Crystalline Structure oj Copper. 



357 



The face (ill) of the copper crystal, prepared as described 

 above, was first investigated. The chamber was set at 25° 40' 

 ( = 2 X 12° 50'). The crystal face, being adjusted so that the 

 rays fell on it at a small glancing angle, was turned so that 

 this angle assumed in turn a series of values between 6° and 

 20°, in order to find if, in some position, it reflected the 

 X rays. This had to be done because the true orientation of 

 the plane (111) in the crystal was not known with any 

 exactness. At none of these angles was there a reflexion into 

 the chamber, the simple cubic arrangement of the copper 

 atoms being therefore ruled out. 



On setting the ionization chamber at 16°, however, a 

 marked effect was found. Fig. 1 shows the current in the 

 chamber for a series of angles at which the crystal was set. 



Fig. 1, 





5° 10° 15' 



Cl/iA'C/A'S /1A/Ci.£ Of S?ArS O/VCri^TAL FACC ... 



Between 8° and 11° 30' there is a marked increase in the 

 ionization current, which rises to a maximum at 9° 30'. 

 Now if the crystal were perfect, the range of angles at which 

 it reflected the X rays would be limited to some 30' at most. 

 The fact that the crystal reflects over such a wide range of 

 angles shows that its planes are distorted to an extent of 

 several degrees, instead of being strictly parallel. As the 

 crystal is turned round, one set of planes after another comes 

 into the reflecting position and causes an ionization current 

 in the chamber. From the curve we deduce that when the 

 •crystal is set at 0° 30' the area of its face so oriented as to 

 reflect is larger than at any other angle. 



A scries of readings are now taken with the chamber at 

 various angles " #," and the crystal in each case at the angle 



Q 



" 9+1° 30'/' in order always to make use of this larger 

 reflecting area. The results are shown in the curve for the face 



