62 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
distilled over and over again in glass retorts, black spots always make their appear¬ 
ance in the residue unless the operation is carefully shielded from the action of dusty 
air. If sulphur is distilled and then exposed to air so that dust can settle on it, 
then, on repeating the distillation, black spots make their reappearance. We have 
only once succeeded in absolutely preventing the presence of black spots in a retort 
from which sulphur was distilled, but we have noted over and over again that just 
in proportion to the precautions that are taken to exclude dust, so do the black spots 
become smaller and fewer, and the colour of sulphur suddenly cooled from a high 
temperature clearer and clearer. 
That pure sulphur has no smell we know, because we have prepared a sample by 
precipitation of hyposulphite, and after five distillations no smell could be detected 
from it. This observation is important, for we have large quantities of sulphur twice 
distilled from Sicilian roll sulphur which leaves no residue when burned away in a 
platinum dish, which remains bright yellow on cooling quickly, which will yield no 
gas when heated in vacuo up to near its boiling-point, and which generally might be 
considered to be satisfactory had it not the well-known sulphury smell, which, by the 
way, we consider to be possibly due to traces of chloride of sulphur. The smell is 
not to be removed by exposure in vacuo at any temperature and during many hours, 
and samples of sulphur which possess it had better be left on one side. 
Pure sulphur, of course, leaves no residue on burning in a platinum basin, but we 
have never succeeded in reaching this stage with sulphur which is burned in the 
ordinary way. We have, however, got as far as this—that the combustion of 
200 grams of sulphur in a platinum dish will leave a stain so small that it cannot be 
observed unless the platinum is brightly polished. The stain disappears at a red 
heat, and may be safely attributed to the dust which gets into the dish during the 
time the sulphur is being introduced, and while the process of burning is being 
observed. In order to obtain the maximum residue from any sample of sulphur, the 
last cubic centimetre or so must be boiled off very carefully and without overheating* 
The residue which, with our purest sulphur, such as we have used in electrical 
experiments—is after all a mere stain, is far beyond the powers of the balance to 
detect. It is dark in colour and burns off at a red heat, indicating that it is com¬ 
posed of the same organic compound which was discovered by Mitsch erlich. We 
have observed that after a sample of sulphur has been exposed to the air during 
electrical experiments, it will give a comparatively large residue, although when first 
prepared the residue could only be observed by close attention. Those samples of 
sulphur which when suddenly cooled appeared of a buff colour, always gave com¬ 
paratively large residues, and these were the samples which had been most exposed to 
the action of air. Of course ordinary sulphur leaves a large quantity of earthy residue 
besides, which can be got rid of by about two distillations—but we are not discussing 
such preliminary purifications. Impurities which can be detected by the means above 
described may be considered to be the more obvious impurities, and do not give rise 
