and the subterraneous Lakes of' Pur-ace. 115 



The good curate of the village of Purace thought to render 

 a great service to his parishioners, as well as to the inhabitants 

 of the town of Popayan, in causing, as he said, the chimneys 

 of the volcano to be cleaned now and then. Pie ordered the 

 Indians to take away the crust of sulphur which rises in form 

 of a dome above the crevice. This crust has acquired some- 

 times, as they affirm, a thickness of as much as four feet in 

 less than two years. It lessens without doubt the opening by 

 which the vapours of sulphurous acid escape; but it may be 

 conceived that the elastic force of these vapours is such that, 

 if the opening were entirely stopped up for some moments, 

 it would sooner break the new arch than produce commotions 

 by acting against the rocky sides of the volcano. For several 

 years the lakes, which represent in miniature the crater-lakes 

 of our extinguished volcanos, seem each to preserve the same 

 level of their luie of water; which proves that the evaporation 

 is equal to the infiltration of the waters of snow and rain : 

 but this equilibrium has not always been equally steady. 

 About the year 1790 the Boca grande caused partial inun- 

 dations. I dwell on this phaenomenon, because it seems to 

 throw some light on a problem of the geology of volca- 

 nos, which has not been sufficiently examined: I mean the 

 ejections of water and mud. At Vesuvius these ejections 

 are only apparent, and come neither from the interior of the 

 crater nor from the lateral crevices. An immense electric 

 tension manifests itself in the atmosphere which surrounds 

 the summit of the volcano at the time of great eruptions. 

 Flashes of lightning cleave the air; the aqueous vapours 

 thrown out by the crater are cooled ; thick clouds envelop the 

 summit during the continuance of this storm, confined to a 

 little space; the water descends in torrents, and is mixed with 

 the tufaceous substances which it drags with it*. These ef- 

 fects, purely meteorological, have given rise to the traditions 

 about boiling waters that issued from the crater of Vesuvius 



♦ M. cle la Condaminc {Memoires de V Academic 1754, p. 18) had already 

 cxi>ressed very precise ideas on the cause of these phaenomena, which are 

 found e(jually well explained in the Stnrin deW hwcndio dti 1737, published 

 l)v theAcademy of Naples. I saw in my last journey to Naples, (December 

 1 82-2,) the ravages caused by the torrents of water from the side of Ottajano, 

 at the foot of Vesuvius. They had transported into the plain, not only 

 mud, but masses of lava 48 feet in circumference and iJ.i feet high. See 

 the excellent description of these pha-nomena by MM. Monticelli and 

 Covelli. {Sloria del Vesuvio dcg/i nnni 1H21— 18i2;J, p. «J1— i>8.) Phil. 

 Map. vol, Ixiii. p. 46. lly the mixture of the rain and the volcanic cin- 

 ders, there is formed in the air {I.e. p.!H) a kind of pisolites with concen- 

 tric layers, which I also found on the plain of 1 lanibato, anion}; the ancient 

 ejections of the Carguaira/o. The inhabitants of the province of Quito 

 call these pisolites rmtli /lai/stoiiei. 



V 2 in 



