num 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 many 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 elimination, 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-sensitiveness.
B. On the change of sign of response in a receiver due to a variation of radiation intensity.
C. On the physico-chemical changes produced in a sensitive substance by the action of electric radiation, and on the radiation-product.
D. The phenomena of electric reversal and of radio-molecular oscillation.
E. On "fatigue" and the action of mechanical tapping and other disturbances by which the sensitiveness of a fatigued receiver may be restored.
