201 
ON THE SOLEATARA, ETC., OF NAPLES. 
than half an inch in diameter. Into this I carefully inserted 
the bulb of a thermometer, and found the mercury rise slowly. 
By dint of a little gentle force I succeeded in inserting the 
thermometer tube several inches through soft powdery earth, 
and found the temperature continue to rise till it showed 
60° C. 
There is a very curious result obtained at all these fumaroles. 
The vapour they emit is chiefly steam, with a small but per- 
ceptible proportion of gases, generally acid. The gases con- 
sist partly of sulphuretted hydrogen, as is evident by the smell. 
Occasionally there is sulphurous acid in the vapour. There is 
also carbonic acid, which, under certain circumstances, seems 
very abundant. In the grotto del Oane this is the principal 
gas, and it comes off readily at a temperature many degrees 
above the mean temperature of the air ; so that the cavern in 
which the poor dogs are stifled for the amusement of tourists 
is itself a warm-air bath. Under ordinary circumstances the 
vapour is just visible; but when a lighted or smouldering 
match, the burning end of a cigar, or a piece of burning tinder 
is brought near and in contact with the issuing vapour, this 
vapour becomes visible, and fumes to an extent altogether in- 
consistent with the amount of burning material. At the great 
vent of the Solfatara, where the steam is always visible, a few 
twigs burnt at the entrance of the vent will produce a volume 
of rolling clouds of smoke rising to a considerable height and 
continuing for many minutes. As soon as the effect has sub- 
sided, it may be reproduced by the same means. 
Fumaroles of the same nature I found to exist in great abun- 
dance on the walls of the craters formed during the great erup- 
tion of last year on the eastern slopes of Etna. They then 
occasionally emitted chlorine in sensible quantity, and the same 
may be remarked in the interior of the crater of Vesuvius, which 
was in a state of semi-activity at the time of my visit. They 
are familiar enough in all active volcanic districts, and are the 
small vents relieving the superficial and nearly exhausted 
energy, just as the great throats of Etna and Vesuvius belch 
out steam and gases, with a few scorise, from time to time, 
when the pressure in the larger and deeper cavities they are 
connected with becomes too great in the intervals between 
important eruptions. 
But these gentle and long-enduring indications of subter- 
ranean fire are not without some result on the rocks adjacent. 
In the most recent cases, such as the cone of eruption of the 
last year just alluded to, the work is seen to be very rapid, and 
may easily be traced, though, of course, it is partial and confined 
to the part of the cone through which the gases have passed. 
We there see the nature and amount of the chemical action, 
VOL. v. — no. xix. . p 
