and aeriform matter in cavities found in certain crystals. 371 
sured ; but there seemed to be nearly the same relation be- 
tween the space filled by fluid, and that containing aeriform 
matter ; and in all of them the contraction of the globule of 
aeriform matter was evidently greater, and in one instance 
to less than — of its original bulk. 
The fourth crystal that I experimented upon was of un- 
known locality ; but l have reason to believe that it was 
from Guanaxuato, in Mexico, as it strongly resembled some 
that Mr. Heuland showed me from that place. The cavity 
in it was extremely small, and when pierced into, under dis- 
tilled water, the globule of gas, from being •§- of an inch in 
diameter,* diminished so as to be less than ~ ; so that its rare- 
faction was much greater in this than in the other instances; 
the water was too small in quantity to be minutely examined ; 
it seemed to be nearly pure, producing a cloudiness barely per- 
ceptible in solutions of nitrate of silver and muriate of baryta. 
It was an interesting point to ascertain whether the same 
circumstances occurred in productions found in rocks which 
have been generally considered as of igneous origin, such as 
the basaltic rocks in the neighbourhood of Vicenza, the chal- 
cedonies of which so often afford included water. I found it 
much more easy to make experiments of this kind, and to 
procure specimens, which were abundantly supplied to me 
from the same sources as those I have just referred to; and 
though some of these specimens proved to be permeable to 
the atmosphere, and to have been filled with water artificially, 
* I have not thought it necessary to refer to the heights of the barometer and 
thermometer in these experiments, as it is impossible to gain any other than general 
results, upon quantities in which differences arising from atmospheric temperature 
and pressure, would be quite unappreciable. 
