256 



PHYSICAL GEOGRAPHY. 



in are speedily suffocated ; and it takes its name from the dogs which are placed in it 

 by way of experiment. The cave and its neighbourhood appear to have undergone 



Grotto del Cane. 



some considerable changes ; for, from Pliny's reference to the mephitic gas, it would 

 seem to have been fatal to human life, which is not possible now, owing to the small 

 height of the stratum, unless an individual threw himself upon the surface of the 

 floor. According to some ancient accounts, bubbles were constant upon the lake Agnano 

 close by, occasioned by the escape of gas, of which there is no appearance now ; so that 

 the quantity of deadly air exhaled has been mucK reduced, if dependence may be placed 

 upon these authorities. A similar instance, on a much larger scale, is seen in the crater 

 of the extinct volcano of St. Leger, or of Neyruc in the south of France, on the banks of 

 the Ardeche, amid the great number of volcanic remains of that region. This crater 

 exhibits a cultivated, and in part inhabited, district, which is surrounded like an amphi- 

 theatre by the ancient walls of volcanic debris. Its soil is one vast sieve for the ascent 

 of carbonic acid. Perforations have been made in it to facilitate the emission of the gas, 

 and guide it from the fields, to which its contact is very injurious. The height of the 

 bed of gas, over the ground of these holes, has been found to be, in the most favourable 

 circumstances, about one foot and a half. Changes of weather have the most important 

 influence on it ; and in violent rains the whole mass of gas is absorbed. The quantity of 

 this gas, which issues from the soil of the whole neighbourhood, has a very striking 

 influence on the health of the inhabitants who work in these fields ; and if the proprietors 

 do not yearly clear out these perforations, their harvest is lost by means of the poisonous 

 vapours. Another example occurs near Pyrmont, where there is a cavern of mephitic 

 gas, named Dunsthohle, which exhibits the same phenomena as the dog grotto near 

 Naples ; and of a kindred kind is the extraordinary valley in the island of Java, called 

 by the natives Guwo-upas, poisoned valley, which is without vegetation, and strewed 

 with the skeletons of human beings, quadrupeds, and birds, being generally half-filled 

 with a noxious gas which destroys life in a few minutes. 



In addition to the cavities which are the handiwork of nature, immense subterranean 



