238 DESCRIPTION OF A NITRE CAVE. 



to rush into the mouth of the cave; and whenever during the 

 winter, any portion of air in the main avenue, where the pas- 

 sage leads oflj is accidentally heated by fires, or by carrying 

 torches or lamps through the cave, as this heated air cannot 

 escape by the mouth of the cave (for the arch descends to- 

 wards the mouth) it ascends into this chamber, and rises to the 

 ceiling, where it must remain until the external air and that 

 in the passage and avenue acquire a higher temperature than 

 the air in the chamber. This chamber then is constructed pre- 

 cisely upon the principles of the Russian vapour bath, so mi- 

 nutely described by count Rumford. 



During the winter season, the walls and floor of this cave re- 

 main perfectly dry; but in summer, innumerable drops of 

 water collect upon the rocks and trickle down upon the floor 

 which sometimes becomes as moist as a bed of mortar. This 

 is particularly the case during very hot weather when the at- 

 mosphere is loaded with vapours. I collected a quantity of 

 the liquid condensed upon the rocks, and found that it possess- 

 ed the same properties with the liquor obtained by lixiviating 

 the earth on the floor of the cave. It would appear from this 

 fact, that the nitric acid is formed in the cave and is conden- 

 sed upon the rocks, the lime of which it dissolves. But in 

 what manner this nitric acid is formed, I confess myself whol- 

 ly ignorant, as there are no substances in a state of putrefaction 

 within the cave which could yield the requisite supply of ni- 

 trogene gas. It is to be remarked, that the whole of the wa- 

 ter condensed upon the rocks, does not taste of the nitrate of 

 lime. A great part of it is quite insipid, although dropping 

 upon earth which is rich in nitre, and many parts of the cavern 

 have been found so completely filled with clay, that it is not 

 easy to conjecture how it was possible for atmospheric air to 

 reach them, and this clay too, is strongly impregnated with 

 nitrate of lime. The depth of the earth on the floor of this 

 cave has never yet been ascertained. In some places the work- 

 men have dug down fifteen feet and the earth even at that 

 depth still contains nitre. It is commonly supposed that through- 

 out the cave, every bushel of earth contains at least one pound 

 of nitre. In many places it will yield more than two pounds 



