104 Dr Brewster on the Existence of Tzvo New Fluids 
n o, which was now uniform, was not that of total reflexion, nor 
yet that of the expanded fluid, but of an intermediate intensity, 
corresponding to that of a dense vapour, with a refractive power 
much lower than 1.211, 
There is another set of phenomena of exquisite beauty to an 
optical observer, which seem to arise either from the decompo- 
sition of the fluid, or the condensation of gaseous matter in the 
vacuity. 
When heat is applied to the cavity, the new fluid has its sur- 
face in a state of constant agitation, resembling, in the closest 
manner, a surface into which a fluid is discharging itself by 
drops. When the vacuity is just filled up, one or more drops 
quit the point where the vacuity disappeared, and pass along 
the surface of the cavity, like a drop of oil adhering to it m 
close contact, and never mixing with the fluid. Each of these 
drops begins in a short time to spread circularly, and to exhibit 
within its disc an immense number of close coloured rings. By 
slow cooling the drops become thinner, and the rings less nu 
merous, and more completely displayed, till they entirely dis- 
appear at a particular temperature. When the cooling is ef- 
fected quickly, the matter which composes the thin plate that 
exhibits the rings, discharges itself* rapidly in gaseous bubbles. 
Sect. VI. — On the Phenomena of the two New Fluids when ta- 
ken out of the Cavities. 
Erom the extreme minuteness of the cavities in topaz, our 
author’s first attempts to extract the fluid were not attended 
with much success ; but he at last fell upon a method by which 
he has opened more than a hundred cavities. 
When the most expansible of the new fluids first runs from 
the cavity upon the surface of the topaz, it neither remains 
still, like the fixed oils, nor disappears, like evaporable fluids. 
Under the influence, no doubt, of heat and moisture, it is in a 
state of constant motion, now spreading itself in a thin plate 
over a large surface, and now contracting itself into a deeper 
and much less extended drop *. These contractions and ex- 
* A round hemispherical drop often stretches itself into a plane of more than 
twelve times its original area. 
