THE VAETATTON OF ANGLES OBSEliVED IN CEYSTALS. 
488 
Another crystal of potash-alum, liaving l)een adjusted near the surface of the 
solution, was immersed to a considerable depth ; the image from a brilliant facet was 
found to move through 0° 22' (as measured l)y tlio micrometer eye-piece) in tlie 
direction of a battening of the foim. 
A third crystal gave near the surface of the liquid 
208° 46i', 
CO, 208° 431', 
208° 30'. 
At a greater depth 
208° 39', 
208° 32'. 
Eaised to the surface aerain 
oj^oj = 0° IGl/, 
= 0° 13 A, 
_ /vO r~! 
”o^ 138° Ilf', 
Op 137° 58V. 
Depressed again 
138° 8', 
_o^ 137° 58f'. 
0,0^ 
= 0° 131' 
4 ? 
OyOp 
0° 91', 
The above experiments indicate that in crystals of potassium- or ammonium-alum 
the vicinal faces, which invariably replace the octahedron planes, belong to different 
forms {lihl} of flat triakis-octahedra, and tliat, as the crystal grows, these continually 
change their inclinations; the faces a, /3, y, wliich replace an octahedron face o, 
generally belong to tlie same form, and they make an angle of from 2' to 30' with o ; 
wheir the vicinal facets at the toj) of the crystal beloiig to a different form from tliose 
at the l:)ottom they generally make a larger angle than the latter with o ; similarly 
as the crystal is immersed deeper in the solutioji tfie vicinal form becomes flatter ; 
during tlie growth of a crystal in a cooling solution the vicinal form usually becomes 
more acute ; hut at different times quite different vicinal faces may grow at the same 
temperature. 
(7.) The Vicinal Faces on Sodium Chlorate, Zinc Sulphate, and 
Magnesium Sulphate. 
(a.) Sodium Chlorate. 
Cubes of sodium chlorate give multiple images which, in accordance with their 
symmetry, are usually two images in a horizontal plane yielded by one pair of cube 
