ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR, 
105 
ment was put back into the oil bath, and again heated to 145°, after which the tem¬ 
perature might be considered uniform. We desired to avoid distorting the plates by 
having any want of uniformity in the temperature. The bath was then allowed 
to cool slowly to 95° C., when the sulphur crystallized, but was still soft. The tem¬ 
perature was then raised to 118° Cl., kept at this for about an hour, and the gas then 
turned off, and the bath allowed to cool slowly. It takes six or seven hours to reach 
the temperature of, say, 30° C. In order to have a check on the nature of the 
sulphur employed, two auxiliary aluminium plates were prepared at the same time, 
and furnished with a film of sulphur, under identical conditions with the actual 
experimental plates. The next day these plates were forced apart, and about 1 gram 
weight of the sulphur tested by carbon bisulphide. It was found that the sulphur 
was all but absolutely soluble. We would not like to say that it was not perfectly 
soluble, for the minute particles left could only be seen when the test-tube was 
in bright sunlight, and they might have been merely dust motes. However, our 
impression was that a trace of insoluble sulphur was present; it could not have 
amounted to more than about ’01 milligram, so that the film may be considered to 
contain, say, 99,999 parts of soluble sulphur out of 100,000. 
After the observations were complete, and the plates taken apart on October 25, it 
was found that 3 sq. millims. ought to be allowed for the space taken up by the 
bubbles. The film appeared to be perfectly soluble, nothing in the way of residue 
could be detected, but only about 1 decigram was available for testing. The melting- 
point was taken five times, and found to be from 120’8° C. to 121’4° C., consequently 
the sulphur was prismatic. A microscopic investigation showed that there were 
larger crystals present in the film cemented together by smaller ones, or, possibly, by 
uncrystalline matter. We were-so fortunate as to secure the assistance of an expert 
mineralogist, Professor David, in the microscopic examination of the film. The 
investigation of the film at the end of the experiments, so kindly undertaken by 
Professor David, involved grinding pieces of the film down to very fine slices, in the 
same manner as rock sections are ground down, and subsequent examination by the 
polarizing microscope. The results were as follows :— 
(1.) The greater portion of the film, probably the whole, is crystalline and 
anisotropic. 
(2.) Most of the crystals, but not all, have their principal axes lying in the plane 
of the film. 
(3.) So far as could be seen, most of the crystals appear to have their principal axes 
parallel. 
(4.) The question as to whether all the crystals were monoclinic was investigated 
with great care by observing the extinction angles, and comparing the action of the 
film with that of fresh and aged monoclinic crystals. The result was, that the 
crystals had certainly ceased to be monoclinic, and within the limit of observation 
were entirely octahedral. The “ extinction angle ” of monoclinic sulphur is at 45° to 
MDCCCXCVI.—A. P 
