Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

56. Attraction in Liquids.—In a liquid the attraction between the particles is very feeble compared with that in solids. The attraction of the particles of steel is in strength about three million times that of the particles of water. We make the estimate in this way: We find that a steel wire will sustain a weight equal to 39,000 feet of the wire. But a drop of water hanging to the end of a stick can not be more than one-sixth of an inch in length; that is, water will hold together by the attraction of its particles only to this extent, which is a little less than the three millionth part of the length of steel wire which could hang without breaking.

57. Freeness of Movement of the Particles of Liquids.—There is one prominent characteristic of liquids which is probably not entirely owing to the feeble attraction of their particles—I mean the freeness with which these particles are moved among each other. This is owing probably in part to some peculiar arrangement of the atoms in making the particles of a liquid. I will illustrate this in a coarse way. If the atoms of lead in shot were so arranged as to make irregular jagged forms, they could not readily be moved among each other. We suppose the ultimate atoms of a liquid to be so arranged in the formation of particles as to make them not only round but very smooth. Hence comes the great ease with which they circulate among each other.

Fig. 9.

Fig. 10.

58. Globular Shape of Drops of Liquids.—As the particles of a liquid move thus freely among each other, their attraction disposes them to assume a globular or round shape. The reason of this can be made plain by Figs. 9 and 10. The outside of a perfect sphere is all at the same distance from the centre. So all the circumference of a circle is at the same distance from the centre, as represented in Fig. 9. But this is not true of all parts of the surface of a cube or of a square: a, for example, is farther from the centre than b is. Now in a drop of liquid all the particles are attracted toward the centre, for in that line from each particle lies the largest number of particles to attract it. This can be made obvious by taking some point in the drop, as represented in Fig. 10, and drawing lines from it through the centre and in other directions. If a be the point in the drop, it is plain that the line from it through the centre is longer than a b or a c. Therefore a particle, a, will be attracted toward the centre rather than in the direction a b or a c, because there are more particles in the direction of the centre, and the more particles there are the stronger is the attraction. But this is not all. The particles in the line a c, tending to make a go toward c, are balanced by the particles in the line a e, tending to make it go toward e. The two lines of particles therefore together tend to make it go in a middle line between them, that is, toward the centre, just as two strings pulling equally, the one to c and the other to e, would make a body, a, move in a middle line between these two directions. The same can be shown of the two lines of particles a b and a d, and so of any other two alike in situation on each side of the line through the centre. The tendency of every particle is, then, to go toward the centre, and it would go there if there were not particles between to prevent it. You see how this would operate in the case of the particles on the surface of the drop. As these are all striving, as we may say, in obedience to attraction, to get to the centre, none of them will be raised up into an angle or a point, as would be the case if the drop were in the shape of a cube. If this should be done it would show that some of the particles were not as strongly attracted toward the centre as others are, which is an impossibility.

59. The Globular Form in Different Liquids.—The disposition to form a sphere is seen more distinctly in mercury than in any other liquid. If you drop a little of it upon a plate it separates into globules, which roll about like shot. Why can not the same thing be done with water? Why do the drops of water hang upon the window-pane, showing only in an imperfect way their disposition to the globular arrangement? It is because the particles of water have a greater attraction for other substances, and less attraction for each other, than the particles of the quicksilver have. Water sometimes exhibits its disposition to the globular form in full on the leaves of some plants, and rolls about in balls like mercury. This is because there is something on the surface of the leaf which repels rather than attracts the water. If you put your finger, however, on one of these drops, it will spoil it, and your finger will be moistened, because there is an attraction between the particles of your skin and of the water. Take another illustration of this difference in attraction. If you drop a little oil upon the surface of water it will float about in round drops. This is because the water repels the oil, as the surface of some kinds of leaves does water. But when oil is spilled upon wood or cloth its particles have so strong an attraction for their particles that they unite with them, instead of gathering up into little round companies as they do on the surface of water.

60. Manufacture of Shot.—We have a beautiful example of the tendency of fluids to the globular arrangement in the manufacture of shot. The melted lead is poured into a large vessel in the top of the shot-tower. This vessel has holes in its bottom, from which the metal falls in drops. Each drop, as it whirls round and round in its fall, takes the globular form. By the time that it reaches the end of its journey, about two hundred feet, it becomes so far cooled as to be solid, and as it is received in a reservoir of water, its globular form is retained. Bullets can not be made in this way, because a quantity of melted lead sufficient to make a bullet will not hold together in a globular form.

61. Globular Form of the Earth and the Heavenly Bodies.—It is supposed that the sun, moon, earth, and all the heavenly bodies were once in a liquid state, and that they owe their globular shape to this fact. As they whirled on in this condition in their course, the different solids were gradually formed, and at length they acquired their present state. How all the mighty changes could be effected in our earth, converging it from a liquid into a body with a solid crust, having such various substances in it, and so variously arranged, with its depressions containing water, and the whole covered with its robe of air fifty miles in thickness, we can not understand. And yet there are some portions of the process which chemistry and geology have revealed to us, giving us some glimpses of the wonders which, during the lapse of ages, God wrought in our earth in preparing it for the habitation of man.