Penny Blood: Serial Sensation and Working-Class Entertainment — page 260: what you’re looking at

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

CH. x1x.} THE VOLCANO IN ERUPTION. 243 the water-substance in it through decrease of pressure, the same identical water being supposed to remain always engaged in the same portion of lava’. But ebullition would help to raise and eject the lava. In the equation which we have arrived at, connecting the pressure p of the lava at the level of the surface of the crust with the proportion 1 — V of water-substance, which the magma contains as it exists below when subject to the pressure of the crust, if we assume a value for one of these quantities, we can — find the other. The equation in question is’, 1 Pog 20" — P= 1. yo? p gpk Vp Probably in different localities, and at the same locality at different times, these quantities, p at the surface, and 1— V, are different ; and ‘it is to the connection between the quantity of water and this pressure that the well-known fact 1s due, that a long continued state of repose in a volcano is followed by an exceptionally violent eruption, and wee versd. In order that the pressure at its base may bring the lava to the surface of the crust, it 1s requisite that the mean density of the column should be lowered by a certain amount of water being vesicularly mingled with it. But the vesicles rise through it, and even- tually leave the column of lava deficient in enough water-sub- stance to enable it to be raised to the surface. At the same time the magma around the base of the funnel has been to a certain extent exhausted of its superabundant water, and thus an eruption will have the effect of diminishing the proportion of water in the magma, or 1— V. Exceptionally powerful erup- tions might be expected to occur when a volcano had been quiescent for a long while, and the water had become again equably diffused, and regained its average proportion, beneath the vent; or when a volcano has newly burst through the crust at a place where the water-substance had never, or at least had not for a long period, been drained away. We have of course no means of knowing with any degree of certainty the values of either of these quantities, p at the surface, or 1 — V. 1 p. 189. 2 p. 191. 16—2 GOR GIOO SS (CO)