82 ATOMS, MOLECULES, 



air, is taken up by the strings. It may be regarded 

 as absorbed, each string constituting itself thereby 

 a new centre of motion. Thus also as regards the 

 tightly-locked atoms of molecules on which waves of 

 light or radiant heat impinge. Like the waves of 

 sound just adverted to, the waves of ether select those 

 atoms whose periods of vibration synchronise with their 

 own periods of recurrence, and to such atoms they 

 deliver up their motion. It is thus that light and 

 radiant heat are absorbed. 



And here the statement, though elementary, must 

 not be omitted, that the colours of the prismatic spec- 

 trum, which are presented in an impure form in the 

 rainbow, are due to different rates of atomic vibration 

 in their source, the sun. From the extreme red to the 

 extreme violet, between which are embraced all colours 

 visible to the human eye, the rapidity of vibration 

 steadily increases, the length of the waves of ether pro- 

 duced by these vibrations diminishing in the same pro- 

 portion. I say * visible to the human eye,' because 

 there may be eyes capable of receiving visual impression 

 from waves which do not affect ours. There is a vast 

 store of rays, or more correctly waves, beyond the red, 

 and also beyond the violet, which are incompetent to 

 excite our vision ; so that could the whole length of the 

 spectrum, visible and invisible, be seen by the same eye, 

 its length would be vastly augmented. 



I have spoken of molecules being wrecked by a 

 moderate amount of heat of the proper quality : let us 

 examine this point for a moment. There is a liquid 

 called nitrite of amyl frequently administered to 

 patients suffering from heart disease. The liquid is 

 volatile, and its vapour is usually inhaled by the patient. 

 Let a quantity of this vapour be introduced into a wide 

 glass tube, and let a concentrated beam of solar light 



