and aeriform matter in cavities found in certain crystals. 373 
I endeavoured to find some calcareous secondary rocks, or 
crystals belonging to them, containing cavities, on which 
experiments of the same kind might be made ; but in a num- 
ber of trials I have as yet found none impermeable to the 
atmosphere; and the cavities of such, when bored, are al- 
ways found to contain atmospheric air in a common state of 
density. 
I was surprised to find that this was the case even with 
cavities in calcareous spar in the centre of a lime-stone rock ; 
yet these cavities which contained atmospheric air did not fill 
with water when the stone was placed in water under an ex- 
hausted receiver. When however it was dry, and placed in 
a receiver alternately exhausted and filled with hydrogene, 
the air that was produced by piercing the cavities, was found 
mixed with hydrogene; proving that the substance of the 
stone was permeable to elastic fluid. 
I hope soon to be able to make further researches on this 
subject ; but in reasoning upon the vacuum, or rarefied state 
of the aeriform matter in the cavities of these rock crystals 
and chalcedonies, it appears difficult to account for the phe- 
nomenon, except on the supposition of their being formed at 
a higher temperature than that now belonging to the surface 
of the globe; and the most probable supposition seems to be, 
that the water and the silica were in chemical union, and sepa- 
rated from each other by cooling. 
Water in the temperature of the arctic winter is constantly 
a crystallized body. As a fluid, its solvent powers are in- 
creased as its heat becomes higher, and when elastic, the 
density of its vapour is exalted in proportion to its heat ; so 
that an atmosphere of steam, supplied from an indefinite 
