392          ON  THE  CHANGE  OF   KEFRANGIIULITV   OF   UUHT.
For some time there will be a tendency one way, then for Home time a tendency the other way, and so on, the opposite tendencies balancing each other in the long run. The lengths of the times during which the tendency lies in one direction, will depend upon the periodic times of the molecular and ethereal vibrations, being on the whole greater or less according as the two periodic times are more or less nearly equal But since no external forces actually act to keep the amplitudes constant, when the ethereal vibrations are favourable to disturbance the molecule* is further disturbed, and therefore its periodic time is diminished; and when they are favourable to quiescence the disturbance, of the molecule is checked, and therefore its periodic time is increased. If, then, the ether be vibrating more rapidly than the molecule, when the action is favourable to disturbance the periodic time of the molecular vibrations is rendered more nearly equal to that of the ethereal vibrations, and therefore the time during which the action is favourable to disturbance is prolonged ; but when the action is favourable to quiescence, the etlert is just uk*. reverse. Hence, on the whole, there is a balance outstanding in favour of disturbance. Hut if the ether be vibrating more slowly than the molecule, it appears from similar reasoning that there will be a balance the other way. Hence, it is only when the periodic time of the ethereal vibrations is less than that of the molecular, that the latter vibrations can be kept going by tin* former.
232. But it will probably be objected to this explanation, that when a periodic disturbing force. aiFects the mean motion of a planet, the mean motion is a, maximum, not, when the force tending to augment it is a maximum, but. at a time later by a quarter of the period of the force, namely, when the, force vanishes in changing sign; and that in a similar manner the change in the periodic time of the vibrations of a disturbed molecule will affect equally the duration of the time during which the action is favourable to increased disturbance, and that during which it. is favourable to quiescence, or more exactly will not alter either, since the effects in the first and second halves of those times will neutralize each other. The answer to this objection is, that we, must not treat a molecule as if it were isolated, like a heavenly body, since it is continually losing its motion by communication,