CHAPTER IX. 



PROPERTIES OF CORPUSCLES: CATHODE RAYS. 



In the absence of any of the ionizing agencies we have 

 so far considered, the very high resistance of the air can 

 be overcome only by the application of intense electrical 

 forces. Instead of an invisible leakage we then obtain a 

 sudden spark, as one sees any day in the discharge of an 

 electrical machine or in a lightning flash. But this spark 

 discharge is due to the fact that the air or gas is at the 

 normal pressure of the atmosphere, about 15 Ibs. "to* the 

 square inch. Just so soon as we begin to decrease this 

 pressure the phenomenon begins to change; and this 

 change goes on progressively until its character becomes 

 absorbingly interesting. 



The less the pressure the easier it is to form these little 

 negatively electrified particles under the influence of elec- 

 tric forces. If an electric machine or induction coil be 

 connected with metallic terminals fused into a glass bulb, 

 so arranged in connection with an air pump that the air 

 can be gradually withdrawn, one of the most beautiful 

 experiments in physical science makes evident the maxi- 

 mum properties of corpuscles. When the air has been 

 withdrawn until it has a pressure of about one ten-thou- 

 sandth of an atmosphere, the first effect noticeable in the 



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