206 Professor J. J. Thomson on Electric Discharge. [Jan. ID, 



constant cuiTent tliroui!:h the discharge tube. We can calculate what 

 the electric force must be to produce from the molecules of A alone 

 the number of ions required to cany this current ; having found the 

 electric force on this supposition, we can, knowing the current, tind 

 the rate at which the electric forces would be doing work in the tube. 

 If this rate of work is less than that required to make B luminous, 

 the cuiTent will be carried by the ions of A alone, and the spectrum 

 of B will not be developed ; if the rate of work on this supposition 

 is greater than that required to make B luminous, the spectrum of B 

 will appear, and it must take a share in carrying the current. Let us 

 suppose that we have so much of A present that the rate of work is 

 not sufficient to develop the spectrum of B, and consider what will 

 happen as the proportion of A is diminished. In order to supply the 

 number of ions required to carry the given cun-ent from the smaller 

 number of molecules of A, the electric force, and therefore the rate 

 of work in the tube, must, on the supposition that the cuiTent is 

 wholly carried by A, increase, and if we continually diminish the 

 amount of A present, the rate of work will at last reach a value 

 sufficient to make B luminous, with the given cun'ent. This stage 

 will give the smallest quantity of A which can, for the given cm-rent, 

 wholly swamp the spectrum of B. The rate of work done in the 

 tube will depend on the current going through it, and also on the 

 pressure of the gases, so that both these quantities will influence the 

 proportion of the gas B required to make its spectrum visible. 



[J. J. T.] 



