1901.] Effect of Light and Electric Radiation on Matter. 159 



But very much quicker or slower rates may easily be obtained by 

 means of the different-sized pulleys. The spot of light from the 

 galvanometer is thrown down on the paper by an inclined mirror. 

 The movement of the galvanometer spot takes place at right angles to 

 the direction of motion of the paper. There is a guide rod at right 

 angles to the motion of paper, along which the recording pencil is 

 moved. The excursion of the galvanometer spot can thus easily be 



Fig. 1. — The Eecording Apparatus. P is the platform moving on rails and carrying 

 the squared recording paper. B is the guide bar. C, the clockwork. P, the 

 pulley. M, the mirror to reflect the galvanometer on the platform. 



followed with a pencil, and it is quite easy to do this, when the 

 fluctuation period is more than 2 seconds. In the experiments to be 

 described, this period varied from 2 seconds to several minutes. A 

 curve is thus directly obtained, with conductivity or electromotive 

 variation as ordinate, and the time as abscissa. The curves given 

 in the accompanying papers are exact copies from the direct records. 



4. Transition of a Molecular Receiver from Non-recovering to Self-recovering 



Condition. 



When a substance is strained by radiation there is produced a 

 sudden variation of conductivity. The substance will recover from 

 the strained condition (1) if it has not been overstrained by an exces- 

 sive stimulus, or (2) if its electric elasticity be very great. I have 

 found in general that on careful adjustment of a receiver there is a 

 tendency to self-recovery if the intensity of incident radiation is not 

 too strong. In the case of substances which are, electrically speaking, 

 highly elastic, such as K, there is an automatic recovery even when 



M 2 



