the Nature of Spectra. 79 



of the higher harmonic vibrations, to which the more-refran- 

 gible rays correspond, increase with the augmentation of the 

 amplitude ; and in fact Lecoq de Boisbaudran finds that, in 

 the nitrogen-spectrum, at a higher temperature the blue lines, 

 corresponding to the double octave of certain vibrations, come 

 out, at a lower the red and yellow, as a fifth of the same vibra- 

 tions. Similarly, lithium chloride in the flame of the Bunsen 

 burner shows a very bright red and a very faint orange line ; 

 before the blowpipe the latter increases in brightness much 

 more than the former, though without becoming equal to it ; 

 if, however, the induction-spark strikes npon a solution of 

 lithium chloride, the orange line is much brighter than the 

 red, and the blue lithium-line becomes very bright. Many 

 other instances might be mentioned. 



By mere elevation of temperature, also, as F. Lippich and 

 afterwards Pfaundler have argued on theoretical grounds, a 

 widening of the lines may also be produced by the molecules 

 of the gas having each a high velocity, partly directed towards 

 the observer, partly away from him. In far higher measure, 

 however, do such widenings of the lines take place when we 

 increase the pressure of the gas ; the individual vibrating 

 aether envelopes of an atom can then only for a brief period 

 carry on their motions undisturbed, since they are mostly 

 within the compass of the sphere of action* of the other atoms. 

 This conclusion is confirmed by the experiments of Wiillner, 

 G. Camician, and others. 



Interferences accompanying great Differences of Progress. — 

 The time during which an optic motion of the aether of the 

 individual backward -and-f or ward-rushing molecules and atoms 

 takes place undisturbed I have endeavoured to ascertain by the 

 following considerations : — 



If two rays of light are to interfere, they must start from 

 the same point ; and during the time which elapses between 

 the instants when the first and the second ray are emitted no 



* The diameter of this action-sphere is the length to which, reckoned 

 from the centre of the atom or molecule, the aether possesses a state dif- 

 fering from that which it possesses in free space ; as the temperature rises 

 the length will increase at those molecules which are composed of a plu- 

 rality of atoms. On the other hand, the so-called molecular diameter 

 denotes, in the kinetic theory of gases, the distance up to which the centres 

 of two molecules can approach each other when they strike one another — 

 a distance which essentially varies with the force of the collision, and 

 hence may diminish with rise of temperature, as the experiments on the 

 friction of gases show. The diameter of the sphere of action will in every 

 case be greater than this molecular diameter, since the particles pushing 

 against one another must first pass through a portion of the action- sphere 

 before their motion reverses its direction. 



H2 



