202 Prof*. Millikan and Mr. Winchester on Influence of 



although the time required to attain this position was some- 

 times varied as much as five or six fold. This result, taken 

 in connexion with the independence both of the discharge- 

 rate and of the " velocity of projection upon temperature, 

 seems to establish quite conclusively Lenard's view as to the 

 mechanism of the emission of the electrons. 



8. The Order in ivhich the Metals exhibit Photo-elect ric 

 Effects. — The order in which the metals exhibit the power of 

 discharging negative electricity is commonly supposed to be 

 the same as the order in which they are found in the Volta con- 

 tact series * (an order which is in general the same as that of 

 decreasing solution pressures), the more strongly electro- 

 positive metals showing the larger photo-electric effects. 



This conclusion has been based chiefly upon Elster and 

 Geitel's work upon the highly electro-positive metals which 

 they arrange in the following order with respect to decreasing 

 photo-electric sensitiveness : — Rubidium, potassium, sodium, 

 lithium, magnesium, thallium, zincf. 



So far as we are able to discover, until very recently 

 no one has attempted to arrange the commoner metals in 

 the order of decreasing photo-electric sensitiveness, but k 

 veryirecent paper by Ramsay and Spencer records such an 

 attempt. These investigators conclude that, " considered 

 generally, the rates of discharge of all elements are in the 

 same order as their electro-potentials "£. They note, how- 

 ever, that there are five or six notable exceptions to the rule. 

 Their experiments, like those of all their predecessors, with 

 one exception, who have worked upon the order of the 

 metals with reference to this phenomenon, were conducted in 

 air, and give therefore, in our opinion, no indication what- 

 ever as to the effect of the ultra-violet light upon the metals 

 themselves ; for it has been repeatedly shown that the amount 

 of discharge from a given metal under the influence of a 

 given source depends upon the nature of the surrounding 



3 as §- 



The results recorded in Table II. show that, in a vacuum, the 



relative rates of discharge hear no relationship ivJiatever to the 

 Volta contact series. In general it is the more highly electro- 

 positive metals, that is those having high solution pressures, 



* See Stark's Die Elect ric it at in ~Gasen, p. 110 (1902) ; also J. J. 

 Thomson's ' Conduction of Electricity through Gases/ p. 12 (1903) ; also 

 Bicliat and Swvngedauw's report at the " Congres International de 

 Physique/' vol. iii. p. 168 (1900). 



t Elster & Geitel, Wied. Ann. xliii. p. 225 (1891). 



i Ramsay & Spencer, Phil. Mag. xii. p. 407 (1906). 



§ Wulf, Ann. d. Phys. ix. p. 951 (1902) ; also Varley, Phil. Trans, ccii. 

 p. 439 (1904). * 



