229 Fluids in the Cavities of Minerals. 
mencement of evaporation from the new fluid below, and 
arises from the attenuated vapour which attaches itself to the 
roof of the cavity. As the heat increases, the brown spot en- 
larges, 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 dis- 
appear, by retiring to the centre, but only to reappear with 
new lustre. During the application of heat, the circle e fgh 
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 di ions when 
they are again formed. 
When the vaporisation is so feeble as to show 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 effect is produced by the approximation of a heated 
dy. When the heat reaches the fluid, however, it makes it 
throw off fresh vapour, and the rings again a 
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 evapo- 
ration immediately causes them to disappear, till the tempera- 
ture again rises. 
When the temperature is perfectly uniform, the rings re- 
main stationary, and it is interesting to observe the first ring 
produced by the vapour swelling ont to meet the first ring at 
he margin of the fluid, and sometimes coming so near it, that 
the darkest parts of both form a broad black 
As the heat increases, the vacuity V advances to the sum- 
mit AB, and disappears at 7940, exhibiting several curious 
phenomena which we have not room to describe. One of 
these, however, is so singular that it deserves to be particu- 
larly noticed. After the vacuity V e fg h has disappeared 
entirely, a b spot comes from the summit AB, and takes 
its station in the centre of the ring of new fluidadcd. This 
brown tint sometimes rises to higher orders of colours ; but 
fluid with which it is in immediate contact. It might, how- 
ever, be a fluid substance, arising either from the decomposi- 
of the fluid itself, or from the condensation of gaseeus mat- 
within the vacuity ; though this is not very probable, from 
