170 LIGHT SCIENCE FOR LEISURE HOURS. 



were eruptions the sole cause of danger. Poisonous 

 vapours, such as are emitted by volcanic craters after 

 eruption, appear to have exhaled, at times, from ex- 

 tensive tracts on Pithecusa, and thus to have rendered 

 the island uninhabitable. 



Still nearer to Vesuvius lay the celebrated Lake 

 Avernus. The name Avernus is said to be a corrup- 

 tion of the Greek word Aornos, signifying ' without 

 birds,' the poisonous exhalations from the waters of 

 the lake destroying all birds which attempted to fly over 

 its surface. Doubt has been thrown on the destructive 

 properties assigned by the ancients to the vapours 

 ascending from Avernus. The lake is now a healthy 

 and agreeable neighbourhood, frequented, says Hum- 

 boldt, by many kinds of birds, which suffer no injury 

 whatever even when they skim the very surface of the 

 water. Yet there can be little doubt that Avernus 

 hides the outlet of an extinct volcano ; and long after 

 this volcano had become inactive, the lake which con- 

 cealed its site 'may have deserved the appellation of 

 " atri janua Ditis," emitting, perhaps, gases as de- 

 structive of animal life as those suffocating vapours 

 given out by Lake Quilotoa, in Quito, in 1797, by 

 which whole herds of cattle were killed on its shores, 

 or as those deleterious emanations which annihilated 

 all the cattle in the island of Lancerote, one of the 

 Canaries, in 1730.' 



While Ischia was in full activity, not only was 

 Vesuvius quiescent, but even Etna seemed to be 

 gradually expiring, so that Seneca ranks this volcano 



