200 
DR. A. E. H. TUTTON ON THE 
singular that even in the very numerous (over 30) crops prepared in the course of the 
work now described no other than these same four forms have been observed on this 
member of the great monoclinic series. 
The crystals are bright green in colour. 
Crystal System. —Monoclinic. Class No. 5, holohedral-prismatic. 
Ratios of Axes .— 
a:b : c = 0'7467 : 1 : 0'5059. Values of vom Rath, 0'7453 : 1 : 0‘5060. 
Axial Angle. — /3 = 104° 27'. Value of vom Rath, 104° 53'. 
Forms Observed. —c {001}, p {110}, q {Oil}, and r' {201}. 
Habit. —Tabular parallel to c {001}, to short prismatic parallel to p{ll0}. A 
common intermediate type is shown in fig. 1. 
Eleven small crystals were measured, selected from four different crops. Greater 
difficulty than usual was experienced in obtaining crystals free from serious striation 
of the p {110} and c {001} faces, and a very large number of crops were grown, from 
two entirely distinct preparations, before satisfactory small crystals were produced. 
The striation affects the values of the angles pp and cp particularly, rendering pp 
( = 2 ap) considerably - too large, pp ( = 2 bp) conversely too small bo the same extent 
(possibly as much as five minutes), and cp to a less degree (2' or 3} too large. This 
fact was established by measuring a number of crystals affected by the striation and 
comparing the results with those derived from the perfect crystals used in the 
measurements recorded in the table, which represent the true value of the angles. 
The unusual prevalence of such striation of the p and c faces is also responsible for the 
more than usual discrepancy between the observed and calculated values of the angle 
cr', in cases like that of the salt under consideration, in which the r' faces were 
perfect (yielding brilliant single images of the signal) and in which the c faces, when 
adjusted for the measurement of this orthozone, gave also indubitably placed images, 
all the images of the bundle coalescing for this zone in the same vertical line. 
The faces of q {011} were usually relatively much smaller than those of the basal 
pinakoid c{00l}, as shown in fig. 1, and the faces of r'{201} were usually also small, 
although generally somewhat larger than the q faces. Occasionally, in particular 
crops, the q faces were relatively larger and the c faces correspondingly less. 
Of the only four forms observed, c {001} and jo{ 110} thus usually largely 
predominate and determine the habit. 
