522 



THE GEOLOGIST. 



peculiar odour, which has sometimes been attributed to sulphuretted 

 hydrogen, or arsenuretted hydrogen ; but, if I mistake not, I)r. 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 Methano opened 

 in fiery eruptions, at the close of which an agreeable odour was difi'used 

 in the night time. It is very remarkable that the latter was observed 

 during the volcanic eruptions of Santorino in the autumn of 1650, when, 

 according to Ludwig Iloss, an an indescribable pleasant odour " fol- 

 lowed the stinking smell of sulphurous vapours. The same jplea^ant 

 odour has been also noticed by Kotzebue, during an eruption of the 

 newly-formed volcano Umnack, in the year 1804; and, during the 



