Results of Crystal Analysis, 93 



a peculiarity with the geometry of the lattice will make the 

 assumption of molecules almost a necessity. 



Let us consider the arrangement of atoms in the (110) plan 

 of zircon, fig. 5. The triangle (aba) has a Zr atom in two 

 of its corners and a Si atom in the third. Now an oxygen 

 atom d belonging to the Zr atom a will — on account of the 

 geometry found for the lattice — have equal distances to the Zr 

 atom b and the Si atom c (fig. 5). Now the chemical-affinity 

 forces must necessarily be different for (Zr-O) and (Si-O), 

 and if such forces were acting between the oxygen atom d 

 and any of the atoms c and b, we cannot explain an equi- 

 librium position of the oxygen atom d which makes the 

 distances (d-b) and (d-c) equal. 



§ 11. r Jhe Structure of Thorite, 



As mentioned in the introduction, the mineral thorite 

 (ThSiC^) also belongs to the Zircon group. This mineral, 

 however, occurs in the so-called " metamict " form, which 

 indicates a state in which the outer appearance of a crystal 

 is preserved, but the substance itself has in the course of time 

 become isotropic. 



Thus the crystals, when examined with polarized light, 

 give no indication of an optical axis, and it seems as if the 

 atomic arrangement in a lattice has become unstable. 



Now it would be a matter of interest to see how these 

 crystals behave towards Rontgen rays. Do they give any 

 reflexion? Or does a " metamict " crystal possess any trace 

 of its original lattice ? 



I have made a series of experiments with a number of 

 different minerals to investigate this point. A full account 

 of these experiments will be given later. In this connexion 

 I shall only mention that the mineral thorite gave no X-ray 

 reflexion at all, although several very fine crystals were 

 tested. Thus the Rontgen-ray analyses have shown that 

 the lattice of thorite was completely broken down, and only 

 the outer form has been preserved to indicate the atomic 

 framework which once existed in the crystal. All symmetry 

 properties, however, go to support the view, that the atoms 

 must have been arranged in a lattice of the zircon type. 



§ 12. Remarks regarding the Intensities of the Normal 

 Spectrum. 



W. H. Bragg and W. L. Bragg have found that the 

 intensities of a normal spectrum gradually diminish with 

 increasing order. As an average they put the relative 



