MODERN THEORIES OF ELECTRICITY AND MATTER. 109 



the influence of certain exterior conditions, as, for example, the 

 Koentgen rays, for in such conditions a gas becomes conducting. A 

 charged electroscope in connection with a metallic plate in ordinary 

 circumstances loses its charge but slowly. If, however, a stream of 

 Roentgen rays penetrates the air around the plate, the discharge 

 proceeds raj^idly. It is not necessar}^ for the Roentgen rays actually 

 to strike the plate, but suffices that the air be traversed within a dis- 

 tance where the electric field is still sensible. This is shown by con- 

 straining the Roentgen rays to follow a tube impenetrable to them, 

 and thus shielding the plate from their path, so that it is certainly the 

 gas which is modified and rendered conducting. We say that the 

 gas is ionized, some of its molecules having been decomposed by the 

 rays, and that each of these has given rise to the formation of two 

 ions laden with equal electric charges having opposite signs. The 

 ions are put in motion under the influence of the electric field with a 

 velocity which increases with the strength of the field. If the elec- 

 troscope is cliarged positively, the negative ions are drawn toward 

 and discharge it, while the positive ions go in the opposite direction 

 and neutralize the charge found at the extremities of the lines of force 

 which emanate from the plate. 



If the gas which has been under the influence of the rays is left 

 to itself without the action of any electric field to move the ions, its 

 conductivity disappears spontaneously, and we say that the ions 

 have recombined to form neutral molecules. 



There appear to be in the gas movable charged centers, which travel 

 toward the plate of the electroscope. These centers may be inter- 

 cepted by means of a screen of paraffin. The screen should not 

 itself be charged, as may be tested by means of a second electroscope. 

 The positively charged plate of the first electroscope may noAV be 

 covered with the screen, and the Roentgen rays then allowed to act 

 for a time. Negative ions moving toward the charged plate are 

 arrested by the paraffin, and they charge the screen negativel3^ 

 This ma}" be verified by again bringing the paraffin screen near the 

 second electroscope. 



It may be shown that under the action of the Roentgen rays the 

 number of ions produced in a gas in a given time is definitely limited. 



The rate of discharge of the electroscope is measured by the rate of 

 fall of the gold leaves; and it increases with the electric intensity of 

 the charge, as may be easily understood. Therefore, the stronger the 

 electric field and the greater the velocity the less is the chance that the 

 opposite ions draw together. But for a charge sufficiently great, the 

 rate of the discharge no longer depends on the amount of the charge 

 and does not increase as it augments. Under these circumstances there 

 are no longer any recombinations of ions; the}'' are all utilized for 

 conducting the current, which can not exceed what they can carry. 



