and the Nature of the Liquidity of Lavas. 191 
own expansive force from the confining surfaces, might perhaps 
prevent its complete fusion, however high the temperature. 
3. I had long been impressed by the vast volumes of aqueous 
and other elastic vapours evidently discharged from every voleano 
in eruption, and to all appearance the chief agents in the expul- 
sion of lavas from the bowels of the earth. That this vapour is 
liable to be developed in every part of the mass of lava is shown 
by the formation of vesicles throughout its substance wherever 
the pressure is so reduced as to permit their expansion; for 
instance, in the superficial portions of a current, and in some 
lava-currents throughout the entire mass. 
The experiments of Mr. Knox, related in a paper read before 
the Royal Society in 1824*, had taught me that water in an 
appreciable quantity is mechanically combined with the elemen- 
tary particles of all the crystalline rocks of igneous origin. The 
questions therefore arose,—Might not the water thus intimately 
disseminated through a mass of crystalline lava, although at an 
intense temperature, remain unvaporized, owing to the still 
greater intensity of the pressure by which it is confined while 
yet within the bowels of the earth? And would it not, under 
these circumstances, exert an intense expansive force upon all the 
_ confining molecular or crystalline surfaces between which it lies, 
and thus occasion a tendency to separation among these solid 
particles whenever the compressing forces were relaxed, or the 
temperature increased sufficiently, so as to give a certain degree 
of mobility to these particles inter se, and an imperfect liquidity 
to the mass composed of them? And, supposing the intumes- 
cence thus occasioned to raise any portion of this semi-liquid 
matter into the open air, would not the instantaneous absorption 
of caloric from the contiguous particles that must accompany the 
vaporization of this water, and its escape in bubbles or pores and 
through cracks, owing to the nearly absolute cessation of pres- 
Sure, account for the sudden cooling down and setting, or con- 
solidation, of the exposed surfaces, without having undergone 
complete fusion (except in the case of mere superficial films), 
notwithstanding their previous intense temperature, amounting 
even to a white heat ? 
This supposition seemed to me to account satisfactorily, not 
only for the absence of a vitreous texture even in superficial por- 
tions of many lava-streams, and their instantancous consolida- 
tion on exposure, in cellular or porous slabs and cakes, but also 
for several other characteristics of igneous rocks, not easily to be 
reconciled with the idea of their having always issued from the 
earth in a state of absolute fusion ; such, for example, as the 
eracked and vitrified aspect of the felspar-crystals of many tra- 
* Phil. Trans. 1825. 
