158 DR. DAUBENY ON THE ERUPTION OF VESUVIUS. 



assistance of mining implements. The surface temperature of the lava was indeed not 

 high enough to melt lead, but one of Daniell's pyrometers, with an iron rod, left in 

 contact with it for a few minutes, rose more than one degree. It is probable, however, 

 that I had failed in this instance in obtaining the full temperature of the superficies ; 

 for nearly a month afterwards, that is, late in December, after much rain had fallen, 

 I removed the scoriae from another contiguous portion of the bed, and found that a 

 thermometer placed upon it, and merely covered over with a little sand, rose to 390° 

 of Fahrenheit. From the cracks and cavities of this lava much aqueous vapour was 

 still exhaling, and this I succeeded in condensing by means of the same apparatus 

 which I had employed within the crater. 



The condensed steam on examination was found to be impregnated, not only with 

 free muriatic acid, but also with muriate of ammonia ; and as the vapours were col- 

 lected at the very point of their escape from the lava, it can hardly be doubted, that 

 the latter salt is actually present ready formed within the cavities of the stone, 

 having been emitted from the volcano along with the lava itself. The scoriae which 

 cover the surface of the bed are in some places quite incrusted over with beautiful 

 crystals of this sort, some of which are perfectly white, whilst others are of an orange- 

 yellow colour. The latter appears to be owing to the presence of oxide of iron. The 

 quantity of sal ammoniac was large enough to repay the trouble of collecting, and 

 much of it was carried away by the peasants to Naples to be sold to the workers in 

 brass and jewellery. Muriate of soda was also common amongst the substances in- 

 crusting the scoriae, but none could be detected in the vapour emitted at the period 

 of my examination. 



The very same substances I found to be exhaled, during my stay at Naples, from 

 the crater of the Solfatara of Puzzuoli, which differed however in one respect, namely, 

 in that of emitting much sulphuretted hydrogen, from which the vapours of Vesuvius 

 were entirely free. Hence the film of minute crystals of sulphur which forms on the 

 surface of the rock of the Solfatara in the immediate neighbourhood in the Fuma- 

 roles ; whilst from the Vesuvian lava no sulphur in any form was given out at the 

 time of my visit, although amongst the sublimations produced at an earlier stage of 

 the operations, crystals of this body were not uncommon. 



The disengagement of such principles, as water, muriatic acid, and sal ammoniac 

 from a semi-extinct volcano like the Solfatara, is much more intelligible, than its escape 

 from the substance of a bed of lava which has already undergone consolidation. 



In the latter instance, what is the condition in which we are to imagine such bo- 

 dies to exist in the heart of the mass ? Not certainly in a state of chemical union 

 with its constituents, for we cannot conceive any affinity inherent in salts of ammonia 

 or soda for the earthy ingredients of a bed of lava; neither, if in combination with 

 them, would they be separated, as the latter parted with its heat. 



It seems necessary to suppose, that these bodies, being thrown up at the time of 

 the eruption from the interior of the volcano, became entangled within the interstices 



