1853.] OF THE ROYAL INSTITUTION. 331 
in the rocks themselves. Where the iron has been extracted, the 
rock has become a mass of white clay, where the iron is re-deposited 
the mass exhibits the colour produced by iron. But it would weary 
the audience, and thus defeat the object of the lecture, were the 
details thus minutely dwelt upon. Let it suffice therefore to weld 
swiftly together the links of the great chain operations, to which the 
various thermal springs and gaseous eruptions of Iceland owe their 
existence and peculiarities. 
Hydrochloric acid, though playing a far less important part in 
Iceland than at Vesuvius and Etna, is nevertheless present. The 
presence of common salt is proved by the fact of its being found as 
one of the products of sublimation. Now it is a well known fact 
that this substance, exposed to a high heat in the presence of silica 
and the vapour of water, is decomposed; the sodium takes the 
oxygen of the water and becomes soda, the chlorine takes the 
hydrogen and forms hydrochloric acid. There is no difficulty, 
therefore, in accounting for the origin of this gas, as all the con- 
ditions for its formation are present. 
Sulphurous acid and sulphuretted hydrogen play a most important 
part in Iceland ; — how can their presence be accounted for? Leta 
piece of one of the igneous rocks of the island be heated to redness, 
and permit the vapour of sulphur to pass over it. The oxide of iron 
of the rock is decomposed ; a portion of the sulphur unites with the 
iron, which remains as sulphuret ; the liberated oxygen unites with the 
remaining sulphur, and forms sulphurous acid. Let the temperature 
of the heated mass sink till it descends just below a red heat, and then 
let the vapour of water be passed over it ; a decomposition of the sul- 
phuret before formed is the consequence; the iron is reoxidised, and 
the liberated sulphur unites with the free hydrogen tv form sulphu- 
retted hydrogen, and thus the presence of two of the most important 
agents in these phenomena is accounted for. These are experimental 
facts capable of being repeated in the laboratory, and the chronological 
order of the gases thus produced is exactly the same as that ob- 
served in nature. In the active volcanoes, where the temperature is 
high, we have the sulphurous acid ; in the dormant ones, where the 
temperature has sunk so far as to permit of the decompositions just 
described, we have the sulphuretted hydrogen. This accounts for 
the irregular and simultaneous appearance of these two gases in 
various parts of the island. At Krisuvik, for example, exhalations 
of sulphurous acid, sulphuretted hydrogen, steam, and sulphur, * 
burst in wild disorder from the hot ground. The first two gases 
cannot exist amicably together. In Iceland they wage incessant 
war, mutually decompose each other, and scatter their sulphur over 
the steaming fields. In this way the true salfataras of the island 
are formed. 
* In nature the vapour of sulphur is doubtless derived from the action of heat 
upon certain sulphur compounds. 
