102 Dr Brewster on the existence of Two New Fluids 
a considerable brilliancy. The boundaries abed, e fg h , ale 
marked by a well-defined outline, and also by concentric coloured 
rings of thin plates, produced by the extreme thinness of each of 
the fluids at the edges. 
•If We now raise the temperature of the room gradually to 58°, 
we shall observe a brown spot appear in the centre of the vacuity 
V e fg h. This spot marks the visible commencement of evapora- 
tion from the new fluid below, and arises from the attenuated va- 
pour which attaches itself to the roof of the cavity. As the heat 
increases, the brown spot enlarges, and becomes very dark. It is 
then succeeded by white, and one or more rings rise in the centre 
of the vacuity. The vapour then seems to form a drop, and all 
the rings disappear, by retiring to the centre, but only to re- 
appear with new lustre. During the application of heat, the 
circle e fg h is in a state of constant contraction and dilatation, 
like the pupil of the eye when exposed to light, being always 
greatest when the rings disappear, and contracting its dimensions 
when they are again formed. 
When the vaporisation is so feeble as to shew itself only by 
a single ring of one or two tints of the second order, these tints 
may be made to disappear instantly by the slight degree of heat 
arising from a single breath upon the crystal ; and the same ef- 
fect is produced by the approximation of a heated body. When 
the heat reaches the fluid, however, it makes it throw off fresh 
vapour, and the rings again appear. 
If we put a drop of Ether upon the crystal when the rings 
are in a state of rapid play, the cold occasioned by its evapora- 
tion immediately causes them to disappear, till the temperature 
again rises. 
When the temperature is perfectly uniform, the rings remain 
stationary, and it is interesting to observe the first ring pro- 
duced by the vapour swelling out to meet the first ring at the 
margin of the fluid, and sometimes coming so near it, that the 
darkest >parts of both form a broad black band. 
As the heat increases, the vacuity V advances to the summit 
AB, and disappears at 791°, exhibiting several curious pheno- 
mena which we have not room to describe. One of these, how- 
ever, is so singular that it deserves to be particularly noticed. 
After the vacuity V ef gh has disappeared entirely, a brown 
