56 
ì | 
or 
great circle, which is drawn in fig. 10. So with this fact also the last 
impediment to the formerly given explanation of the abnormality 
of quartz has been removed; thus the remarkable deviation of the 
image parallel to (1210) can now be explained without difficulty as 
a result of the mentioned twin-structure of the mineral. While we 
moreover formerly did not succeed in obtaining good images of a 
quartz-plate cut parallel to (OUOL), we have now again made a series 
of systematical experiments with quartz-plates of different thickness; 
it was found that for the purpose of getting well reproducible nega- 
tives it was necessary, to radiate through rather thick erystal-plates. 
The Röntgen-pattern, which is veproduced in fig. 13 on Plate IV, 
and in stereographical projection in fig. 9 on Plate V, was obtained 
by means of a quarlz-plate of 3,75 mm. thickness. In this way the 
Röntgen-patterns of quartz have now become complete, and the 
exactness of the theory is also here proved for quartz, — at least 
in normal cases. 
A beautiful crystal of a pale green brucite: Mg(OH),, from Hey- 
dale, Shetland Islands, Scotland, enabled us to get also an image 
of a basal section of this mineral, which has so simple a chemical 
constitution, and the axial ratio: a@:¢=1:0,5208. It is reproduced 
in tig. 15 on Plate IV, and in stereographical projection in fig. J1 
on Plate V. A study of the influence of heating basal sections of 
this mineral in the furnace formerly described by us, on the Röntgen- 
image, is now planned. 
Finally we have published in fig. 12 on Plates HI and V also 
the results of radiating through a basal plate, 1,09 m.m. thick, of 
the monoclinic-sphenoidical Lithiumsulphate: Li,SO, + H,O. This 
substance, with the parameters: a:6:¢ = 1,6102:1:0,5643, and 
B = 87°29’, is yet an object of the said symmetry-class, not showing 
an optical activity in solution, as was the case in the formerly 
investigated cases *): cane-sugar, tartric acid, ete. Therefore this 
experiment may be considered a welcome completion of the re- 
searches, regarding such monoclinic crystals. In this case too the image 
parallel to (OO1) appears to possess a single vertical plane of sym- 
metry, notwithstanding the fact, that the crystals themselves have 
only a single binary axis. Also this fact proves once more the gene- 
ral justification of the conclusions drawn in that paper. *) 
Laboratories for Physics and for Inorganic and 
Physical Chemistry of the University. 
Groningen, 15 February 1916. 
') These Proceed. 18. 1201 (Jan. 1916). 
