considered as the real sources of the ether-waves? As long as we were confined to the experiments of Leslie, Rumford, and Melloni, it was difficult to answer this question. But, when it was discovered that gases and vapors possessed—in some cases to an astonishing extent—the power both of absorbing and radiating heat, a new light was thrown upon the question.
You know that the theory of gases and vapors, now generally accepted, is that they consist of molecular or atomic projectiles darting to and fro, clashing and recoiling—endowed, in short, with a motion not of vibration, but of translation. When two molecules clash, or when a single molecule strikes against its boundary, the first effect is to deform the molecule, by moving its atoms out of their places. But gifted as they are with enormous resiliency, the atoms immediately recover their positions, and continue to quiver in consequence of the shock. Held tightly by the force of affinity, they resemble a string stretched to almost infinite tension, and therefore capable of generating tremors of almost infinite rapidity. What we call the heat of a gas is made up of these two motions—the flight of the molecules through space, and the quivering of their constituent atoms. Thus does the eye of Science pierce to what Newton called "the more secret and noble works of Nature," and make us at home amid the mysteries of a world lying in all probability vastly farther beyond the range of the microscope than the power of the microscope, at its maximum, lies beyond that of the unaided eye.
The great principle of radiation, which affirms that all bodies absorb the same rays that they emit, is now a familiar one. When, for example, a beam of white light is sent through a yellow sodium-flame, produced by a copious supply of sodium-vapor, the yellow constituent of the white beam is stopped by the yellow flame, and, if the beam be subsequently analyzed by a prism, a black band is found in the place of the intercepted yellow band of the spectrum. We have been led to our present theoretic knowledge of light by a close study of the phenomena of sound, which in the present instance will help us to a conception of the action of the sodium-flame. The atoms of sodium vapor synchronize in their vibrations with the particular waves of ether which produce the sensation of yellow light. The vapor, therefore, can take up or absorb the motion of those waves, as a stretched piano string takes up or absorbs the pulses of a voice pitched to the note of the string. This action of sodium-vapor may be shown by an experiment which startled and perplexed me on first making it, more than twenty years ago. The spectra of incandescent metallic vapors are, as you know, not continuous, but formed of brilliant bands. Wishing, in 1861, to obtain the brilliant yellow band produced by incandescent sodium-vapor, I placed a bit of sodium in a carbon crucible, and volatilized it by a powerful voltaic current. A feeble spectrum overspread the screen, from which it was thought the sodium band would stand