§40. GROTTO DEL CANE. 81 



to pieces between the fingers. I have already shown yon 

 masses of pebbles held together by calcareous spar, from 

 the same locality ; we have, therefore, examples in that 

 ancient bed, both of the conservative and disintegrating 

 effects of carbonic acid— cementing the loose beach into 

 solid blocks by calcareous depositions, and when in a 

 gaseous state, or combined with water, dissolving the 

 granite by its action on the felspar. 



40. Carbonic acid gas in caves. — The escape of car- 

 bonic acid gas through fissures, into mines, wells, and 

 caverns, is of frequent occurrence, and as the specific gravity 

 of this gas is greater than that of atmospheric air, it forms an 

 invisible pool at the bottom of these cavities, and its presence 

 is seldom suspected, till shown by its deleterious effects ; 

 hence fatal accidents often happen to well-diggers and 

 excavators from this cause ;* it is called choke-damp by 

 miners. 



The Grotto del Cane, near Puzzuoli, four leagues from 

 Naples, has for centuries been celebrated on account of 

 the constant evolution, from fissures in the rock, of car- 

 bonic acid gas in combination with much aqueous vapour, 

 which is condensed by the coldness of the external at- 

 mosphere. The floor of the cavern being lower than the 

 entrance, the gas is spread over the bottom like a pool of 

 water, and the upper part is free from any noxious vapour ; 

 the suffocating effects of the carbonic acid is, therefore, 

 not felt by any creature whose organs of respiration are 

 above the level of this mephitic lake ; but if a dog, or other 

 small animal, enters the cave, it instantly falls senseless, 

 and would expire if not speedily removed : the name 

 of the cave is derived from the experiment being often 

 made on dogs, for the amusement of visitors, f It is im- 



* A few pounds of quicklime thrown into a well, quickly absorbs 

 the carbonic acid gas. 



f See Sandys' Travels ; London, 1637. 

 G 



