M. FOUQU&S SANTO BIN AND ITS ERUPTIONS. 535 



water into its two elements. At a later stage, the gases varied inces- 

 santly in composition, and the combustible elements at last entirely 

 disappeared. The emitted gases consisted chiefly of free hydrogen, 

 marsh-gas, hydrochloric, sulphuric, and carbonic acids, sulphuretted 

 hydrogen, and nitrogen. Water played a prominent part in the dif- 

 ferent phases of the eruption. In the state of vapor it was present in 

 all the emissions of volatile matters whatever was their temperature, 

 and it might be regarded as the immediate cause of the explosions. 

 No dry smoke-vent was observed. Coming in its liquid state from 

 hot springs, the temperature and flow of the water varied with the 

 state of the sea. 



Chloride of iron, expelled as a vapor, was found associated with 

 hydrochloric acid ; but the hydrochlorate of ammonia, commonly 

 abundant in eruptions, Avas almost wanting. This fact lends support 

 to the opinion that the ammoniacal product of volcanoes is of organic 

 origin, and is brought by the atmosphere over vents charged with hy- 

 drochloric acid. The distance and the small extent of cultivated lands 

 explain the rarity of ammonia in the air at Santorin. 



Over the central eruptive mouths, the points of the volcano where 

 the incandescence was most lively, the spectroscope showed the pres- 

 ence of volatilized salts of soda and potash. After the eruptive mani- 

 festations had ceased, the salts which were deposited around the ori- 

 fices of the vents were collected and analyzed, and appeared to consist 

 largely of the chlorides of sodium and potassium, the sulphates of soda 

 and potash, and the carbonates of soda and magnesia. Sea-water after 

 evaporation leaves a residuum of analogous composition. These re- 

 sults are interesting in their bearing on the theory that the water of 

 the sea is the immediate ordinary agent of the eruptions. All of the 

 elements usually occurring at volcanic vents were found to be present 

 at the points which were the seat of a volatilization of alkaline salts, 

 lending support to the opinion which had been previously drawn from 

 studies made at Etna, that the vents at their highest temperature pre- 

 sent at once all the chemical elements of the volatilized bodies in the 

 volcanoes, and that the cooler vents are gradually impoverished as 

 their temperature becomes insufficient for the reduction of the erup- 

 tive materials to vapors. 



Some of the volatilized substances in the volcanic conduits are sus- 

 ceptible of reacting on each other and producing fixed compounds. 

 Thus are engendered the hydrated oxides of iron, specular iron-ore, 

 free sulphuric acid, alum, and sulphate of lime, which are met around 

 the vents. 



Certain crystallized silicates, however, generally originate under 

 different conditions. Although they are formed of elements which we 

 are in the habit of considering fixed, they are found in the volcanic 

 vents, on the surface of the rocks, under such conditions that they 

 could have been produced only through volatilization. Such silicates 



