Page:On Radiation.djvu/65

 and the visible ones transmitted by a layer of gas, we conclude that the oscillating periods of the gaseous molecules coincide with those of the invisible, and not with those of the visible spectrum.

It requires some discipline of the imagination to form a clear picture of this process. Such a picture is, however, possible. When the waves of ether impinge upon molecules whose periods of vibration coincide with the recurrence of the undulations, the timed strokes of the waves cause the motion of the molecules to accumulate, as a heavy pendulum is set in motion by well-timed puffs of breath. Thousands of millions of shocks are received every second from the calorific waves, and it is not difficult to see that every wave, arriving just in time to repeat the action of its predecessor, the molecules must finally be caused to swing through wider spaces than if the arrivals were not so timed. In fact, it is not difficult to see that an assemblage of molecules, operated upon by contending waves, might remain practically quiescent, and this is actually the case when the waves of the visible spectrum pass through a transparent gas or vapour. There is here no sensible transference of motion from the ether to the molecules; in other words, there is no sensible absorption.

One striking example of the influence of period may be here recorded. Carbonic acid gas is one of