66 FEAGMENTS OF SCIENCE. 



It requires some discipline of the imagination to 

 form a clear picture of this process. Such a picture is, 

 however, possible, and ought to be obtained. 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 augment 

 the vibration of the molecules, as a heavy pendulum is 

 set in motion by well-timed puffs of breath. Millions 

 of millions of shocks are received every second from the 

 calorific waves; and it is not difficult to see that as 

 every wave arrives 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. 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 of heat. 



One striking example of the influence of period may 

 be here recorded. Carbonic acid gas is one of the 

 feeblest absorbers of the radiant heat emitted by solid 

 bodies. It is, for example, to a great extent transparent 

 to the rays emitted by the heated copper plate already 

 referred to. There are, however, certain rays, com- 

 paratively few in number, emitted by the copper, to 

 which the carbonic acid is impervious; and could we 

 obtain a source of heat emitting such rays only, we 

 should find carbonic acid more opaque to the radiation 

 from that source, than any other gas. Such a source is 

 actually found in the flame of carbonic oxide, where 

 hot carbonic acid constitutes the main radiating body. 

 Of the rays emitted by our heated plate of copper, 



