THEORETICAL. 3^5 



ence of potential between the terminals in the carbon arc of the spark 

 gap in the secondary on an induction coil, or of the electrodes of a 

 vacuum-tube. Under the influence of an electric field these dissociated 

 particles are set in motion. Beside their ordinary gas motion, there 

 will be a component along the length of the tube (in a vacuum-tube) 

 produced by the electrical forces. Although the charges are the same, 

 the negative ions, the electrons, on account of their small mass will 

 acquire a great speed, and by collision with neutral molecules will give 

 rise to fresh ions. The energy acquired, however, wnll be the same for 

 both, since they move through the same difference of potential. The 

 mean free path of the electron will be the greater since it is smaller in 

 size. The mean energ>' at collision, in any case, will be that acquired 

 bv the charged particle in moving through its path under the action of 

 electrical forces. When the electron collides with a molecule several 

 effects will be produced. The first is to increase the kinetic energy of 

 the molecule as a whole, which in turn, by colliding with other mole- 

 cules, will cause a rise in the thermal temperature of the gas. As a 

 result the electrons in the molecules wall be thrown out of their posi- 

 tions of equilibrium, and will execute a series of vibrations. In so 

 doing they will emit radiation in the form of heat or light, depend- 

 ing upon the intensity of the excitation, which in turn depends upon 

 the temperature. Since the mean temperature of the molecules is only 

 about 300° abs. (Warburg, loc. cit.), it will be well to discuss presently 

 the 4.75 fi COo band in this connection. 



When an electron collides with a molecule the second efifect is an 

 acceleration of the former, which, when properly interpreted, means a 

 wave motion, hence periodicity (Stark, loc. cit.). This period will be 

 determined by the time of impact, independently of the chemical nature 

 of the body. Since all possible times of impact are possible, an infinite 

 number of dififerent electro-magnetic waves will be emitted, and we 

 have a continuous spectrum. This is for the free electrons. The elec- 

 trons that remain bound in the molecule will also be set into vibration 

 by the impact of the collision. Since they are thus bound, there will 

 be a relative motion among them, and the period of any one or group of 

 these electrons will be characteristic of the kind of atom. The line and 

 band spectra of the elements are, from this standpoint, due to these 

 electrons within the atom.' In a more recent paper on this subject by 

 Nutting^ the recombination of these dissociated aggregates is empha- 

 sized as being the source of line spectra. 



1 Stark : Ann. der Physik (4), 14, p. 506, 1904. 

 ^Nutting: Astrophys. Jour., 21, p. 400, 1905. 



