122 



SCIENCE 



[N. S. Vol. XLV. No. 1153 



rays from a strong source and to work at high 

 sensibility. The sensibility used in the dif- 

 ferent experiments was varied between 125 

 and 50 divisions per volt and was measured 

 after each reading of the electroscope. 



With the crystals in position a balance was 

 obtained between the two ionization currents 

 in the two chambers. Any change in absorp- 

 tion would then be shown by a corresponding 

 motion of the leaf of the electroscope. In 

 practise it was found unwise to attempt to 



EXPERIMENTS WITH POWDERED CRYSTALS 



The crystallizing dish was filled with the 

 crystals of a given material and placed as 

 shown in Fig. 1. The lead slit was then ad- 

 justed until a small leak was observed in the 

 electroscope and the average of a number of 

 readings taken. The crystals were then re- 

 duced to a fine powder in a mortar and this 

 powder pressed down in the dish to produce the 

 same thickness of layer as in crystal form. 

 The leak was then read as before. 



-i-S 



adjust the balance very accurately when using 

 strong 7-ray sources, since small fluctuations 

 appeared in the movement of the leaf, possibly 

 due to the Schweidler effect. 



A decrease in absorption corresponding to a 

 diminution in reflection in general took place. 

 In most of the experiments a 14 mg. radium 

 standard was used as source of 7-rays. A 



