in the Cavities of Minerals. 105 
tensions are marked by a very beautiful optical phenomenon. 
When the fluid has extended itself into a thin plate, it ceases 
to reflect light, like the most attenuated part of the soap-bubble, 
and when it is again accumulated into a thicker drop, it is co- 
vered with all the coloured rings of thin plates. When one of 
the drops of fluid is very minute and perfectly circular, it re- 
sembles, in the most accurate manner, the small drops which 
pass from the vacuity, and which have been described in the 
preceding section. 
After performing these motions, which sometimes last for 
ten or twelve minutes, the fluid suddenly disappears, and leaves 
behind it a residue of minute and separate particles, which are 
opaque by reflected, but transparent by transmitted light. 
Upon examining this residue with a single microscope held in 
the hand, it again started into a fluid state, and extended and 
contracted itself as before. This was owing to the moisture of 
the hand ; and our author could at any time revive the indurated 
substance, by the approach of a moist body. A portion of the 
fluid, which was taken out of a cavity twenty days ago, is 
still capable of being restored to a fluid state by moisture. 
This portion was shewn to an eminent naturalist, the Reverend 
Dr Fleming of Flisk, who remarked, that, had he observed it 
accidentally, he would have ascribed its apparent vitality to the 
movements of some of the animals of the genus Planaria. 
After the cavity has remained open for one or two days, the 
second fluid comes out of it, and hardens very speedily into a 
yellowish resinous-looking substance, which is perfectly trans- 
parent. This substance absorbs moisture, but with less avidity 
than the other. It is not volatilized by heat. It is not soluble 
in water or alcohol ; but it is rapidly dissolved with efferves- 
cence by the sulphuric acid. The nitric and muriatic acids also 
dissolve it. 
The residue of the first fluid is volatilized by heat ; and it is 
also dissolved, but without effervescence, by the sulphuric, the 
nitric, and the muriatic acids. After standing some time, both 
ihese substances acquire a brilliant lustre, as if some metallic 
body entered into their composition. 
It would be improper to conclude this paper, says our author, 
.without noticing the relations which arc supposed to subsist be- 
