1883.] on Thoughts on Badiation, Theoretical and Practical. 257 



You know tliat the theory of gases and vapours, 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 raj)idity. 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 pro- 

 bability vastly further beyond the range of the microscope than 

 the range 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 vapour, the yellow 

 constituent of the white beam is stopped by the yellow flame, and if 

 the beam be subsequently analysed by a prism, a black band is found 

 in the place of the intercepted yellow band of the spectrum. We 

 have been led, as you know, 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 vapour synchronize in their vibrations with 

 the particular waves of ether which produce the sensation of yellow 

 light. The vapour, 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. I will now show you the 

 action of sodium vapour, in a way and with a result which startled 

 and perplexed me on first making the experiment, more than twenty 

 years ago. You know that the spectra of incandescent metallic 

 vapours are not continuous, but formed of brilliant bands. I wished, 

 in 1861, to obtain the brilliant yellow band produced by incandescent 

 sodium vapour. To this end, I placed a bit of sodium in a carbon 

 crucible, and volatilized it by a 2:)owerful voltaic current. A feeble 

 spectrum overspread the screen, from which I thought the sodium 

 band would stand out with dominant brilliancy. To my surprise, at 

 the very point where I expected this brilliant band to appear, a band 

 of darkness took its place. By humouring the voltaic arc a little, the 

 darkness vanished, and in the end I obtained the bright band which 

 I had sought at the beginning. On reflection the cause was manifest 



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