VOLCANO CLOUDS 577 



(6) A dew-point hygrometer carried along the rim through the smoke 

 cloud showed a lower humidity within the cloud than in the clear air just 

 outside of it. 



Before proceeding to recount our own experience with these phenomena, 

 it may be as well to express our belief that nearly all of these observations, 

 both of Green and Brun, may be perfectly true as recounted above, and 

 still offer no proof that the volcano exhales no water vapor. 



The Explanation of the Volcano Cloud 



Green's observation that the great white cloud appears but intermit- 

 tently may be explained by a somewhat closer observation of the condi- 

 tions of formation of the cloud without assumptions of any kind about its 

 possible water content. For example, we noted, during several months of 

 constant observation, that the visible cloud does not rise directly from the 

 surface of the liquid lava, but rather from cracks in the inclosing banks, 4 

 shattered, as they always are, by alternations of heat and cold as the liquid 

 lava rises and falls in the basin. When the lava is high enough to com- 

 pletely flood the floor of the basin, these cracks are closed and all the gases 

 emitted emerge directly from the surface of the lava into the atmosphere 

 and have the temperature appropriate to the surface of the liquid (1,000 

 to 1,200° centigrade). At this temperature the gases (sulphur and hydro- 

 gen, for example) burn promptly on contact with the oxygen of the air 

 and remain nearly or quite invisible. A thin blue haze can sometimes be 

 distinguished above a bursting bubble 5 when conditions are exceptionally 

 favorable, but this haze is so thin that spectators watching for it from 

 the rim will generally disagree about its existence. 



This is the condition of no cloud (plate 17) described by Green, and 



* Cf. plates 22, 23, 24, and 27. 



Observations confirmatory of the conclusion that the smoke cloud when present does 

 not rise from the liquid lava, but from the shattered floor and talus surrounding the 

 basin, have been recorded by other writers. 



For example, Prof. W. T. Brigham, Director of the Bishop Museum, Honolulu, who 

 for fifty years has been one of the most careful observers of volcanic pbenomena in the 

 Island of Hawaii, writes as follows ("Volcanoes of Kilauea and Mauna Loa on the 

 Island of Hawaii," Honolulu, 1909) : 



Page 28 : ". . . It should be noticed how small the supply of steam in the active 

 outpour of Kilauea really is." 



Page 28 : "When the pit Is empty of molten lava, the smoke is often most abundant." 



Legend to plate 45 : "Lava pool below the rim of Halemaumau. . . . Little vapor 

 rises from the portion which is active." 



Legend to plate 50: "There is little escape of steam from the take surface." 



William Lowthian Green ("Vestiges of the Molten Globe," vol. il) writes (p. 170) : 

 "Smoke, vapors, and #ases seem to aviso from the oiitkvs of eruption and orifices in the 

 neighborhood of molten lavas on Hawaii, and not from the lavas themselves." 



5 Cf. Frank A. Perret : "The Circulatory System in the Ilalenuuunau Lava Lake during 

 the Summer of 1911." Am. Jour. Sci. (4), vol. 35, 1913, p. 341. 



