AND ETHER WAVES. 83 



be sent through the tube along its axis. Prior to the 

 entry of the beam, the vapour is as invisible as the 

 purest air. When the light enters, a bright cloud is 

 immediately precipitated on the beam. This is entirely 

 due to the waves of light, which wreck the nitrite of 

 amyl molecules, the products of decomposition forming 

 innumerable liquid particles, which constitute the cloud. 

 Many other gases and vapours are acted upon in a 

 similar manner. Now the waves that produce this de- 

 composition are by no means the most powerful of those 

 emitted by the sun. It is, for example, possible to 

 gather up the ultra-red waves into a concentrated beam, 

 and to send it through the vapour, like a beam of light. 

 But though possessing vastly greater energy than the 

 light waves, they fail to produce decomposition. Hence 

 the justification of the statement already made, that a 

 suitable relation must subsist between the molecules 

 and the waves of ether to render the latter effectual. 



A very impressive illustration of the decomposing 

 power of the waves of light is here purposely chosen ; 

 but the processes of photography illustrate the same prin- 

 ciple. The photographer, without fear, illuminates his 

 developing-room with light transmitted through red or 

 yellow glass; but he dares not use blue glass, for blue light 

 would decompose his chemicals. And yet the waves of 

 red light, measured by the amount of energy which 

 they carry, are immensely more powerful than the 

 waves of blue. The blue rays are usually called che- 

 mical rays a misleading term ; for, as Draper and 

 others have taught us, the rays that produce the 

 grandest chemical effects in Nature, by decomposing 

 the carbonic acid and water which form the nutriment 

 of plants, are not the blue ones. In regard, however, 

 to the salts of silver, and many other compounds, the 

 blue rays are the most effectual. How is it, then, that 



