82 ON THE STRUCTURE OF ATOMS. 



own determinate length of wave, though all colors travel with 

 the same speed, and together form white light. In every inch 

 of the progress of light there would be 38,000 waves of red and 

 59,000 waves of violet, and intermediate numbers for every 

 shade of the other and intermediate colors. 



Now in the passage of white light through the gases or vapors 

 of the simple elements, as is the case with the light from the sun, 

 certain colors are absorbed or held back, and the spectroscope, 

 which simply spreads light out into its component colors, shows 

 just what colors each elemental gas retains, by the dark bands 

 which cross the spectrum. This is an infallible test by which 

 any gaseous element may be known whenever interposed before 

 any luminous substance. Some of the elemental gases cut out 

 but few bands of colors, others a great many. Hydrogen for in- 

 stance has but four dark bands, while in the spectrum of the 

 vapor of iron there have been counted something like a thousand 

 bands. Now each of these numerous bands indicates that some 

 infinitesimal portion of the atom of iron has the power or the 

 freedom to vibrate exactly in unison with the rays thus absorbed, 

 and that these particular vibrations of the light are communicated 

 to parts of the iron atom instead of passing on their way unob- 

 structed. But it is impossible to suppose that a single solid 

 atom has the power to, and does thus actually vibrate in a thou- 

 sand different periods at the same time. This power can only be 

 conceived of as belonging to a thousand or more sub-atoms or 

 portions of which the atom of iron may be composed. 



But if the elemental gases are in a state of incandescence, 

 that is are themselves emitting the light, as is shown in the vola- 

 tilization of substances in the electric arc-light, then they send 

 forth only the same rays which by the former supposition they 

 absorbed. That is, four bright lines would be all the light we 

 should get from incandescent hydrogen, while something like a 

 thousand very fine bright lines would represent the light from 

 glowing iron vapor. In this case the thousand portions of the 

 iron atom which have the power to vibrate in the same periods 

 with certain light-waves, are the only ones that when set in motion 

 by high heat can communicate motions to the ether which come 



