586 Prof. J. J. Thomson on the Emission of 



mined by a reading microscope passing through the side of 

 the box ; after a reading had been taken, shutters were put 

 before the window and the eyepiece of the microscope covered 

 with a cap. The readings only took a second or two, but even 

 in that time the red light produced an appreciable leak in the 

 electroscope ; this part of the leak can, however, easily be 

 separated from that taking place in the dark as the latter is 

 proportional to the interval between two readings, while the 

 former is independent of this interval. 



To test the efficiency of the means taken to exclude the 

 light, some of the experiments on the rate of leak were made 

 in a photographic dark room, others with the case exposed to 

 the light of the laboratory ; the leak was the same in the two 

 cases. Another proof of the exclusion of light is that a 

 sensitive photographic plate placed in the case for 48 hours 

 was not fogged. 



When the leaves of the electroscope were charged with 

 positive electricity there was, even in the dark always, a 

 small leak of electricity from the leaves, while there was 

 no perceptible leak when the leaves were negatively charged. 

 The positive leak was entirely stopped by a transverse 

 magnetic field : this proves that it is due to negative cor- 

 puscles emitted by the rubidium or the alloy of Na and K. 



On some occasions the positive leak was abnormally large. 

 This was traced to the presence in the tube containing the 

 electroscope of minute quantities of hydrogen ; it was lound 

 that this gas had an extraordinary influence on the emission 

 of corpuscles from electropositive metals. To investigate 

 this more fully, an arrangement was added to the glass vessel 

 containing the electroscope, by which bubbles of hydrogen 

 could be admitted from time to time into the vessel. The 

 admission of a very small quantity of hydrogen produced 

 temporarily a very large increase in the rate at which 

 electricity escaped from the positively charged leaves of 

 the electroscope, the rate of leak after the admission of the 

 hydrogen being often ten times its previous value. The 

 increase in the leak rapidly died away, and after about 

 20 minutes the leak resumed its original value ; the admission 

 of a fresh supply of hydrogen, however, sent it up again. 

 The admission of small quantities of air or carbonic acid did 

 not produce any appreciable increase in the leak. It would 

 appear that while these electropositive metals are absorbing 

 hydrogen, the rate of emission of negatively electrified 

 particles is greatly increased. 



The influence of hydrogen on the emission of corpuscles 



