soon as conductivity was also found to be destroyed when an electric current was passed through the gas—a phenomenon for which a parallel may be found in electrolysis. For if the ions were removed from an electrolytic solution by the passage of a current. the solution would cease to conduct as sufficient electricity had passed to remove them all; and it may be supposed that the conducting agents which are produced in a gas by exposure to X-rays are likewise abstracted from it when they are employed to transport charges.
The same idea may be applied to explain another property of gases exposed to X-rays. The strength of the current through the gas depends both on the intensity of the radiation and also on the electromotive force; but if the former factor be constant, and the electromotive force be increased, the current does not increase indefinitely, but tends to attain a certain "saturation" value. The existence of this saturation value is evidently due to the inability of the electromotive force to do more than to remove the ions as fast as they are produced by the rays.
Meanwhile other evidence was accumulating to show that the conductivity produced in gases by X-rays is of the same nature as the conductivity of the gases from fames and from the path of a discharge, to which the theory of Giese and Schuster had already been applied. One proof of this identity was supplied by observations of the condensation of watervapour into clouds. It had been noticed long before by John Aitken[1] that gases rising from flames cause precipitation of the aqueous vapour from a saturated gas; and R. von Helmholtz[2] had found that gases through which an electric discharge has been passed possess the same property. It was now shown by C. T. R. Wilson,[3] working in the Cavendish Laboratory at Cambridge, that the same is true of gases which have been exposed to X-rays. The explanation
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