326 



CHARLES PACKARD 



is about 50 mm. may be strikingly demonstrated in a dark room 

 by holding a willemite screen where the rays may fall upon it. 

 The gamma rays may be similarly shown falling on the shelf B. 

 The luminescence of the screen is about the same on both shelves. 

 Secondary beta rays are probably produced in the lead and fall 

 upon the shelf B but the effects they produced, as distinguished 

 from those of the gamma rays, was not studied. 



The material to be radiated was placed in small glass cells, 

 open on top, and provided with mica bottoms. The cells were 

 placed on the shelves B and C. In some experiments the cell 



Text figure A 



was placed directly above the radium capsule and at varying 

 distances from it. The material was thus exposed to both beta 

 and gamma rays. By varying the thickness of the mica bottoms 

 of the cells it was possible to screen out the slower beta rays or 

 to utilize them all. With a thickness of 0.1 mm. of mica it was 

 found that very few of the slow beta rays were absorbed. A 

 thickness of 0.10 mm. of mica is sufficient to screen out the 

 slower rays, so that only the more rapid ones could affect the 

 material. 



These experiments were made at Woods Hole during the sum- 

 mer of 1914. For the preparation of radium and for the electro- 

 magnet with the lead device for holding the radium I am deeply 

 indebted to Dr. Robert Abbe of New York City. I take pleasure 

 here in expressing to him my hearty thanks. 



