400 
DR. A. K H. TUTTON ON THE 
The forms given in the list other than a and p are the same as were observed by 
Topsge. The only four angular measurements which he obtained were as follows :— 
pp = (110): (llO) = 71° 21', cp -(001): (TlO) = 101° 32', cr' = (001): (201) = 63° 1', 
and pq = (110): (Oil) = 85° 30', all of which he states were only approximate, owing 
to the crystals becoming dull and decomposed immediately after removal from the 
mother liquor. They differ considerably from the accurate values now obtained. 
Cleavage. —It was not possible to determine the cleavage with accuracy, on account 
of the rapid decomposition of cleaved surfaces, but it would appear that the usual 
cleavage parallel r' {201}, developed throughout the series, is also shared by 
potassium ferrous selenate. 
Volume. ' 
Relative Density. —The following results were obtained in six determinations by 
the Retgers immersion method. The first four were with crystals belonging to one 
crop, and were carried out immediately after removal from the mother liquor and 
careful drying with a soft handkerchief; the last two were carried out on another day, 
with another of the crops, the crystals used being also transferred directly from the 
mother liquor to the immersion liquid, after a few moments occupied in the drying 
with a handkerchief. The results are seen to be eminently concordant. A different 
room was employed for the work of making the liquid of exactly the same density 
as the crystal, the temperature of which was higher than that of the laboratory ; for it 
is inadvisable to work at a temperature much more than 5° below 20° C. (the standard 
temperature of all the author’s density comparisons), on account of the large 
magnitude of the correction for the coefficient of expansion of the crystals which is 
then involved. The crystals were not at all adversely affected during the rapid work 
at the temperature of 13" — 15°, as they were immersed in the heavy protective liquid, 
being quite as transparent at the finish as when immersed, and several of them were 
employed immediately afterwards in the goniometrical work. 
I. Density for 13°’0/4° . 
. 2-4931 
For 20 /-[ 
. 2-4914 
II. 
15°-0/4° . 
. 2A944 
„ 20°/4° . 
. 2-4932 
i—i 
i—i 
i—i 
13°-3/4° . 
. 2-4930 
,, 20°/4° . 
. 2-4913 
IV. 
o 
o 
»—H 
. 2A939 
„ 2074” . 
. 2-4925 
V. 
13°-6/4° . 
. 2-4955 
„ 2074" . 
. 2A939 
VI. 
15°-4/4° . 
. 2-4953 
„ 2074" . 
. 2-4942 
Mean . . . 2‘4928 
Mean 
> 2'4921 
\ 2'4941 
As it is probable that the higher mean value for the second of the two different 
preparations is nearer the truth for cavity-free crystals, the value accepted for 20°/4° 
is 2 494. 
Molecular Volume. — ^ = -- 1 - = 210'39. 
d 2-494 
Molecular Distance Ratios {topic axial ratios).— 
X : ^ : w - 6‘2230 : 8-3085 : 4‘1908. 
