1871-] Molecules, Ultimates, Atoms, and Waves. 175 



bright spectral lines are very numerous, part of the effect 

 may be due to the various degrees in which the atoms com- 

 posing the ultimate are influenced by the atomic attraction, 

 instead of its being wholly due to differences in the intrinsic 

 inertia of the atoms. The diverse rates of vibration may 

 be partly owing to mere differences in the position of the 

 atoms within the limits of the ultimate, as in the case of 

 incandescent solids and liquids. Among atoms having the 

 same intrinsic inertia, those at and near the surface of the 

 ultimate will have slower rates than those in the interior. 

 Indeed, each ultimate may be regarded as a minute incan- 

 descent mass, whose atoms are vibrating at different rates — 

 only the rates are much less various than in the case of 

 solids and liquids; so that only vibrations of certain definite 

 rates are communicated by the atoms to the ether — giving 

 rise to the definite spectral lines. Doubtless, however, 

 where these lines are few in number, differences in the in- 

 trinsic inertia of the atoms themselves are probably more 

 efficient in causing diversities in the rates of vibration, than 

 mere differences in the position of the atoms relatively to 

 each other, within the limits of the ultimate which they 

 unite to constitute. 



When the elementary gases or vapours act as absorbents 

 on white light passing through them, they produce in the 

 continuous spectrum of the transmitted light dark lines, 

 exactly corresponding in position to the bright lines which 

 they themselves generate. This effect cannot be explained 

 otherwise than by the transference of the vibratory motion 

 passing through the ether to the particles of the gas or 

 vapour. But were it transferred to the chemical ultimates 

 in their integrity, they being all of the same weight in the 

 same element, could take up only the vibrations performed 

 at one single rate or some octave of that rate. The cir- 

 cumstance, however, that they absorb definite lines in di- 

 verse and distinct parts of the spectrum, shows that the 

 motion must be taken up by different atoms having in 

 virtue of their intrinsic inertia a tendency to vibrate at 

 those different definite rates. For it seems highly improbable 

 that one and the same individual atom, or that atoms all 

 exactly alike in their intrinsic inertia, could select and 

 assume different definite rates of vibration from among the 

 vast number of rates presented in the continuous spectrum. 



The appearance of the lines, whether bright or dark, is 

 much influenced by the temperature and pressure to which 

 the gas or vapour is subjected. Increase of temperature 

 augments the brightness or blackness of the lines, and in 



