336 PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 



will Strike them at a greater or less obliquity. Two types of 

 investigation are evidently possible. If X-rays from the same 

 source are used in the examination of different crystals, the spac- 

 ings of their molecular structures may be compared, or if the 

 same crystal is used in the examination of X-rays from dift'erent 

 sources, the X-rays themselves may be compared. Many sub- 

 stances under the impact of X-rays, are known to emit secondary 

 X-rays characteristic of the substance, depending, probably, on 

 their molecular spacing and arrangement. By comparative ex- 

 periments the length of an X-ray wave is found to be of the 

 order of one hundred millionth of a centimeter, or about one 

 one-thousandth the average length of a ware of light. The 

 molecular spacings appear to be of approximately the same order 

 of magnitude. This new science of crystallography is yet in its 

 beginnings, but gives much promise for the future. 



The discovery of the X-rays, with their effect upon a photo- 

 graphic plate, their power of exciting fluorescence and phos- 

 phorescence, and their penetrative power, aroused wide interest, 

 especially the latter quality, which made it possible to render 

 visible or to photograph the bones in the hand, or coins in an 

 opaque leather purse. As fluorescence and phosphorescence 

 could be aroused in other ways, it was natural to look for other 

 kinds of radiation, recognisable by their fluorescent or photo- 

 graphic effects. Henri Becquerel in 1896 obtained evidence of 

 such rays from uranium compounds, rays which were contin- 

 uously emitted without electrical or other excitation, ?Md which 

 were capable of penetrating opaque objects, of affecting a photo- 

 graphic plate, and of discharging electrified bodies, very much 

 as the X-rays do. 



The Curies of Paris, passing in review a great variety of 

 substances to discover possible radio-active properties, found in 

 pitchblende, an ore of uranium, radiant powers far greater than 

 that of uranium itself. They succeeded with much labor in iso- 

 lating several highly active substances, to the most important of 

 which they gave the name of radium. 



Rutherford, then at McGill University, showed that radium, 

 as also the other radio-active substances, emits three kinds of 



