Results of Crystal Analysis. 421 



The molecular element had four C atoms arranged tetra- 

 gonallv in a plane perpendicular to its axis, which, as in 

 the previous cases of tetragonal crystals, give a natural 

 explanation of the fact that the ratio c/a is smaller than 

 unity. 



Stereoscopic reproductions of a model of the lattice are 

 shown in PI. IX. (IS) and (c). 



Part II. — Remarks concerning the Structure of Xenotime. 



§ 7. In the previous paper (III.*) I gave the results of an 

 analysis of the crystal xenotime (YPOj, and found a lattice 

 which in the most satisfactory way would account for the 

 spectra observed. 



I mentioned that to get reflexion from this mineral 

 was attended by considerable difficulties, on account of an 

 internal .transformation of the structure. Fairly good 

 reflexions, which permitted a fairly accurate determination 

 of the relative intensities, we only got from the faces (110) 

 and (101). From the (100) face we only got the first- 

 and second-order reflexion very weak ; and from the (111) 

 face we merely obtained one single maximum, which we 

 took to be that of the fourth-order spectrum. For the (001) 

 face we did not obtain any reflexion at all. 



More recently I have greatly improved my experimental 

 arrangements, partly by using a new Hontgen-ray bulb 

 and partly by increasing the sensitiveness of the electrical 

 arrangements for measuring the ionization current ; and I 

 undertook a new series of measurements of the reflexions 

 from xenotime under these improved conditions. 



The result was very satisfactory, and we succeeded in 

 observing several weak maxima which we had not pre- 

 viously been able to detect. Thus we could measure the 

 three first maxima of the (111) face and determine the 

 spectrum from the base, which previously had given no 

 detectable reflexion. 



The spectra of the five faces (100), (110), (001), (101), and 

 (111), as we finally found them, are given in Table VI. and in 

 fig. 9. 



Comparing the spectra of xenotime with those of zircon, 

 we notice that the analogy between them has now become 

 much more marked. 



* L. Vegard, Phil. 31 ag. xxxii. (1916) p. 505. 



