DEPARTMENT OF TERRESTRIAL MAGNETISM. 305 



the film of a photographic plate produce unages; and the experiments of 

 Russell and others, especially S. Saeland/ have furnished strong evidence that 

 the effect on the plate in at least most cases is due to the presence in or on the 

 objects of hydrogen peroxide. It was supposed by E. F. Mace^ that such 

 images were produced only in a magnetic field ; but observations published by 

 Bauer and Swann' have confirmed the earlier investigators, who used no mag- 

 netic field, and have shown that if the magnetic field has any effect it is only 

 to produce modification in the intensity of the images. If these images were 

 due to the emission of free electrons, the magnetic field might in some cases 

 have an appreciable effect upon them. The field would scarcely be expected 

 to alter the rate of emission of hydrogen peroxide from the objects or the sensi- 

 tiveness of the photographic plate, though it might affect the distribution of 

 the gas, pulhng it into or pushing it out from the stronger parts of the field 

 and thus affecting the photographic images. 



Wlien the object on the film is active, a positive image is foraied beneath 

 it, the remainder of the plate being less affected or unaffected unless other 

 active materials are present. If the object is not active, and the walls of the 

 in closure or other objects contained therein are sufficiently active, the film 

 is only partially darkened beneath the object, unless it is too thin, but the film 

 is darkened over the rest of its extent. The two effects are often superposed. 



In two experiments made with glass objects in glass bell-jars, by the method 

 used in the Department's former experiments, no certain effect of the field 

 could be detected. The method was then radically altered in order to apply 

 more intense and controllable magnetic fields and diminish the difference 

 between the treatment of the plate in the magnetic field and the plate simul- 

 taneously exposed out of the field. Two quite similar vacuum cells were made 

 from brass and copper, corresponding parts from the same stock, to hold the 

 two plates, and they were connected by a long brass tube. One cell was usually 

 placed in the field of a powerful electro-magnet, the other at a distance from 

 the magnet. Instead of using two photographic plates, one plate was always 

 cut in two, and half placed in each cell. The two halves were always devel- 

 oped together. The cells with their contents could be interchanged. The 

 temperature near each cell was measured, and the two temperatures were 

 usually kept nearly the same. The maximum magnetic intensity was much 

 greater than in the earlier experiments, exceeding 6,000 gausses. 



In most of the experiments the field was nearly uniform over the film, and 

 the intensity was normal to the plate. Experiments were made in this way 

 with similar objects of copper, brass, glass, zinc, and lead laid upon each plate. 

 In some experiments the brass top of the cell was directly above the film, with 

 nothing intervening except the test objects and more or less air. In other 

 experiments the brass top was lined beneath with a sheet of copper. In still 

 others ttiis sheet was covered with a thin layer of rosin. In some experiments 

 the metal objects were freshly sandpapered on the sides touching the films, 

 so as to give positive images. In others the active state of the objects had 

 disappeared, or nearly disappeared, so that the objects cast shadows, with or 

 without positive effects inside them. Experiments were made at different 

 pressures and with exposures ranging from a few hours to many days, and 

 many good photographs were obtained. The results do not indicate any 

 certain effect of the magnetic field. 



1 Ann. d. Phys., vol. 26, p. 899 (1908). ^ Sci. Amer., p. 411 (Nov. 4, 1916). 



' Phys. Rev., vol. 9, p. 563 (1917). 



