I'L' I'n.f. J. J. Thomson. On the Propagation of (lie [Jan. 1 ">, 



]>oiiit of view, the shortening of tubes of electrostatic induct ion) 

 is effected by the dissociation of the molecules into atoms, in other 

 words, that " chemical decomposition is not to be considered as an 

 accidental attendant on the electrical discharge, but as an essential 

 feature of the discharge, without which it could not occur " ('Phil. 

 Mag.,' vol. 15, 1883, p. 432). Free atoms must, on this view, exist 

 in the path of the discharge to serve as the ends of the tubes of force 

 as they shorten. If, however, we take this view of the discharge of 

 electricity, the chemical decomposition attendant on the discharge 

 along the positive column cannot consist of the consecutive inter- 

 change of atoms between adjacent molecules, for, since on this view 

 each atom would have to move up to the one in the adjacent mole- 

 cule, the velocity of the atoms would have to be that of the discharge 

 of the positive column, viz., about half that of light. The existence 

 < f a wind in the tube of this velocity is, a priori, unlikely, and the 

 following calculation will show that it would require the expenditure 

 of more energy than we have at our disposal. 



Let us take the case of the discharge of a parallel plate condenser, 

 the distance between the plates being 1 cm. Let F be the electro- 

 motive intensity between the plates, K the specific inductive capacity 

 of the gas ; then the energy per square centimetre of area of the con- 

 denser plate is 



Let N be the number of atoms required to discharge unit area of 

 the condenser ; then, if a is the density of the electricity on the con- 

 denser and 6 the charge on each atom in electro-magnetic measure, 



N = a. 



If m is the mass of one of these atoms, r the velocity with which 

 the atoms move, their kinetic energy is 



If ]S" is the number of atoms in one gramme of the substance, then, 

 if the charges on the atoms are the same as that deduced from electro- 

 lytic considerations, 



Ne = 10* and Nw = 1. 

 Now 4 = KF. 



Making these substitutions, we find that the kinetic energy of the 

 atoms is 



1 KFpa 

 8 10* ' 



