Crystalline Structure of Copper. 359* 



three principal faces of the crystal is that which would exist 

 for a face-centred cubic lattice. In such a lattice 



^(100) 1^(110)1 ^(111) — 1 



1 2 



We have already seen that the absolute values of the angles. 

 at which reflexion occurs are those to be expected if the 

 copper atoms lie on a face-centred lattice. It is further to 

 be observed that the 1st, 2nd, and 3rd order peaks reflected 

 from the faces of the crystals are in every case quite normal, 

 the first order being greater than the second, and that 

 greater than the third. This, as has been shown in former 

 papers (Proc. Roy. Soc. A. vol. lxxxix. p. 472) implies a 

 regular arrangement of reflecting planes, equally spaced and 

 identical in nature. Lastly, as a check, a search was made 

 for spectra at half the angles at which the first spectra of 

 fig. 2 occur, in the case of the planes (100) and (110). 

 This search gave a negative result. Taking all this into 

 consideration, there can be little doubt that tlie atoms 

 of a copper crystal are arranged on a face-centred cubic 

 lattice. 



The results are interesting in that they show that con- 

 siderable accuracy of measurement can be obtained with the 

 X-ray spectrometer, even when the crystal itself is highly 

 irregular. If the cubic symmetry of the copper crystal had 

 not rendered this unnecessary, it would have been possible 

 to measure the axial ratio of the crystal within 1 per cent, of 

 the truth, although the faces of the crystal were distorted by 

 many degrees. Since the crystal is so irregular, only a 

 fraction of its surface reflects at any one angle, and therefore 

 the electroscope had to be very sensitive when measuring the 

 small ionization current. This explains the very obvious 

 irregularities of the curves in fig. 2. The dots in this figure 

 represent a set of readings. All these curves were repeated 

 several times, some with different crystal faces. Some curves 

 were more irregular than others, but all agreed closely in the 

 positions of the spectra. 



I wish to take this opportunity of again thanking Mr. Hut- 

 chinson for his kind help, both in supplying material and in 

 aiding with his advice the preparation of the various crystal 

 faces. I wish to thank Professor Sir J. J. Thomson for his 

 kind interest in the experiments. I am indebted to the 

 Institut International de Physique Solvay for a grant with 

 the aid of which the apparatus used in these experiments was 

 purchased. 



