Page:Popular Science Monthly Volume 1.djvu/216

206 the motes entirely absent, the beam would be unseen. It falls upon a concave mirror (a glass one silvered behind will answer), and is gathered 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 perfectly 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 transparent 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 better 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-particles. 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 afterward 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.