MINERAL CAVITIES AND THEIR CONTENTS. 
By W. N. HARTLEY, F.R.S.E. 
KING S COLLEGE, LONDON. 
^ ^ HERE are few enquiries in natural science more calculated 
to awaken curiosity than those relating to the changes 
which the matter composing the surface of the globe has under- 
gone. The imagination is excited by the magnitude of the 
operations, by the obscurity of the phenomena, and the remote- 
ness of the time at which they occurred ; and all the intel- 
lectual powers are required to be brought into activity to find 
facts or analogies, or to institute experiments by which they 
may be referred to 6 known causes.’ ” 
The foregoing passage was written by Sir Humphry Davy, 
when President of the Royal Society, in the year 1822, and it 
was certainly never more full of truth than at the present day, 
notwithstanding the advances in physics, chemistry, and 
geology which have since been made. 
It formed the preface to a paper on the aqueous and gaseous 
contents of the fluid-cavities of minerals. These cavities, which 
are most commonly found of large size in rock crystal, were 
relieved of their contents by boring holes in them with a dia- 
mond, the specimens being submerged in mercury or in water. 
Wires were inserted to displace the gases, and capillary tubes 
to draw off the liquid, much in the same way that vaccine 
matter is drawn off from a vesicle. The liquid was found to 
be in almost every case either pure water or some dilute saline 
solution containing a sulphate or a chloride of sodium or potas- 
sium. The gaseous portion was nitrogen, generally in a rarefied 
condition, but having in one case a tension equal to ten times 
that of the atmosphere. It was undoubtedly proved that the 
brown viscid liquid in one crystal was mineral naphtha. 
Sir David Brewster, in the following year, announced the 
discovery of a remarkable liquid — or, as he then said, two liquids 
