Hussakitc. — Kraits and Rci finger. $i 
The crystals, which were measured, possess vitreous histre ; 
those more or less rounded have a greasy or pearly lustre. 
The color is yellowish white, honey yellow, brown, or dark 
brown. 
Hardness=5. 
The specific gravity^ determined by means of the hydro- 
static balance, was found to be 4.587 at 20" C. 
Perfect cleavage j^arallel to m (no). 
Fracture uneven ; streak white to yellowish wliite. 
In a thin section, perpendicular to the c axis, hussakite 
shows in convergent polarized light a normal unaxial interfer- 
ence figure. The character of the double refraction is positive. 
From a transparent crystal about i cm. long and 0.5 cm. 
thick Voigt and Hochgesang of Gottingen ground a prism 
with the refracting edge parallel to the c axis. This prisni 
was used to determine the indices of refraction for lithium, 
sodium, and thallium lights. The method employed was that 
of minimum deviation. The results are as follows : 
w 
Lithium light 1.7 166 
Sodium " 1.7207 
Thallium " 1-7244 
The images produced bv the sodium and lithium lights 
were very sharp and hence these observations are re- 
liable. The image of the ordinary ray for thallium light was 
not as well defined as for the other colors but this determin- 
ation may, nevertheless, be considered as fairly accurate. The 
image of the extraordinary ray for this same color was very 
much blurred and hence two separate determinations were 
made, namely, 1.8183 and 1.820Q, which differ considerably 
from one another. The value given above is the mean 
of these two observations. 
If we consider the determination for lithium and sodium 
lights as reliable and then calculate the indices of refraction for 
thallium light by means of Cauchy's formula 
. n = A + >i-,' 
we obtain 
w = 1.7245 e = 1. 8194 
as the theoretical values for this color, which agree quite 
closely with the observations given above. 
e 
e-w 
1.8113 
0.0947 
1-8155 
0.0948 
1.8196 
0.0952 
