Penny Blood: Serial Sensation and Working-Class Entertainment — page 258: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

cH. xIx.] THE VOLCANO IN ERUPTION. —_—241 we regard it, the hypothesis of a local and periodical heating of the rocks appears to be surrounded by great difficulties. Such a rise of temperature can be produced only in one of three ways; (1) by mechanical work being locally converted into heat, or (2) by local chemical reaction, or (3) by heat being transferred from some hotter neighbouring region. The first is Mallet’s hypo- - thesis, and is believed to be insufficient. The second was Davy’s, and has been long abandoned. There remains only the third. Now there are but two ways in which heat can be in such a case conveyed from hotter to cooler regions, and these are by convection and by conduction. If the former be the mode appealed to, then we have necessarily a fluid con- dition of the underlying rocks, which is our hypothesis. But if conduction be the process involved, then we must have an unfused mass, beneath, and adjoining, the area affected, at a temperature higher than is requisite to fuse other rocks; which is in itself improbable. But that such a condition of excessive temperature should be local and temporary, is still more so, because the tendency of conduction would be to equalize the temperature along isogeotherms approximately parallel to the outer surface. | There is however, the theory of Richthofen to be considered. Instead of invoking a rise of temperature, he supposes a tem- porary increase of fusibility. He appears to think that there are heated couches of rock, in a condition which he calls “slowing lava’,’ but which are not in a state of fusion. Fissures being formed in the crystalline crust above this, water gains access to it, and it enters into a state of “ aqueous fusion,” and thereby becomes increased in volume, and ‘this expansion would immediately cause a motion of the masses rendered liquid, in the direction of least resistance, that is, upwards in the fissure, and would, if continued for a sufficient time, make the same overflow on the surface of the crust, even if unassisted by other ejecting agents, such as the vapour of water *.” The difficulties attending the accession of superficial water 1 «Natural system of Volcanic Rocks,”’ p. 66. 2 Ibid. p. 36. F. | 16 COLA @ NOO (CO)