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PSEUDO-PHOTOGRAPHIC EFFECTS 75
not absolutely settled. E. Legrady x carried out some experiments with various metals and gases, and from the results he obtained he came to the conclusion that metals themselves have no action upon the photographic plate, their action depends upon the presence of moisture. Nitrogen gas also is found by him to increase the action. Since he obtained decided action in pure hydrogen gas he decided that the action of the metals cannot be ascribed to the generation and subsequent action of hydrogen peroxide. Also since he found hydrogen to have no action upon the plates in the absence of the metals, the active hydrogen must be in some way different from ordinary hydrogen ; perhaps it is ionised. He considers this idea to be greatly strengthened by results obtained when further experiments were carried out with spongy platinum and palladium.
Action on Pith, etc., after being exposed to Sunlight. — Russell found that pith may be in contact with a photographic plate at a fairly high temperature for forty-eight hours and yet no trace of action be visible, yet if the pith has been exposed to sunlight for two or three hours it will then give a dark picture. The same action occurs with old printed matter, pure indiarubber, and many bodies which under ordinary conditions are but slightly active. It is not even necessary to expose these bodies to the action of sunlight ; blue rays can produce a similar effect.
Becquerel Rays. — As stated elsewhere, Rontgen discovered the X-rays in 1895. In 1896 Becquerel made the discovery that salts of uranium emit invisible radiations which affect photographic plates and cause the surrounding atmospheric air to become a conductor of electricity. Since that time a number of substances have been found to emit these so-called Becquerel rays, and they are usually spoken of as radioactive bodies, the best known of them being radium.
1 Zeiischr. fur Wiss. Photo., 6, 1908.