106 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
the long axis of the crystal, in the case of the film sulphur the extinction angle had 
changed to about 8° with the original monoclinic axis. As the extinction orientation of 
the supposed octahedra is unknown, and the crystals are too small to observe singly, 
no further conclusion can be obtained. 
We desire to express our thanks to Professor David for the foregoing valuable 
addition to our knowledge of the structure of this film. It clearly consists either 
of a form of sulphur intermediate between monoclinic and rhombic sulphur with 
extinction angles of 8° or 9°, and which still preserves the melting-point of prismatic 
sulphur, or, on the other hand, it may perhaps turn out that the octahedra formed 
in this way are merely very unstable and return to the monoclinic state so rapidly 
that in melting-point experiments the reconversion always occurs before the melting 
can take place. Aged monoclinic crystals of sulphur from various sources were 
found to behave in the same way. Against the instability theory we have the fact 
that the melting-point was the same, whatever the initial temperature of the glycerine 
bath. During the electrical observations the film was probably partly normal mono¬ 
clinic and partly “ aged ” monoclinic. 
No quartz rods were used to reduce the surface conduction. The film was dried 
for some days in position in the oil bath by sulphuric acid and phosphorus pentoxide. 
The colour of the sulphur was a pure lemon-yellow. 
Most probable area of film, 15G'415 — '03 sq. centims. 
A = 156'385 sq. centims. 
Thickness of Film .—The threads were slightly conical and broken, so that the 
thickest parts were '024 centim. in diameter, as has been stated. When the plates 
were taken apart, a series of measurements of the film by the spherometer were 
made. Bits of the film were taken from each of the sides near the edge, and a bit 
from the middle. The bit to be measured was placed on the spherometer plate in 
a marked position—with careful dusting—and then covered by a small square of 
plane parallel glass, not larger than the fragment of film employed. The spherometer 
was well oiled and got into a steady state of temperature by handling before the zero 
was obtained—precautions very necessary, but generally omitted. Ten settings were 
taken on each portion of the film. The five means of the readings were, in inches, 
•337670 
•336990 
•337840 
•336470 
•336979 
Grand mean . . *3371898 
