206 THE POPULAR SCIENCE MONTHLY. 



the motes entirely absent, the beam would be unseen. It falls upon a 

 concave mirror (a glass one silvered behind will answer), and is gath- 

 ered up by the mirror into a cone of reflected rays ; the luminous 

 apex of the cone, which is the focus of the mirror, being about fifteen 

 inches distant from its reflecting surface. Let us mark the focus 

 accurately by a pointer. 



And now let us place in the path of the beam a substance pei-fectly 

 opaque to light. This substance is iodine dissolved in a liquid called 

 bisulphide of carbon. The light at the focus instantly vanishes when 

 the dark solution is introduced. But the solution is intensely trans- 

 parent to the dark waves, and a focus of such waves remains in the 

 air of the room after the light has been abolished. You may feel the 

 heat of these waves with your hand ; you may let them fall upon a 

 thermometer, and thus prove their presence ; or, best of all, you may 

 cause them to produce a current of electricity, which deflects a large 

 magnetic needle. The magnitude of the deflection is a measure of the 

 heat. 



Our object now is, by the use of a more powerful lamp, and a bet- 

 ter mirror (one silvered in front and with a shorter focal distance), to 

 intensify the action here rendered so sensible. As before, the focus is 

 rendered strikingly visible by the intense illumination of the dust-par- 

 ticles. We will first filter the beam so as to intercept its dark waves, 

 and then permit the purely luminous waves to exert their utmost 

 power on a small bundle of gun-cotton placed at the focus. 



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 incompetent 

 to explode the cotton, while the waves of the full beam are competent 

 to do so ; hence we may conclude that the dark waves are the real 

 agents in the explosion. 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. Hence it is the dark waves, and they 

 only, that are concerned in the ignition of the cotton. 



At the same dark focus sheets of platinum are raised to vivid 

 redness ; zinc is burnt up ; paper instantly blazes ; magnesium wire is 

 ignited; charcoal within a receiver containing oxygen is set burning; 

 a diamond similarly placed is caused to glow like a star, being after- 

 ward gradually dissipated. And all this while the air at the focus 

 remains as cool as in any other part of the room. 



To obtain the light-waves we employ a clear solution of alum in 

 water; to obtain the dark-waves we employ the solution of iodine 

 above referred to. But, as before stated, the alum is not so perfect a 

 filter as the iodine; for it transmits a portion of the obscure heat. 



