460 THE POPULAR SCIENCE MONTHLY. 



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 dart- 

 ing to and fro, clashing and recoiling endowed, in short, with a mo- 

 tion 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 immedi- 

 ately 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 ab- 

 sorb 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 experi- 

 ment 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 vola- 

 tilized it by a powerful voltaic current. A feeble spectrum overspread 

 the screen, from which it was thought the sodium band would stand 



