14 INORGANIC EVOLUTION. [CHAP. 



vibrating and still pass on to one's ear at the other end of the room as 

 if nothing had happened to them. 



Now apply this to light. Suppose we have at one end of a room a 

 vivid light source giving us all possible waves of light from red to 

 violet. This we may represent as before by 



. Sly w a m Y L^ 



Also suppose that we have in the middle of the room a screen of 

 molecules, say a sodium flame, capable of emitting yellow light, 



What will happen 1 Will the light come to our eyes exactly as if 

 the molecules were not there ? No ; it will not. What then will be 

 the difference 1 The molecules which vibrate at such a rate that they 

 give out yellow light, keep for their own purpose filch, so to speak, 

 from the light passing through them the particular vibrations which 

 they want to carry on their own motions, and we shall have 



\V7 n PET) (f\i /TS\ nET) 



\J 'J LED vLfl ( ^) Lr\i 



as a result ; the light comes to us minus the vibrations which have thus 

 been utilised, as we may put it, by the screen of vapour. We have, in 

 fact, an apparently dark space which may be represented thus : 



- w a [ Y \& 



In the spectroscope we see what would otherwise be a continuous 

 spectrum, with a dark band across the yellow absolutely identical in 

 position with the bright band observed when the molecules of the 

 vapour of which the screen is composed radiated light in the first in- 

 stance. It is not, however, a case of absolute blackness, or absence of 

 that particular ray, for the molecules are set in vibration by the rays 

 which they absorb, and therefore give out some light, but it is so feeble 

 as to appear black by contrast with the very much brighter rays coming 

 direct from the original source. 



This great law may be summed up as follows : Gases and vapours, 

 wJien relatively cool, absorb those rays which the)/ themselves emit wlien 

 incandescent ; the absorption is continuous or discontinuous (or selective) 

 as the radiation is continuous or discontinuous (or selective). 



I have referred to this matter at. some length because in our light 

 sources, in the sun, an.d in most of the stars we have light from a more 

 highly heated centre passing through an envelope of cooler vapours, 

 and on this account absorption phenomena are produced. 



