278 DYNAJVIICAL GEOLOGY. 



liave been melted to produce the supply, since all rocks of the supercrust 

 contain traces of moisture (page 205). But this is not the chief source of 

 the vapor of water. 



Among the other materials of the vapors, sulphurous acid (SO2) is probably the most 

 abundant. It has the smell of burning sulphur. It is always present, and probably comes 

 from iron sulphides in the melted rocks, since they are often sparingly present in the solid 

 lavas. Hydrogen is sometimes present ; it may come from the dissociation of the elements 

 of vrater (H2O), or from any oxidation in the lava in which the oxygen iised is derived 

 from water. Hydrochloric acid (HCl) is one of the gases when sea water gains admission 

 to the hot lavas. Carbonic acid (CO2) may be emitted if any limestone (CaO.COa) exists 

 below in proximity to the melted lavas. Carbonic oxide (CO) has been detected by W. 

 Libbey in spectroscopic observations of the flames, 1, 2, or 3 feet high, that appear about 

 the lava vents of Kilauea. The above, with more or less of atmospheric air, are the 

 chief gases of the melted lavas. There are other vapors given out by solfataras, but these 

 take no part in the eruptive work of the volcano. 



The mechanical work of the vapors is due almost wholly to the vapor 

 of water. In view of the relation on Hawaii between times of eruptions 

 and the rainy season, and between length of lava-column above the sea and 

 projectile force, there is strong probability that fresh waters are in many 

 volcanoes the chief agent. Whenever subterranean waters in their descent 

 below the surface approach the hot rocks about the lava-column, they are con- 

 verted into steam ; and the amount of steam generated from even a small con- 

 tinued supply of water would be so large (in view of the fact that at the 

 ordinary pressure one cubic foot of water will yield 1700 cubic feet of steam) 

 that it could not all escape outward through the rocks, but in part would be 

 forced into the rising lavas of the conduit. Moreover, it has been experi- 

 mentally demonstrated by Daubree (1879) that, under such circumstances, a 

 molecular absorption of the steam would take place against any pressure 

 outward that might exist within the heated column. 



3. Effects of vapors. — (a) Projectile effects. — The escape of vapor and its 

 expansion encounter resistance in consequence of the cohesion of the liquid 

 material, which resistance is proportional to the strength of this cohesion, 

 or is conversely as the degree of liquidity. Water, in boiling, lets very 

 small bubbles of steam through easily ; and the elastic force of the steam 

 of the bubble makes low jets, one or two inches in height. But the elastic 

 force of a small bubble of vapor is too feeble to break its way through lava ; 

 enlargement, therefore, goes on until the force is sufficient to overbalance 

 the resistance ; then comes the break of the liquid lava-shell of the bubble, 

 and the projection of its fragments vertically or nearly so into the air, — 

 vertically, because the shell is thinnest at top. The projectile force thus 

 depends largely on the size of the bubble, or, what is equivalent, on the 

 viscidity of the liquid lava, and on the rate of supply of vapors seeking to 

 escape. 



On account of the remarkable liquidity of basaltic lavas, the jsrojectile 

 force required to break a way through may be so small as to throw the lava 



