522 
THE GEOLOGIST. 
peculiar odour, whicli has sometimes been attributed to sulphuretted 
hj-drogen, or arsenuretted hydrogen ; but, if I mistake not. Dr. Percy 
has satisfactorily proved that in many black limestones no sulphuretted 
hydrogen is contained ; and it appears more probable that this odour is 
of organic nature, and due to bituminous substances contained in the 
limestones we speak of. 
Every schoolboy is aware that when two pieces of quartz are rubbed 
smartly together in the dark, they produce a sort of electric light, or 
phosphorescence, which is, to a certain extent, a reproduction en petit 
of the grand phenomenon of sheet- lightning. A strong odour is 
emitted at the same time, and this, although I have made no actual 
experiment to prove it, I believe to be due to ozone — a peculiar con- 
dition or state of oxygen gas, which, though quite devoid of smell in 
its natural state, becomes, under the influence of the electric spark, and 
in various other circumstances, remarkably odoriferous, whilst, at the 
same time, its chemical properties are completely changed. The elec- 
tricity produced by rubbing together the two pieces of quartz acts, it 
would seem, upon the oxygen of the air which surrounds them, and 
produces an odour of ozone. 
The strong-smelling substances, sulphurous acid, hydrochloric acid, 
sulphuretted hydrogen, and perhaps chlorine, are present in active 
volcanos and solfatara. Hydrochloric acid is very common, for 
instance, at Vesuvius, where it is condensed by the aqueous vapour 
into an acid liquid ; it is also found in certain mineral waters, and now 
and then it is evolved from beds or strata of rock-salt. Chlorine is 
frequently discovered in the pores of certain ancient volcanic products, 
such as those of the Puy-Sarcourg, in Auvergne. Sulphurous acid is 
extremely common in volcanic eruptions of all descriptions, and in the 
gaseous emanations of solfatara, &c., whilst sulphuretted hydrogen 
(hydrosulphuric acid) is most frequently perceived in dormant vol- 
canos and certain mineral waters. 
Pure carbonic acid, which is acknowledged to be the most important 
of all gaseous emanations, both on account of the abundance with 
which it is evolved and the number of localities in which it presents 
itself, is completely devoid of smell. The same may be said of nitrogen 
gas and proto- carbide of hydrogen, whilst deuto-carbide of hydrogen 
has a slight but very peculiar odour. 
In mud-volcanos and salzes we have a production of sulphurous acid, 
carbides of hydrogen, naphtha, or other bituminous and odoriferous 
substances, besides certain gases which are devoid of smell. 
A fact which is perhaps little known is, that naphtha is also present in 
ordinary volcanos ; and this was actually perceived by the ancient writer 
Strabo, who relates that the elevated dome-like hill of Methane opened 
in fiery eruptions, at the close of which an agreeable odour was diffused 
in the night time. It is very remarkable that the latter was observed 
during the volcanic eruptions of San torino in the autumn of 1650, when, 
according to Ludwig Ross, an "an indescribable pleasant odour " fol- 
lowed the stinking smell of sulphurous vapours. The same pleasant 
odour has been also noticed by Kotzebue, during an eruption of the 
newly-formed volcano Umnack, in the year 1804; and, during the 
