>B 



Fig. 3. 



LENGTHENING THE VISIBLE SPECTRUM. 93 



alone the luminous, but also the thermal rays, are united at this point. 

 We become convinced of this fact by holding a cigar at the focus : it 

 is at once ignited, begins to smoke, and bursts into flame. In conse- 

 quence of the concentration of ^ C 

 the caloric rays, the most varied 

 inflammable bodies may be ig- 

 nited at this luminous point, /x^ 

 Paper is perforated and charred ^/ 

 in a moment, zinc consumes 

 with a bright violet flame. 

 Very thin, blackened platinum 

 is brought to a white heat, and 

 emits an intense white light. "VVe place a test-tube filled with water 

 within the focus ; it begins at once to burst into bubbles, and com- 

 mences to boil. Are these observed occurrences effected by the rays 

 of heat or of light, emanating from the incandescent carbon-points ? 

 We answer this question by placing a body in the course of the rays, 

 which, although it transmits the luminous rays, absorbs the thermal 

 ones. Such a one is a concentrated solution of alum in water. We 

 place a glass vessel, filled with this perfectly transparent solution, be- 

 tween the two reflectors, and in this manner sift the rays emanating 

 from the carbon-points. The luminous focus is still there, but the 

 ebullition of the water in the test-tube ceases at once. We remove the 

 vessel, and ebullition is resumed with violence. Those rays, therefore, 

 that caused the boiling were absorbed by the alum solution. This had 

 meanwhile been raised in temperature, and, if left sufficiently long, it 

 would begin to boil. We return the solution into the path of the rays, 

 and place white paper within the focus. It is illumined brightly, but 

 not consumed. We repeat the experiment with gun-cotton wrapped in 

 white paper. It might lie there for a hundred years without exploding. 

 We remove the vessel, and explosion occurs at once. We continue the 

 experiment with black paper, by bringing it into the focus of the rays 

 sifted through the solution, when it is at once perforated and ignited. 

 Gun-cotton wrapped in black paper explodes almost instantly. Why 

 is it that the same rays that left white paper intact at once ignite 

 black ? The luminous radiation transmitted by the solution is not 

 absorbed, but reflected, by the white paper. It is brightly illumined, 

 but not heated. Black paper, however, absorbs these rays, is heated 

 thereby, and ignites. 



The preceding experiments convince us that the combustion and 

 heating of bodies in the focus are solely caused by the dark rays emit- 

 ted by the carbon-points. We confirm this conviction by introducing 

 into the path of the rays a body transmitting the dark radiation with 

 the greatest facility, while completely absorbing the luminous one. 

 According to Tyndall's experiments, this condition is complied with 

 to a very high degree by a solution of iodine in carbonic disulphide. 



