1895.] Mineralogy. 653 
General Notes. 
MINERALOGY.’ 
Vicinal Planes and the Variation of Crystal Angles.— 
Miers’ has measured by means of a specially constructed goniometer® 
_ the changes in the form of crystals during their growth. Potash and 
ammonium alum is a substance whose apparently octahedral crystals 
are subject to noticeable variations in the size of the octahedral angle, 
and whose faces are sometimes vicinal in character. Miers began an 
investigation to determine whether the angles subject to variation 
have different values at different stages in the growth of the crystal, 
and if so, whether the faces change their inclination during growth, 
provided the crystal is held fixed. He has made the following import- 
ant observations : 
(1.) The faces of the regular octahedron are never developed on 
alum growing from aqueous solution. 
(2.) The reflecting planes (which are often very perfect) are those 
of a very flat triangular pyramid (trisoctahedron). 
(3.) The three faces of this triangular pyramid may be very un- 
equal in size. 
(4.) The trisoctahedron which replaces one octahedral face of a 
crystal may be different from that which replaces another face of the - 
same crystal. 
(5) During the growth of the crystal the reflecting planes change 
their mutual inclinations; the trisoctahedron becomes in general more 
acute, i. e., deviates more from the octahedron which it replaces as the 
crystal grows. 
(6.) This change takes place, not continuously, but per saltum, each 
reflecting plane becoming replaced by another which is inclined to it 
by a small angle (generally about three minutes). 
(7.) During growth the faces are always those of trisoctahedrons; 
but, if for any reason, as rise of temperature, re-solution occurs, icosi- 
tetrahedrons are developed. 
‘Edited by Dr. Wm. H. Hobbs, University of Wisconsin, Madison, Wis. 
? Abstract of paper read before the British Association. Nature, 50 pp. 411- 
412. (August 23d, 1894.) 
3 See these notes, March, 1895. 
