142 Professor Silliman's Geological Notes. 



now containing Lake Agnano, since that has not been active 

 in the historic period. 



In another cavity excavated in the hill, near to the Grotto 

 del Cane, is an abundant flow of carbonic acid, accompanied, 

 as is said, by ammonia ; but we did not perceive any odour 

 of this gas, nor any deposit of carbonate of ammonia or of 

 sal ammoniac on the walls of the place. 



2. Suljphur Lake of the Campagna, near Tivoli. 



The celebrated sulphur lake of the Campagna, near Tivoli, 

 the Aquas Albulae of the ancients, still retains its interest to 

 the geologist, as the most remarkable of all the thermal 

 sources of Italy. Whether we regard the constancy of the 

 flow from it, both of water and of gas, the area of the lake, or 

 the important part which it and similar craters appear to 

 have played in the formation of the travertine of the Cam- 

 pagna — we must admit that it is worthy of attentive consi- 

 deration. 



In the days of Father Kircher, this lake was described as 

 being over one mile in circuit. Now, the lake is not over 

 500 or 600 feet in diameter, and its area is constantly dimi- 

 nishing. The baths of Agrippa, which are now at a short 

 distance from the shore, shew the extent to which this con- 

 traction has taken place in that direction, since they were 

 erected in the waters of the lake, which were much esteemed 

 by the ancient Romans for their hygienic efi'ects. These 

 baths were frequented by Augustus, and enlarged by Zeno- 

 bia ; the latter spent her Homan servitude in Tivoli ; and in 

 allusion to this, they are still called " Bagni della Regina.'' 

 At present, the waters are conducted from this lake by a 

 canal artificially cut in the travertine by Cardinal Ippolito 

 d'Este, former Governor of Tivoli, for a distance of two 

 miles. This canal is nine feet wide and four feet deep, and 

 there is a constant discharge of the surplus waters through 

 it in a very rapid current, the volume of which is remarkable 

 when we notice the small size of the source. 



The strong odour of sulphuretted hydrogen announces the 

 existence of this lake a long time before the traveller reaches 

 it. The water was examined by Sir Humphrey Davy, who 



