produced by Collisions of Atoms or\ Molecules. 323 



the " ionization " could not be anything else than dissociation 

 o£ electrions from ponderable atoms. This kind of disso- 

 ciation might be produced in a very hot gas by mere impacts 

 between the atoms of the gas itself, with the large translational 

 velocities to which high temperatures are due. Or it might 

 be produced by extraneous bodies, such as the " a. " or " f$ " 

 particles shot out with high velocities from radioactive 

 substances. We are now however chiefly concerned with 

 the motions of ether produced by collisions of atoms, in 

 circumstances less abnormal than those in which dissociations 

 and recombinations are largely influential. 



§ 21. The pulses described in §§ 11, 12, as due merely to 

 mutual collisions between ponderable atoms (without con- 

 sideration of electrions whether present or not), constitute a 

 kind of motion in the ether, which, if intense enough to pro- 

 duce visible light, would, when analysed by the spectroscope, 

 show a continuous spectrum without the bright lines, which, 

 when seen, prove the existence of long-continued trains o£ 

 sinusoidal vibrations of particles of ether, in the eye perceiving 

 them, and therefore also in the source, and in all the ether 

 between the source and the eye. On the other hand, the 

 vibrations of electrions referred to in § 13 would, if intense 

 enough, produce bright lines in the spectrum. 



§ 22. There is another kind of vibration in the source, 

 which might produce, and which probably does produce, bright 

 lines in the spectrum. If there are two or more ponderable 

 atoms in the molecule of a glowing gas, not dissociated by the 

 violence of the collisions, each atom of the molecule must have 

 a vibratory motion, of which an isolated ponderable atom is 

 incapable ; and these vibratory motions of the atoms of a group 

 must give rise to blight lines in the spectrum, when the 

 frequency of the vibrations in any one, or in all, of the 

 vibrating modes, is between four hundred million million and 

 eight hundred million million per second, if we take this as 

 the range of frequencies for visible light. 



§ 23. The spectroscopic phenomena to be accounted for in 

 a dynamical theory of light include continuous spectrums, with 

 large numbers of bright lines superimposed on the more or 

 less bright background of continuous spectrum. Even when 

 every care has been taken, in artificial sources of light, to 

 eliminate influence of more than one of the substances com- 

 monly called chemical elements, the number of bright lines is 

 generally very large : indeed we are not sure that we have 

 been able to count the whole number of those which are 

 presumably due to any single element. 1 



§ 24. In a glowing monatomic gas, with just one electrion 



