152 Royal Institution. 



of iron, while the spring does not contain a trace of it. It is, how- 

 ever, an experimental fact that the oxide of iron has been dissolved 

 with the rest. How is its disappearance to be accounted for ? The 

 very rock from which it was originally extracted possesses the 

 power of re-precipitating it, when by further contact with the rock 

 the solution which contains it has its excess of acid absorbed and has 

 thus become neutral. In this way the aqueous sulphurous acid acts 

 as a carrier to the iron, taking up its burden here and laying it down 

 there ; and this process of transference can be clearly traced 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 account- 

 ing for the origin of this gas, as all the conditions for its formation 

 are present. 



Sulphurous acid and sulphuretted hydrogen play a most important 

 part in Iceland ; — how can their presence be accounted for ? Let a 

 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 reoxidized, and 

 the liberated sulphur unites with the free hydrogen to 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 observed 

 in nature. In the active volcanoes, where the temperature is high, 

 we have the sulphurous acid ; in the dormant ones, where the tem- 

 perature has sunk so far as to permit of the decompositions just de- 

 scribed, 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 sul- 

 phurous acid, sulphuretted hydrogen, steam and sulphur *, burst in 

 wild disorder from the hot ground. The first two gases cannot 



* In nature the vapour of sulphur is doubtless derived from the action 

 of heat upon certain sulphur compounds. 



