450 
POPULAE SCIENCE REVIEW. 
that the heat of the sun does not exceed this extent^ and_, 
therefore^ if these Plutonic facts and fancies be true,, we must 
have within the solid crust a heat equal to the actual combus- 
tion of the sun itself ; and at the full depth of this solid skin- 
crust a temperature of 80,,400° F. — utterly incompatible, 
according to my ideas, with the present rigid and quiescent 
condition of the eartVs surface. Surely, too, if there were 
such a universal and permanent central heat, the volcanic 
evolutions of gases and steam and the outflow of lava should 
be continuous and persistent — the pot should be always boihng 
and the porridge all alike. Instead of this, the essential 
character of volcanic action is intermittence, and lavas 
everywhere differ in composition. If the central heat melted 
the hypogene or nethermost rocks of the earth^s solid crust 
into lava, as has been supposed heeause the constituents of 
granitic rocks, for example, and lavas are of the same classes 
of minerals — namely, mainly silicates of magnesia, alumina, 
soda and potash; such as hornblende, felspar, augite, &c. — then, 
in that case, the central heat has had to re-act upon portions 
which, in the cooling of the whole mass of the globe, had become 
solidified hy loss of caloric, and this could hardly be the case 
without an increase of the temperature of the supposed interior 
molten core; and where is this increment of heat in an 
admittedly gradually cooling globe to come from ? 
There is only one way of finding out the source of volcanic 
eruptions — to examine their products, and thence to deduce 
the manner and mode of their formation. This brings us to 
the consideration of the chemical theory, for by the chemical 
analysis of’ the mineral ejections, and the study of the chemical 
nature of the gaseous emanations, it must be that we shall 
arrive some day at the means of making lavas synthetically. 
Now, all lavas consist in the main of two principal mineral 
ingredients, hornblende and felspar; the first a silicate of 
magnesia, the last a compound silicate of alumina and soda, or , 
potash. Augite may be regarded as the same as hornblende, 
for it has nearly identically the same composition. Iron exists 
in lavas as the colouring matter, and may have acted as a flux 
during the melting. Obsidian and pumice have a like com- 
position with felspar. Now, here we certainly have as the bases 
of these chief lava-minerals, metals capable of violent ignition. 
In hornblende, the base silicon — ^highly combustible- — in the 
state of silicic acid, just as it would be after combustion, i.e., 
combined with oxygen (Si02). Magnesium has been lately 
made too familiar for its illuminating power to necessi- 
tate any further reference. The combustibility of calcium 
is well known in the brilliant lime-light.^^ In felspar 
we have either a soda or a potash base, and the ignition 
