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. [Illustration: Fig. 9.] [Illustration: 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