulk—that is, the particles of air will be forced nearer together. If alcohol and water be commingled, the mixture occupies a smaller space than did the separate liquids; their particles have, therefore, approached closer to each other. If iron be hammered, it will be driven into less compass, the metallic particles being forced into closer relation. A

certain amount of heat added to bodies in either the solid, liquid, or gaseous form, will cause a certain degree of expansion-that is, will cause the constituent particles to recede from each other; and, when the heat is withdrawn, the particles again approach.

27.-It is concluded from such facts as these that matter consists of exceedingly minute particles which are never in absolute contact, but are surrounded by unoccupied spaces, in which they are free to move under the action of forces. These ultimate separated material points, which are of great minuteness, are termed molecules, a word signifying a small mass. To the physicist molecules are not imaginary, but actual things with weights and magnitudes, and which do not change in the physical transformations of matter. Molecules play a prominent part in modern physical theory; and have made familiar the phrases molecular attractions, molecular forces, molecular constitution of matter. The chemical aspect of molecules will be considered in the chapter on Theoretical Chemistry, Part II.

28. Divisibility of Matter.-The division of matter may be carried to an amazing extent. Gold may be drawn out as a coating upon silver wire until the 492-thousand-millionth part of an ounce is still visible, with its proper metallic color and lustre. It has been estimated that, in a drop of the blood of the musk-deer, such as would remain suspended upon the point of a fine needle, there are one hundred and twenty millions of globules. But these examples of the divisibility of matter bring us only to the threshold of a world of wonders. Microscopic researches. have introduced us to a realm of life peopled with animate beings, which are born, grow, reproduce their kind, and die; and yet so minute that many millions of them heaped together would not exceed in size a grain of sand.

We will now notice some of those forms of force which are exerted between bodies only when in contact, and which are known as molecular attractions. They are mani

fested in the forms cf matter, solid, liquid, and gaseous, which are known as states of aggregation.

§ 2. Adhesion and Cohesion.

29. Their Differences.-Though the molecules of a solid are separated, yet it does not crumble to pieces. They are held together by a force which reaches across their interstices and binds them in a fixed relation. When this force unites bodies dissimilar in kind, it is called adhesion. The sticking of chalk to a black-board, of mortar to bricks, of glue to wood, etc., are examples of adhesion. The same force, when acting between particles of the same kind, is termed cohesion. The form, solidity, hardness, elasticity, brittleness, malleability, and ductility of solids, are the result of various unknown modifications of cohesive force. There is also a mutual attraction among the particles of liquids. In a drop of liquid, cohesion attracts the particles. into a rounded figure, against the influence of their weight, which would spread them out; pendant drops still further exemplify the same force.

FIG. 7.

30. Adhesion of Liquids to Solids.--If a glass rod be dipped in water, the liquid will rise round it above its level in the vessel (Fig. 7), and, when withdrawn, it will be wet. But, if the same rod be dipped in mercury, there is an apparent repulsion (Fig. 8), and the rod when withdrawn is dry. If a rod of gold be dipped in the mercury it is wetted, or covered with a mercurial film. The wetting in this case shows an attraction between the liquid and the solid, and that it is sufficiently strong to produce adhesion. But there may be attraction without wetting; glass is not wet by mercury, and still they are attracted, as may be thus shown. Suspend a flat, circular plate of glass to the arm of a balance,

counterpoise it, and lower the plate (Fig. 9) over a cup of mercury. No matter how near the glass approaches, while there is no contact, there is no attraction. But, as soon as they are made to touch, a slight adhesion occurs, sufficient to lift a portion of the mercury above its level in the ves

sel, the amount of which may be exactly measured by the number of weights required to be placed in the opposite scale-pan to separate them.

31. Conditions of Wetting.-If the adhesive force of any solid for any liquid exceeds half the cohesive force of the liquid particles for each other, the solid will be wet. Thus, the adhesion of gold for mercury and of water for wood exceeds half the cohesive force of the mercurial and watery particles for each other, consequently water wets wood, and mercury wets gold. But, if the adhesion of the solid be less than half the cohesion of the liquid, wetting does not follow contact, as is exemplified by glass and mercury.

32. Capillary Attraction.-If glass rods with small apertures, open at both ends (Fig. 10), be dipped in water, the liquid immediately rises through the orifices to a height which increases in proportion to the smallness of the openings. The same thing may also be beautifully exhibited by placing two plates of glass (Fig. 11) upon their edges in a dish of colored water, one end being joined, and the other slightly separated. The influence of the graduallyapproaching sides of the glass in attracting the liquid up

ward is seen in the course of the curve.

From the circum

stance that this effect is best produced by tubes with very

fine apertures, the attraction that causes these phenomena is called capillary (from capillus, a hair).

FIG. 12.

33. Reversed Capillarity.-If, now, a glass tube be dipped in mercury, we have again a disturbance of liquid equilibrium, but the effect is reversed. The interior column of mercury is depressed below the outside level, and its surface exhibits a convex shape, as seen in Fig. 12. The same thing occurs if the tube be greased and dipped in water, and in all cases where the liquid cannot wet the solid. The common belief, that depression in this case (as in that of the glass and mercury) is caused by repulsion, is quite erroneous. We have proved that, instead of repulsion, there is a strong attraction between glass and mercury. The reversed capillary action simply results from the preponderance of the cohesive over the adhesive force. In every body of fluid, Convex Liquid Sureach particle is kept in place by the mutual action of all the surrounding particles. But, if a column of fluid be separated from the surrounding mass by interposing the walls of a tube, the sides of which exert no equivalent adhesive force, the cohesion of the mass below draws down the upper and outer particles, and produces a roundness or convexity at the top.

face.