424 COSMOS. 



sources of information (the geological and geographical 



been recognised by Deville in the trachytes of the soufriere, together 

 with the crystals of labradorite (Comptes rendus, t. xxxii, p. 675), while 

 Gustav Rose even iound hexagonal-dodecahfidra of quartz in the tra- 

 chytes of the volcano of Arequipa (Meyen, Reise um dieErde, Bd. ii, s. 23). 

 The phenomena here described, of the temporary ejection of very 

 various mineral productions from the fissure-openings of a sou- 

 friere, remind us very forcibly that what we are accustomed to deno- 

 minate a solfatara, soufriere or fumarole, denotes properly speaking 

 only certain conditions of volcanic action. Volcanoes which have once 

 emitted lava ; or, when that failed, have ejected loose scoriae of con- 

 siderable volume, or finally the same scoriae pulverised by trituration, 

 pass on a diminution of their activity, into a state in which they 

 yield only sulphur sublimates of sulphurous acid and aqueous vapour. 

 If as such we were to call them semi- volcanoes, it would readily convey 

 the idea that they are a peculiar class of volcanoes. Bunsen, to whom 

 along with Boussingault, Senarmont, Charles Deville and Danbree 

 science is indebted for such important advances for their ingenious 

 and happy application of chemistry to geology, and especially to the 

 volcanic processes, shows " how, when in sulphur sublimations, 

 which almost always accompany volcanic eruptions, the masses of sul- 

 phur in the form of vapour come in contact with the glowing pyroxene 

 rocks, the sulphurous a^id is generated by the partial decomposition of 

 the oxyde of iron contained in those rocks. If the volcanic action then 

 sinks to a lower temperature, the chemical action of that zone then 

 enters into a new phase. The sulphurous combinations of iron and 

 perhaps of metals of the earths and alkalies there produced, commence 

 their operation on the aqueous vapour, and the result of the alternate 

 action is the generation of sulphuretted hydrogen and the products of 

 its decomposition, disengaged hydrogen and sulphur vapour." The 

 sulphur fumaroles outlive the great volcanic eruptions for centuries. 

 The muriatic acid fumaroles belong to a different and later period. 

 They seldom assume the character of permanent phenomena. The 

 muriatic acid in the gases of craters is generated in this way ; the 

 common salt which so often occurs as a product of sublimation in vol- 

 canoes, particularly in Vesuvius, is decomposed in higher temperatures, 

 itnder the co-operation of aqueous vapour and silicates, and forms 

 muriatic acid and soda, the latter combining with the silicates pre- 

 sent. Muriatic acid fumaroles which, in Italian volcanoes, are not un- 

 frequently on the most extensive scale, and are then generally accom- 

 panied by immense sublimations of common salt, seem to be of a very 

 unimportant character in Iceland. The concluding stages in the chro- 

 nological series of all these phenomena consist in mere emanations of 

 carbonic acid. The hydrogen contained in the volcanic gases has 

 hitherto been almost entirely overlooked. It is present in the vapour, 

 springs of the great solfataras of Krisuvik and Reykjalidh in Iceland, 

 and is indeed at both those places combined with sulphuretted hydro- 

 gen. When the latter corne in contact with sulphuric acid, they are 

 both mutually decomposed by the separation of the sulphur, so that 



