STIFLING CAVERNS. 77 



carbonic acid, and the numerous jets in the Eifel, 

 taken altogether, are said, on a rough reckoning, 

 to throw up about five million cubic feet, or about 

 three hundred tons of gas a-day. To get as much 

 of tins gas by burning wood, it would be necessary 

 to use up daily twelve thousand cubic feet of 

 hewn wood. 



However, these enormous quantities of unbreath- 

 able gas are very quickly dispersed, as they escape 

 into the open atmosphere, and have no perceptible 

 effect on the condition of the air. In places, that 

 are more shut in, the carbonic acid collects about 

 the ground, because it is heavier than the air, and 

 for this reason, if not mixed with the air, falls 

 through it like water. In caves and in deep hol- 

 lows, in which there is always a plentiful and unin- 

 terrupted discharge of carbonic acid, such as the 

 famous Grotto del Cane near the Lake of Ag- 

 nano, a few miles from Naples, and the Dunst- 

 holile near Pyrmont, the ground is always covered 

 to a depth of some feet with this heavy gas. If a 

 large soap bubble is allowed to fall through the 

 air of these caverns, it begins, on reaching the top 

 of the layer of carbonic acid, to sway up and down, 

 like a piece of wood which is let fall from some 

 height into the water, and than it quietly floats 

 upon the gas until it bursts. Animals placed in 

 the vapour must be stifled, unless their heads reach 

 above its upper limit. 



