16 THE FOKMS OF WATEB IN 



action here rendered so sensible. As before, the focus 

 is rendered strikingly visible by the intense illumina- 

 tion of the dust particles. We will first filter the 

 beam so as to intercept its dark waves, and then per- 

 mit the purely luminous waves to exert their utmost 

 power on a bma-11 bundle of gun-cotton placed at the 

 focus. 



43. No effect whatever is produced. The gun-cotton 

 might remain there for a week without ignition. Let 

 us now permit the unfiltered beam to act upon the 

 cotton. It is instantly dissipated in an explosive flash. 

 This experiment proves that the light-waves are incom- 

 petent to explode the cotton, while the waves of the 

 full beam are competent to do so ; hence we may con- 

 clude that the dark waves are the real agents in the 

 explosion. 



44. But this conclusion would be only probable ; for 

 it might be urged that the mixture of the dark waves 

 and the light- waves is necessary to produce the result. 

 Let us then, by means of our opaque solution, isolate 

 our dark waves and converge them on the cotton. It 

 explodes as before. 



45. Hence it is the dark waves, and they only, that 

 are concerned in the ignition of the cotton. 



46. At the same dark focus sheets of platinum are 

 raised to vivid redness; zinc is burnt up; paper in- 

 stantly blazes ; magnesium wire is ignited ; charcoal 

 within a receiver containing oxygen is set burning; a 



