Page:Elementary Text-book of Physics (Anthony, 1897).djvu/244

230 The emission of light or, in general, of radiant energy from a body affords a demonstration of the existence of some motion in those parts of a body which are so small that the motion cannot be directly perceived by ordinary observation; for we can explain radiance only as a motion in a medium through which it travels, and it is evident that this motion cannot be due to the mere presence of a substance, but must be set up by the motion of matter.

We may first apply the kinetic theory to the distinction between solids, liquids, and gases. Each molecule of a solid is supposed to be retained within a certain small region by the action of the surrounding molecules and to move within that region. The phenomenon of crystallization leads us to think that molecules in a solid have certain determinate forms and an arrangement in the body; their motions, therefore, are such that they do not overstep the limits of this arrangement, and we think of their motion as vibratory, using the word vibratory in a rather loose sense. The molecules of a liquid have no fixed position in the mass, but are free to move from one point to another; they are in very close proximity to one another, as appears from the phenomena of capillarity, and exert considerable forces on one another. The chief difference between solids and liquids consists in the absence in the latter of any definite arrangement ; we may think of the molecules of a liquid as rotating and as gliding past each other, and can characterize their motion as rotatory. The great increase in volume exhibited on the change of a mass of liquid into vapor shows that the molecules of a vapor or gas are farther apart than those of a liquid. They are so far apart that their mutual actions due to molecular forces have very little influence on their motions, except during the excessively short period within which any two of them come close together or undergo an encounter. A molecule of a gas is therefore thought of as moving in a succession of short rectilinear paths, the direction of which is in general changed at each encounter. We may therefore characterize the motion in a gas as translatory. The consideration of this translatory motion is