454 Prof. J. C. Bose. On Electric Touch and tlu: Molecular 



on the electro-positive character of the metal; but potassium, one of 

 the most electro-positive metals, exhibits an unusual increase of resist- 

 ance, a property which it will be shown in a future paper it shares 

 with some of the most electro-negative elements. 



There is one property, however, which at first would seem to be in 

 some way related to the sensitiveness of metals — the volatility of 

 metals under the cathodic stimulus, investigated by Sir William 

 Crookes,* who gives the following list of metals, arranged according to 

 their volatility : — 



Palladium 108 



Gold 100 



Silver 80-68 



Lead 75-04 



Tin 56-96 



Brass 51-58 



Platinum 44 



Copper 40 



Cadmium 31*99 



Nickel 10-99 



Indium 10-49 



I Iron 5*5 



j Magnesium 1 very 



j Aluminium j slight 



In this list the substances which are most volatile, e.g., Pd, Au, Ag, 

 are not very sensitive, whereas Fe, Al, Mg, which are least volatile, 

 are strongly sensitive. But the above series does not exactly coincide 

 with the series of electric sensitiveness. Again, the volatility of 

 platinum is retarded in hydrogen gas, but an experiment carried out 

 to determine the sensitiveness of platinum in hydrogen failed to show 

 any great increase in the sensitiveness. 



None of the above suppositions give any satisfactory explanation of 

 the numerous anomalies in the contact-sensitiveness of metals. It 

 then appeared that the observed effect is not due to a single cause but 

 to many causes. An observer studying the dilatation of a gas under 

 reduced pressure, and ignorant of the effect of temperature, will 

 doubtless encounter man}^ anomalies. In the phenomena of contact- 

 sensitiveness the . variables are, however, far more numerous, and the 

 different possible combinations are practically unlimited. It therefore 

 became necessary, by a long and tedious process of successive elimina- 

 tion, to find out the causes which are instrumental in producing the 

 observed effect ; the results obtained throw some light on this intricate 

 subject. The following are some of the principal directions in which a 

 systematic inquiry was carried out : — 



A. On the difference between mass action and molecular or atomic 



action, with reference to the phenomenon of contact-sensi- 

 tiveness. 



B. On the change of sign of response in a receiver due to a varia- 



tion of the radiation intensity. 



Crookes, ' Roj. Soc. Proc.,' vol. 50. 



