98 Dr Brewster on the Existence of Two New Fluids 
the brilliancy of total reflexion, the separating surface of the 
new fluid ABCD, and the cavity, exhibited a faint grey tint, 
while the surrounding portions of the solid topaz were compa- 
ratively black. The variations which the vacuity V undergoes 
by heat are now finely seen, and, at a temperature of 80 a , it va- 
nishes in a brilliant speck, leaving the whole of the cavity 
ABCD of the same uniform tint as in Fig. 4. 
The phenomena now described are not so distinctly seen 
when the stratum m n is deeply seated beneath the surface of 
the topaz, in consequence of the duplication and overlapping of 
tlie "mages formed by double refraction. 
This inconvenience, however, may be nearly removed by 
making the plate of topaz very thin ; or it may be entirely re- 
medied, in plates of any size, by making the incident rays RS 
pass along one of the resultant axes of the topaz, while the re- 
flected rays SL pass along the other resultant axis. 
In order to compare the angle at which total reflexion took 
place at the upper surfaces of the fluid and cavity, with that 
which would have taken place had the fluid been water, a drop 
of water was placed on part of the lower surface of the plate 
AB, Fig. 2., and it was found that the light reflected at the 
same angle of incidence, was much more brilliant from the se- 
parating surface of the new fluid and the cavity, than from the 
separating surface of the topaz and the water, a result which 
indicated, in the most unequivocal manner, that the new fluid 
had a refractive power inferior to water , and that it differed in 
this respect from every other known fluid. 
With a specimen of Amethyst , our author was enabled to de- 
termine, that the refractive power of the expansible fluid was 
about 1.211. 
In the remainder of this section, the author describes analo- 
gous phenomena in Cymophane, Quartz- Crystals from Quebec, 
and Amethyst from Siberia, the last of which is a specimen of 
very great interest from the cabinet of Mr Allan. In these 
crystals the vacuities reappear as follows : 
Cymophane, - - 83°| Fahrenheit. 
Quartz from Quebec, different cavities in the same 
specimen, - - » 76° 80° 125° 
Amethyst from Siberia, • - - 83° § 
