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 consequence of the concentration of 
Fig. 3. the caloric rays, the most varied inflammable bodies may be ignited at this luminous point. 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. We place a test-tube filled with water within the focus; it begins at once to burst into bubbles, and commences 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, between 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 emitted 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.
