408 REPORT— 1890. 
temperature was 60° C. The distance of this tunnel from the Boeca 
Grande is 860 m. which equals a diminution of 0°11186° per metre; and 
as the sewer will pass at 740m. from the same point of maximum 
temperature, we consequently should find in it a temperature of 73:40°. 
In one of my excursions I found a fumarole, about a hundred metres 
to the S.E. of Cariati, from which was issuing a current of vapour with 
slight blowing, sufficiently to be sensible at half a metre distant from the 
opening. With an outside temperature of 15° C. I found the tempera- 
ture of the issuing vapour 30° C. at the mouth of the fumarole. This. 
fumarole occurs at a much greater height than the level of the sewer, and 
slightly to the north. 
Practical results ——All these facts would indicate that the sewer will 
have to traverse very hot rocks with abundant humidity, because the 
bottom level is a very short distance above the drainage level, as 
indicated by the neighbouring wells to be water at a very high tempera- 
ture. As the conditions require that the tunnel shall in this neighbour- 
hood be of considerable length without laterai openings, some doubt 
must be raised in our minds whether, should there be an abundant 
escape of vapour under pressure, artificial ventilation could render the 
continuance of the work practicable. 
3. Exhalation of poisonous or deleterious gases or vapours.—We have 
seen that by the compacter rocks, and by the interstices of the friable 
rocks, as the gravelly pozzolana and the trachytic scorie, we might 
encounter exhalations of very high temperature. But these same exhala- 
tions might contain not only aqueous vapour, but also other substances. 
At the Solfatara the vapour, according to Breislak and many other 
investigators, contains, besides the aqueous components, much sulphurous 
acid, ? sulphuretted hydrogen, arsenic sulphides, arseniuretted hydrogen, 
carbonic acid, and traces of ammonia. 
We find these compounds most abundant at the Bocca Grande, and 
in proportion to the distance therefrom the exhalations gradually lose 
most of the other compound, the aqueous vapour only remaining. 
In studying the region where these exhalations decompose the rocks, 
we find that it is limited principally to a line directed from N.W. to 
S.E.; which line, if prolonged 8.H. of the crater of the Solfatara and the 
outer walls on the same side, would pass by Monte Dolce, where a fissure 
exists in the yellow tuff. At one time it was possible to penetrate into 
the cleft for some 10 to 15m. from the Pozzuoli road, and with more 
limited dimensions even into the mountain. From this cleft there 
constantly issued hot vapours, and all the surface of the tuff walls 
was covered by crystals of gypsum, which would seem to indicate that if 
not at present, at least a short time ago, the vapours contained compounds. 
of sulphur, which becoming oxidised into sulphuric acid had attacked 
the alkaline and earthy constituents and formed fresh compounds, of 
which the gypsum has remained in consequence of its feeble solubility. 
This cleft has lately been filled up and built over in making the tunnel 
of the Cumana Railway. 
Practical results —Besides the high temperature the exhalations into 
the workings might not only consist of simple steam, but may contain 
different gases or vapours of an irritant or irrespirable nature, which 
could be got rid of only by powerful ventilation. 
Besides this difficulty we know that in the vicinity of the Solfatara 
these exhalations rapidly decompose every mineral substance, and there- 
