ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR. 
65 
•03 milligram of selenic acid present, or say ‘014 milligram of selenium, or about one 
part in a million. As at first we under estimated the delicacy of the selenium test, 
it may be worth while to indicate how the test should be performed in order to give 
the best results. The points are : (l) a large quantity of hydrochloric acid should be 
used ; (2) the liquid after some minutes’ boiling should be cooled before the sul¬ 
phurous acid is added; (3) the mixture should then be well shaken ; (4) on heating 
to boiling, a precipitate will appear in two or three minutes if selenium is present— 
at the limit a yellowish rose-colour will be produced, but no actual precipitate. The 
essential point is to have the volume of liquid as small as possible. For instance, 
with one cubic centimetre of liquid ‘01 milligram of selenic acid may be detected 
with certainty, and still smaller quantities in proportion as the volume of liquid is 
smaller. 
After testing for selenium, and taking the most refined precautions as to re-agents, 
the Marsh’s test for arsenic was applied. In no case was arsenic discovered. We 
found by direct experiment that the minimum amount of arsenious oxide we could 
detect by our appliances was '02 milligram, and that by the smell of the gas only. 
The test, however, as made by the nose is just as certain as that made by the eye 
when a mirror is produced; the latter, however, is less delicate. With respect to 
precautions, we will only say that the instructions of Fresenius were carefully 
followed, and that this was not the first occasion on which we had made the test."" 
We may conclude that if arsenic is present in the sulphur, it is only so to an extent 
of about one part in a million. The selenium test will be observed to be more delicate 
than the arsenic test—a fact not generally recognized—and this when the test is 
made by noting the smell of the evolved hydrogen. If a recognizable mirror be 
required, the arsenic test is a good deal less delicate. 
A test for tellurium and selenium was made for one of us in 1888 by Dr. Helms, 
on a sample of sulphur recovered from thiosulphate. These results were negative. 
As regards arsenic and selenium, we wish to go further, and to show that it is very 
improbable that they could have been present even in quantities much less than we 
could have detected. The results of the enquiry will apply alike to sulphur recovered 
by the Chance process and to sulphur recovered in the laboratory from commercial 
sodium thiosulphate (assuming the latter to have come from soda waste). For this 
purpose it is necessary to consider the Leblanc process in some detail. 
The sulphuric acid employed in the salt-cake process is the source from which 
arsenic and possibly selenium may be derived. The greater part of the arsenic is got 
rid of in the salt-cake furnace in the form of chlorides. With respect to selenium, 
we believe that it will be almost all precipitated in the sulphuric acid chamber, but 
as the acid is not filtered, as a rule, this cannot be regarded as a purification. 
* We may note here that the decomposition of arsine by strong sulphuric acid is a test which, though 
not quite so delicate as the mirror test, is not by any means a bad one. It is not usually given as a test 
at all. (The sulphuric acid must be kept concentrated.) 
MDCCCXCVI.—A. K 
