PHILOSOPHICAL SOCIETY OF WASHINGTON. 145 



Smallness of Atoms. — The extreme minuteness of the atoms is evi- 

 denced not alone by the necessary limitations of their orbital excur- 

 sions under ordinary conditions, and by their inconceivable rapidity 

 of oscillation, but even still more strikingly by the vast number of 

 molecules which may be chemically combined and compacted within 

 the volume of an elementary molecule, — still observing the law of 

 Avogadro. 



From such considerations we may infer that the dimensions of 

 the ultimate atoms are probably as much below that of the com- 

 posite molecule, as this is beneath a visible magnitude : or in other 

 words, that were the molecule an object to be seen, the highest 

 power of our best microscopes would utterly fail to detect its con- 

 stituent atoms. 



The Constancy of the Atomic Periods. — "We have learned from the 

 fixity of the spectral lines (whether luminous or dark) that what 

 may be called the tones or pitches of these resonant particles are 

 very accurately maintained through an enormous range of ampli- 

 tude ; that is, that the respective periods of the atomic orbits (in- 

 finitesimally brief as they appear to our slow-moving thoughts) are 

 quite unaffected by their radii, or their rates of velocity. The evi- 

 dence of these uniformities of period in descending temperatures 

 is found in the stability of gaseous absorption lines under all de- 

 grees of cold producible ; these lines remaining dark when taking 

 up the motion of the incandescent back-ground, simply because the 

 amplitude of the oscillation is not sufficient on the whole to impress 

 our sense of vision. And although at very high temperatures both 

 the number and the distinctness of the spectral lines may be con- 

 siderably affected, their position (as long as visible) is not at all 

 disturbed. That new lines should appear at increasing tempera- 

 tures is not surprising, since in every case a certain width of atomic 

 play is required to affect the eye. But that under such circum- 

 stances pre-existing lines should disappear, — as has been established 

 by the researches of Dr. J. Pliicker and Dr. J. W. Hittorf,* — so 



the American Institute, Jan. 27, 1871.) It is instructive to reflect that a 

 wave line of this order (representing an astherial undulation) — executed by 

 the most skillful draftsman or engraver, on any scale whatever, or with 

 any microscopic appliances, could not be distinguished by any process of 

 direct instrumental measurement or verification from a perfectly straight 

 line. 



*Phil. Trans. Roy. Soc. Memoir read March 3, 1864: vol. clv, pp. 1-29. 



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