346 
MINERALS. 
from that dreadful situation, though a little re- 
flection might have shown her it was then too late. 
But nothing could deter her; she ventured forward, 
and had scarcely touched him with her hand when 
the damp prevailed, and the misguided, but faithful 
creature, fell dead by his side. 
This fatal vapour, though as invisible as com- 
mon air, is much heavier, and therefore always 
floats near the surface of the earth, or immediately 
upon any thing from whence it may be extricated. 
This tendency of carbonic acid gas to occupy the 
lowest place, is particularly evident in the famous 
grotto del Cane, near the lake d’ Agnano, in the 
neighbourhood of Naples. This natural curiosity 
is said to be situated on the side of a hill, and that, 
for the amusement of travellers, persons attend at 
the cave with dogs, which in their turns suffer a 
temporary death, from which they recover to the 
astonishment of the spectators. We are assured 
that the poor animals always seem sensible of the 
approach of a stranger, and endeavour to get out 
of the way : however, their attempts being per- 
ceived, they are taken and brought to the grotto, 
the noxious effects of which they have so fre- 
quently experienced. Upon entering this place, 
which is a little cave, or hole, dug into the hill, 
about eight feet high and twelve feet long, the ob- 
server can see no visible marks of its pestilential 
vapour, only to about a foot from the bottom 
the wall seems to be tinged with a colour resem- 
bling that which is given to stagnant w aters. When 
