in the Cavities of Minerals. 9? 
This extraordinary result proved beyond a doubt, that the 
substance contained in the cavity was a new fluid, differing 
from all known fluids in its high expansibility, and resembling 
in this respect a gaseous more than a fluid body. 
In order to confirm this result, our author was desirous of 
examining the other physical properties of this remarkable sub- 
stance. He noticed, in the deep cavities especially, the singular 
volubility of the fluid, and its slight adherence to the sides of 
the cavity, as indicated by the motion of the vacuity V . In 
small cavities containing water, the adhesion of the fluid to the 
stone is so strong, that the air-bubble moves with extreme diffi- 
culty, and even when very large, it often changes its place by 
starts, and remains stationary at the bottom, or in the middle of 
the cavity. In the present case, however, the vacuity moved 
about with great facility and in the cavity, 35 th of an inch 
long, by y\th and of an inch wide and deep, the slightest 
tap of the finger on the microscope caused the air-bubble to 
tremble and oscillate in this microscopic level. Hence the new 
fluid is distinguished by a second physical property, no less re- 
markable than the first. 
Although no doubt was now entertained of the accuracy of 
the conclusion, that the fluid was a new one, yet it was con- 
ceived possible to obtain an approximate measure of its refrac- 
tive power, and thus to put its novelty beyond the reach of a 
doubt. In order to do this, it became necessary to observe the 
manner in which the total reflexion of the upper surface of the 
cavity was modified by the contact of the fluid, and to measure 
the angle at which total reflexion was effected, by the separating 
surface of the fluid and the solid. For this purpose our author 
took a plate of topaz AB, Fig. 2., with a stratum of cavities 
m w, perfectly parallel to the natural surface of the plate. Fie 
then placed upon each surface the rectangular prisms ABC, 
ABD, and introduced between them a thin film of oil of cassia. 
Rays of light RS, RS were then allowed to fall upon the stra- 
tum of the cavities m n, so that the rays reflected from the up- 
per surface of the cavity could be examined by a microscope, 
whose object lens is LL. Upon making this arrangement, the 
stratum of cavities was seen in the most beautiful manner. The 
vacuity V, Fig. 3, of a cavity seen in this way, shone with all 
VOL. IX. NO. 17. JULY 1823. G 
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