No. III. — Pausilipo and the Lago d'Agnano. 261 



two kinds of trees green and in full vigour in the midst of the 

 general destruction of all other plants."* 



It is to exhalations such as these, and there seem to be 

 many more dangerous than that of the Grotto del Cane, as 

 those which occur at Sinuessa, that we must ascribe several of 

 the facts mentioned by the ancients, such as those Tartarean 

 waters, — 



" Quam super baud ullae poterant impune volantes, 

 ** Tendere iter pennis ;" ' 



expressions which apply to Lake Avernus, whose very name, 

 derived from the Greek 'Ao^vo?, seems to indicate the reality of 

 the statement ; but of this we shall have occasion to speak at 

 a future period. It has been asserted that water-fowl are 

 rarely to be seen on Lake Agnano. This, however, is a mistake. 

 For the theory of the evolution of mephitic vapours I may re- 

 fer the reader to Daubeny's work on Volcanos, p. 371 — 378. 



When we advance from this lake towards the base of the 

 Solfatara, we enter a retired glen, and, soon after passing a 

 solitary cottage, reach a muddy rivulet, rolling in a bed full of 

 boulders in soft volcanic strata. This is the water of La Pis- 

 ciarella ; and, by ascending a little way, we reach the hot spring 

 itself, which is now covered with a small hut. Since I last 

 published some remarks on this spring,-|- I have collected the 

 observations of authors upon its temperature, which prove it 

 to be liable to remarkable alternations. Hamilton J declares 

 that he saw the thermometer in the spring rise to the boiling 

 point, though he admits that after rain he found it much 



* Campanie, i. 221. Daubeny (Description of Volcanos, 170,) tells us, 

 that be has been assured that this paradoxical statement is not without 

 foundation. An interesting paper on the influence of gases on plants, by 

 Drs Turner and Christison, was published in this Journal, (vol. viii. 140 ;) 

 but the ingenious authors have not alluded to the influence of natural ex- 

 halations. Sulphurous and muriatic acid gases, however, which they chiefly 

 employed, are those produced by Vesuvius, and in this view the experi- 

 ments are very interesting. Towards the close they mention, that differ- 

 ent plants are very differently affected; and it would be interesting to sub- 

 ject the pear tree to this examination. The inquiry is well worthy of far- 

 ther investigation. 



I See this Journal, No. xiv. p. 2Q5, 



X Campi Phlegrcei, folio, i. 68. 



