FLUIDS IN MINERAL CAVITIES. 331 



even been asserted, that the rolling of thunder is 

 heard by those who approach the spot. Others 

 suppose it to be an air-volcano, like those on the 

 Caspian Sea, and that it is caused by asphaltic 

 soils like those of Mena. It is more probable, 

 however, that it is a sort of carburetted hydrogen, 

 as hydrogen gas is disengaged from the ground 

 in the same district.* 



Grand as the chemical operations are which 

 are going on in the great laboratory of Nature, 

 and alarming as their effects appear when they 

 are displayed in the terrors of the earthquake 

 and the volcano, yet they are not more wonder- 

 ful to the philosopher than the minute though 

 analogous operations which are often at work 

 near our own persons, unseen and unheeded. It 

 is not merely in the bowels of the earth that 

 highly expansive elements are imprisoned and 

 restrained, and occasionally called into tremendous 

 action by the excitation of heat and other causes. 

 Fluids and vapours of a similar character exist in 

 the very gems and precious stones which science 

 has contributed to luxury and to the arts. 



In examining with the microscope the structure 

 of mineral bodies, I discovered in the interior of 

 many of the gems thousands of cavities of various 

 forms and sizes. Some had the shape of hollow 

 and regularly formed crystals ; others possessed 

 the most irregular outline, and consisted of many 

 cavities and branches united without order, but 

 all communicating with each other. These cavi- 

 ties sometimes occurred singly, but most fre- 

 quently in groups forming strata of cavities, at 

 one time perfectly flat and at another time curved. 



* Humboldt's Personal Narrative, vol. iv. p, 254, note* 



