PHYSICAL BASIS OF SOLAR CHEMISTRY. 391 



curious effects were at the same time observed. When 

 the pendulums differed from each other a certain amount, 

 A set B going, but the reaction of B stopped A. Then 

 B set A going, and the re-action of A stopped B. When 

 the periods of oscillation were close to each other, but 

 still not quite alike, the clocks mutually controlled 

 each other, and by a kind of compromise they ticked 

 in perfect unison. 



But what has all this to do with our present subject? 

 The varied actions of the universe are all modes of 

 motion ; and the vibration of a ray claims strict brother- 

 hood with the vibrations of our pendulum. Suppose 

 aethereal waves striking upon atoms which oscillate in 

 the same periods as the waves, the motion of the waves 

 will be absorbed by the atoms ; suppose we send our 

 beam of white light through a sodium flame, the atoms 

 of that flame will be chiefly affected by those undula- 

 tions which are synchronous with their own periods of 

 vibration. There will be on the part of those particular 

 rays a transference of motion from the agitated aether 

 to the atoms of the volatilised metal, which, as already 

 defined, is absorption. 



The experiment justifying this conclusion is now for 

 the first time tobe made before a public audience. I pass 

 a beam through our two prisms, and the spectrum spreads 

 its colours upon the screen. Between the lamp and the 

 prism I interpose a snapdragon light. Alcohol and 

 water are here mixed with common salt, and the metal 

 dish that holds them is heated by a spirit-lamp. The 

 vapour from the mixture ignites and we have a mono- 

 chromatic flame. Through this flame the beam from the 

 lamp is now passing ; and observe the result upon the 

 spectrum. You see a shady band cut out of the yellow, 

 not very dark, but sufficiently so to be seen by every- 

 body present. 



