144 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLET 
Sulphur Plates .—These were made in two ways, and we consider that too much 
care cannot be exercised in this part of the work. 
“ Insoluble ” Plates .—A light sheet zinc mould was prepared, capable of containing 
a plate about two inches greater in diameter than the one to be employed (which was 
made usually of rather larger diameter than the guard-ring), and say four or five 
times as thick. The sulphur, previously filtered as described, through glass wool and 
platinum gauze, was melted and raised to a sufficient temperature by placing the zinc 
mould on a hot flat plate, It was noticed that after the filtered sulphur was cold, 
when it was being broken up for the final melting, it still possessed a strong sulphury 
smell, which in this case suggested gas lime. This shows that there is an advantage 
in distilling the sulphur, although the amount of impurity present may be imperceptible 
on boiling away the filtered sulphur in a polished platinum dish. However, the 
impurity seems to be got rid of by re-melting in the dish, for the smell was not 
detected on breaking up the plates at the close of the experiments. 
The mould having been filled with sulphur at the proper temperature, i.e., after it 
had become very viscous, above 170° C. ? the whole affair was plunged into cold water 
and agitated so as to cool it as quickly as possible. When the plate was sufficiently 
stiff, i.e., after some hours, the mould and sulphur were mounted on a lathe, and the 
bottom of the mould cut completely away, till only a flat surface of sulphur was left. 
This was then reversed on the face plate, and the free surface turned down till we 
reached a homogeneous part of the sulphur, or beyond that if desirable. A round 
cake of the desired size was then cut out. The first plates were then scraped to a 
surface plate ; but it soon appeared that the natural warping of the plates rendered 
tliis an unfruitful labour. Consequently our later practice was to grind the plates 
down on a sheet of glass-paper, glued to a flat (really flat) iron plate, till they were true 
to a straight edge. It was necessary to experiment at once before the plates warped 
too much. 
Soluble Plates .—These cannot be handled so freely as plates consisting partly of 
soluble and partly of insoluble sulphur, however we found the following method gave 
just as perfect results. The mould consisted of a flat plate of “ opal ” glass, which is 
very smooth and generally better than ordinary sheet glass. A ring of plaster of 
Paris was cast on such a plate and dried, then previously annealed sulphur was 
carefully heated to just above the melting-point and poured into the mould levelled 
by the glass plate. The sulphur was caused to cool as fast as convenient, in order to 
prevent the formation of internal cavities. The plate, when cold, left the glass and 
plaster easily, and it was, of course, much larger than requisite. In order to prepare 
the upper surface, the plate was held down flat on the bed of a Brown and Sharpe 
milling machine, and milled at a good speed till it was homogeneous and of the pi’oper 
thickness. This gives just as good a flat surface, or rather better, than any of our 
lathes, and leaves a very fine appearance. Owing to the rapidity of cooling, any 
