66 Dr. L. Vegard on 



analysis of the structure of! this group. The following spectra 

 have been measured: — Magnetite (111), (110), and (100); 

 spinel (111), (110), and (100) ; gahnite (111). The experi- 

 mental results, as well as the lattice constructed from them, 

 were in close agreement with the results of Bragg ; so a 

 more detailed account will be superfluous. 



§ 2. The Structure of Gold and Lead. 



The elements copper, silver, gold, and lead all have 

 crystals which belong to the holohedral class of the cubic 

 system, and from a crystallographic point of view we should 

 expect gold and lead to have a similar space-lattice to that 

 found for copper * and silver f ; but still I think an actual 

 determination will be of interest, as several lattices might 

 give the right symmetry. 



The gold crystals used for the experiments were kindly 

 lent me by Professor W. C. Brogger. The one specimen 

 had the form of an octahedron, but as it had linear dimen- 

 sions of the order of only one millimetre, we did not with our 

 instrument detect any reflexion from it. The specimen used 

 had the common form of a thin plate, twinned about its 

 principal face (111). 



The crystal plate being quite thin, we only got reflexion 

 from the face (111). 



The lead costal was produced artificially. Several 

 methods were tried — e. g., a gradual cooling of the molten 

 substance, and sublimation of the metal in an electric 

 furnace ; but although crystals were formed, they mostly 

 consisted of branches made up of small individual crystals, 

 but we got no crystal face fit for our purpose. 



The method which proved most successful was to let 

 lead precipitate on a piece of zinc from a solution of lead- 

 acetate. 



In this way we got crystal leaves formed in a similar way 

 to the gold plates with the principal face (111), which gave 

 quite a strong reflexion. 



In fig. 1 are given the relative strength and the position 

 of the reflexion maxima for the face (111) of gold and 

 lead as observed with narrow slits (0*4 mm.). The normal 

 variation of intensity with increasing order shows that the 

 1 ' point- planes " parallel to the face (111) are equal and 

 equidistant, and in the simple case of a cubic crystal with one 

 sort of atoms there can only he one lattice, which satisfies this 

 condition and gives the right glancing angle. 



* W. H. Bragg, Phil. Mag. xxviii. (1915) p. 355. 

 t L. Vegard, loc. cit. 



