PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
fi6 
Dr. George Elliott informs us that when burning some samples of sulphur from 
Japan, containing about 1 per cent, of selenium, for the purpose of making sulphuric 
acid, the selenium separated in the denitrating tower and in the chamber, in the 
manner we suggest/" In order to find whether selenium is got rid of in the salt-cake 
process, we burned a quantity of selenium in a properly bent tube, and passed all the 
products of combustion into water, burning the selenium part of the time in a brisk 
current of oxygen, and part of the time with only just enough oxygen to keep the 
selenium alight. Part of the resulting selenious acid was converted into selenic acid, 
and a mixture of these acids and selenium, together with any other of the products of 
combustion of selenium which might have been formed, were added to some sulphuric 
acid in an experiment imitating the salt-cake process. We found that all the 
selenium remaining in the salt-cake did not amount to 1 per cent, of that mixed with 
the sulphuric acid at the commencement of the process, so that the salt-cake process 
gets rid of most of the selenium as well as the arsenic. 
It is possible for some small trace of selenium to get into the black-ash furnace. 
If it does, it is presumably in the condition of free selenium, sodium selenate, or 
selenite. Now the trace of free selenium will probably be volatilized to a great 
extent, and pass off with the other gaseous products. As for the selenite and 
selenate an experiment to be described indicates that they will be decomposed, 
yielding free selenium, with which some of the lime will form calcium selenide. 
We find, however, calcium selenide is decomposed when suspended in water by 
carbonic acid gas yielding free selenium, but not a trace of selenium hydride. These 
facts were established as follows :— 
About 4 grams of precipitated chalk were converted into calcium selenate, and this 
was reduced by hydrogen (pure) to calcium selenide. During the operation much of 
the selenide decomposed into free selenium and lime (?), and there was a considerable 
evolution of selenium hydride. The process was stopped while the evolution was in 
full progress. On washing the contents out of the tube we probably had a mixture of 
free selenium, calcium selenide, calcium selenate, and perhaps other products. A 
stream of carbon dioxide gas (pure) was passed through this mixture, including the 
water, contained in a set of potash bulbs, and as a result a large quantity of selenium 
was deposited, but not the slightest trace of selenium hydride could be discovered 
either by the smell of the emergent gas or by heating the delivery tube red-hot. 
It will be noticed that the great instability of calcium selenide (for we presume that 
the hydrogen selenide came from the reduction of this salt), when heated to redness, 
gives us another reason for suspecting that very little calcium selenide is formed 
in the black-ash furnace—or rather that very little can survive, unless reduction 
* A subsequent note from Dr. Elliott throws some doubt on this, for a red precipitate being noticed 
at the foot of the denitrating tower—when sulphur nearly, if not quite, free from selenium was being 
burned as a source of sulphur dioxide-—an analysis qf the precipitate was made. It then turned out to 
be ferric oxide. 
