1871J Molecules, Ultimates, Atoms, and Waves. 173 



the surface — that vibrate with sufficient energy to generate 

 luminous vibrations in the ether. They are driven to a 

 comparatively considerable distance from their points of 

 rest, and the force by which they are dragged back again 

 being comparatively weak, they take a longer time to perform 

 their excursion. They accordingly generate only red waves 

 in the ether. The light is feeble, and the spectrum it ex- 

 hibits is chiefly confined to the region of the fixed lines A 

 and b. As the motive energy increases, those particles 

 situated a little farther in from the surface begin to vibrate 

 at their proper rates. Their excursions are performed in a 

 more limited space. The force by which they are dragged 

 back to their points of rest is greater, consequently their 

 periods of vibration are quicker. These give rise to orange 

 and yellow waves in the ether, and the spectrum gradually 

 extends towards the line d. A still greater energy sets 

 more of the particles into violent motion. These perform 

 still quicker vibrations, in virtue of their greater adventitious 

 inertia. At last the light becomes quite white, and the 

 spectrum extends continuously from the extreme red to the 

 extreme violet. The particles of the substance are then 

 vibrating at every rate of rapidity embraced within the limits 

 of the visible spectrum, and also at various other rates 

 both slower and quicker than these. This variety of rate 

 cannot be due to diversities in the intrinsic inertia of the 

 particles themselves, and can be explained only by differences 

 in their adventitious inertia, arising from their position in 

 the mass and the various degrees in which they are influenced 

 by the force of cohesion. 



When a substance is thrown into vapour, the operation of the 

 cohesive force is suspended, and the molecules or ultimates 

 are held together merely by the force of gravity. They are 

 free to perform larger excursions ; consequently, while the 

 amplitude of their vibrations is increased, their rapidity is 

 so much lowered that they cease to propagate through the 

 ether vibrations of those rates which fall within the limits 

 of the visible spectrum. It is not till the applied motive 

 energy becomes greatly augmented that a vapour begins to 

 propagate luminous vibrations, and these are then found 

 to be no longer capable of exhibiting a continuous spectrum, 

 but only definite bright lines — showing that there is an 

 entire alteration in the mode of their production. The same 

 remarks apply to permanent gases. 



In some few instances incandescent elemental vapours ex- 

 hibit, in addition to their characteristic bright lines, a diffuse, 

 weak, more or less continuous spectrum. The most probable 



