64 
PROFESSOR R. THRELFALL AND MR, J. H. D. BREARLEY 
and arsenic, however, might occur as volatile impurity, and it was therefore necessary 
for us to ascertain their presence (or absence) by means of a direct experiment. 
After a considerable amount of preliminary work we fell back on the combustion 
of quantities of about twelve grammes of the sulphur to be tested, in, a platinum boat 
in a combustion tube. The products of combustion being mixed with excess of 
oxygen and passed over spongy platinum, the resulting sulphur trioxide was received 
in a large globe cooled by ice and salt. The trioxide was then converted into 
sulphuric acid, and tested for arsenic and selenium. We made two successful 
combustions, using in one case the residue from a distillation of Chance sulphur, and 
in the other the sulphur in the state (twice distilled) in which it was used in our 
experiments. The combustion requires some precaution. In our arrangements, the 
tube employed was of hard glass, and was heated for a length of 80 centims., it was 
1*4 centim. in internal diameter. In this tube 15 centims. were occupied by the 
platinum boat containing the sulphur, this was followed by a “mixing” space of 
red-hot asbestos for 25 centims., and 35 centims. were occupied by spongy platinum 
and platinized asbestos. Oxygen was admitted by two tubes, one only just entered 
the combustion tube and provided the oxygen for the burning of the sulphur in the 
boat; the other ran over the boat and delivered a large excess of oxygen into the 
“mixing” space. Oxygen was observed to be in excess during the whole process. 
By regulating the oxygen supply and the temperature of the sulphur in the boat, 
the combustion can be got to proceed regularly. The combustion tube was bent at 
the delivery end, and dipped into the receiver. At the close of the operation, a 
stream of oxygen was passed for some time, the tube being now red-hot throughout— 
up to the bend—by this means the whole of the contents of the boat were burned 
and all residues chased out of the combustion tube. Finally the part of the tube 
which projected into the globe was washed out with platinum-distilled water and the 
solution added to the solution in the same water of the sulphur trioxide contained 
in the globe. In the first combustion the oxygen supply was not in sufficient excess 
during a few moments and the trioxide was coloured blue by combination with a 
trace of free sulphur. In the second combustion, in which the oxygen was in great 
excess throughout, the resulting sulphuric acid contained sulphurous acid, and the 
excess of oxygen also carried off sulphur dioxide during the process. The conclusion 
seems to he that Sulphur Trioxide is dissociable by spongy platinum at a red heat, 
even in presence of oxygen—this we did not know, it is not mentioned by Lemoine 
(‘Etudes sur les Equilibres Chimiques’). The solution of trioxide from the first 
combustion required to be filtered; the second did not, and was absolutely clear and 
colourless. We remark on this, for there is a chance of selenium separating during 
the solution of the trioxide, in the presence of sulphurous acid. In both cases the 
solution was next boiled with a large excess of sulphurous acid and hydrochloric acid. 
In neither case was there the slightest precipitate. We are satisfied that, under the 
conditions of the experiments, this means that there could not have been more than 
