HEAT. . 293 



No liquid lavas are in any way directly concerned, and hence the eruptions 

 are only semi-volcanic. Their violence may cease in a few hours. 



The eruption at Bandai-san, Japan, in July, 1888, was probably of this kind. The 

 volcano had been extinct for 1000 years. In an hour after it burst out the ash-shower 

 had mostly passed, the pitchy blackness changing so soon to dim twilight ; and in 5 hours 

 all was ended. Kikuchi, who describes the eruption, states that no evidence appeared that 

 liquid lavas contributed to the ejected material, or to any of the results. 



The blowing off of the summits of volcanoes has been attributed to explosive eruptions. 

 Steam has little expansive power after it escapes into the open air. It expends its energies 

 in work where generated, as in a steam-boiler. Where large open craters exist, the volcanic 

 peaks about it would be little moved by the explosion, except through undermining and a 

 collapse. But if the old mountain had been much denuded, and was essentially solid to 

 its summit, an explosion within it might widely scatter the fragments, besides making 

 great excavations at the center. The stones hurled from Bandai-san are said to have 

 struck the trees, on descending, at an angle of about 30°. 



4. Work of the Spent Vapors and Waste Heat of the Volcano : Fumaroles, 



Ovens, Solfataras. 



While the chief part of the spent vapors and heat of the volcano go 

 directly from the boiling or discharged lavas into the air, a portion escapes 

 through fissures about a volcano or a volcanic region. They thus make 

 (1) fumaroles (so named from the Latin fumus, smoke), the greater number 

 of which open upward directly into the air, but some into cavernous places 

 in the crater or in lava-streams ; (2) solfataras (so named from the Italian 

 solfo, sulphur), which are made up of a combination of steaming fissures, and 

 cover large areas with the results of decomposition and deposit from the 

 escaping gases. Fumaroles are common about the walls of active craters 

 and the courses of lava-streams, and the escaping vapors may have all tem- 

 peratures from nearly that of the liquid lava to 212° F. and below. But 

 solfataras are usually more remote from the center of volcanic action, and 

 may occupy regions of long-quiet or essentially extinct craters ; and conse- 

 quently the vapors have a lower temperature. 



Vesuvius has its fumaroles ; but the solfatara of the region is to the west 

 of Naples, over the extinct volcanic region of the Phlegrsean Fields. Kilauea 

 has fumaroles or steaming fissures along its walls and some of large size just 

 west of the Volcano House ; and but a few rods northwest of the same house 

 there is a solfatara region. Both fumaroles and solfataras derive accessions 

 to the vapors from descending waters supplied by rains, and some of the 

 fissures afford only odorless steam. 



The rocks (solidified lavas), acted iipon by the volcanic vapors, consist mostly of 

 silica, alumina, potash, soda, lime, magnesia, and iron oxide ; the presence of potash 

 with little or no soda distinguishes those of the third class (p. 273), and the near absence 

 of potash, those of the second and first classes. 



The chief vapor or gas coming directly from the lavas is, in all volcanoes, sulphurous 

 acid (SO2) ; and with it may be hydrogen and nitrogen. At Vesuvius, chlorine is given 



