Revieics — Prof. Judd — On Volcanoes. 563 



worlv, many new lines of inquiry have been opened up in con- 

 nexion with the subject, and fresh methods of research have been 

 devised and applied to it. More exact observations of travellers 

 over wide areas have greatly multiplied the facts upon which we 

 may reason and speculate, and many erroneous hypotheses which 

 had grown up in connexion with the subject have been removed 

 by patient and critical inquiry." 



From the days of Spallanzani to our own, all the real additions 

 to our knowledge of Vulcanology have resulted from long and 

 patient observations of actual voleanos, both in their active and 

 quiescent states, and by a careful chemical and microscopical study 

 of the minerals they have ejected upon the surface. 



Gases, and especially steam, in a highly-heated state, are un- 

 questionably the great motor agents in all volcanic displays, and to 

 their larger supply and greater compression is doubtless due the 

 intensity and duration of each eruption. 



The presence of water is not caused (as many have supposed) 

 by the sea finding its way by fissures and cracks into the earth's 

 heated interior, and so, by being suddenly converted into steam, 

 giving rise to those subterranean convulsions of which earthquakes 

 and voleanos are the concomitant indications. On the contrary, 

 it appears, both from observations and experiments, that not only 

 vast quantities of aqueous vapour, but also enormous volumes of 

 various gases, are interstitially present everywhere in the earth's 

 crust. The rocks composing it, upon being liquefied, under enor- 

 mous heat and pressure, have the power to absorb many times 

 their own volume of certain gases as well as water-gas. That such 

 is the case we have indisputable evidence in the prodigious quanti- 

 ties of both gases and steam given ofi" during volcanic outbursts and 

 while lava-streams are flowing. Even the volcanic materials of past 

 ages, " which have been consolidated under great pressui'e, such 

 as granites, gabbros, porphyries, etc., exhibit in their crystals innu- 

 merable cavities containing similar gases in a liquefied state. It 

 is to the violent escape of these gases from the molten rock-masses, 

 as the pressure upon them is relieved, that nearly all the active 

 phenomena of volcanoes must be referred ; and it was the recogni- 

 tion of this fact by Spallanzani, while he was watching the pheno- 

 mena displayed in the crater of Stromboli, \Yliich laid the founda- 

 tion of the science of Vulcanology." (p. 357.) 



There is one interesting point dwelt upon by the author in 

 connexion with volcanic phenomena which cannot be omitted. 

 It is the fact that up to the present time, notwithstanding the now 

 considerable number of substances known to occur in meteorites, 

 no element has yet been found in any meteorite which was not pre- 

 viously known as existing in the earth ; and out of the sixty-five or 

 seventy known terrestrial elements, no less than twenty-two have 

 already been detected in meteorites. But as there are a dozen 

 elements which occur in overwhelming proportions in the earth's 

 crust, viz. oxygen, silicon, aluminium, calcium, magnesium, sodium, 

 potassium, iron, carbon, hydrogen, sulphur, and chlorine, making up 



