Sir R. I. Miu'chison on the Vents of Hot Vapour m Tuscany. 55 



iug up large globules from its bubbling surface, the heated matter is 

 ever making au effort to overflow the rim of the little crater*. Where- 

 ever the subterranean vapour escapes from a crack more or less ver- 

 tical, and which presents no impediment, the muddy liquid rapidly 

 attains its maximum heat, which is so intense, that, as M. Lardarel, 

 jun., informed me, no instrument had yet been made to measure 

 accurately the maximum heat beneath the surfacef . It is probable 

 that no active volcano exhibits greater heat at any point where a test 

 can be apjjlied. Twenty-four hours of this process suffice to satu- 

 rate the bubbling mixture with boracic acid, and the stuff is then run 

 off into flat cisterns at a lower level. The fluid is there reduced to a 

 third of its volume by evaporation, hastened by the hot vapour being 

 conveyed in tubes beneath the salt pans, and thus saving the former 

 cost of a great consumption of fuel. After the addition of soda, the 

 desiccation proceeds, and crystals of boracic acid are formed. The 

 violence with which the hot gas issues from any crack, provided it 

 be vertical, is such, that if stones of some weight are thrown upon a 

 narrow gush of it, they are heaved up several feet iuto the air, and 

 heavy flagstones are recpnred to repress the eruptive agent, and con- 

 duct a current of it down to the drying houses and pans. 



It is highly interesting to compare the present issues of the hot 

 gases and the forms of the lagoni, as arranged and controlled by man, 

 with their natural appearance upwards of eighty years ago, when ex- 

 amined and described by Targioni Tozzetti. The thick white and 

 hot sulphureous clouds rising by fits and starts, — the occasional jets 

 of liquid rising from the boiling cauldrons, — the large and brilliant 

 globules as they burst, — the circular shapes of the lagoni, — the in- 

 crustations of sulphur on their banks, — the crackling of the light, 

 pumiceous and hollow ground under foot, — the conversion of the 

 contiguous alberese limestone, then considered a primary rock, into 

 a fftrinaceous or mealy state, — the fumes serving as a true barometer 

 to the neighbourhood J, — the perfect salubrity of the spot to ani- 

 mals, though plants are there withered and blasted ; — all these phse- 

 nomena are nearly the same now as when our predecessor described 

 them. But, on the other hand, some of the former phsenomena are 

 no longer recognizable. There is no more a comitless number of 

 lagoni. We cannot now, as Targioni did, look into dry cavities from 

 which hot blasts only issued, with noises as if from a hundred bellows, 

 and distinguish them from those holes which were then naturally 

 filled with boiling muddy water, discharging gas ; for the mauufac- 



* In his description of the Hawaii Islands of tlie Pacific, Mr. Dana accounts for 

 tlie absence of active eruption and projection of materials into the atmosphere, 

 by the great dimensions of the chief crater, in wliicli the molten matter having a 

 very wide vent, undulates with httlc or no noise, and quietly overllows its lip from 

 time to time. 



t Targioni Tozzetti, the old writer, does not pretend to have ascertained the 

 extreme heat of the vapour ; but Professor PiUa, on what authority 1 know not, 

 places it at 1 10" Reaumur. 



i In rainy weather, or when change is coming on, the vapours cling to the earth 

 •with incrcatied subterranean noise, and in settled tine weather they rise to a great 

 altitude. 



