220 
MR. ROBERT MALLET ON VOLCANIC ENERGY. 
canicityin a given time far greater also, because contraction of the crust is proportionate 
to the rate of cooling, and the heat produced by crushing of the crust proportionate to 
that rate. 
We therefore see here a sufficient cause for the greater height of the mountain-ranges, 
as well as for that of the volcanic craters, in the moon than on our earth. 
This greater height was no doubt further exalted by the diminished action of gravity 
in the smaller globe opposing elevation, as well as by the want of density though pro- 
bable great hardness of the material constituting the moon. 
225. The density of the moon is stated by Hersciiel (‘Astronomy’) to be (>536, our 
earth’s density being taken as unity; and the mean density of our earth being about 5 ’5, 
it follows that that of the moon is about 3’0, which is about that of corundum, sapphire, 
quartz, and hydrous metallic silicates ; so that, with the exception probably of a small 
and possibly metallic nucleus of greater density, the whole may consist of aluminates 
and silicates of great rigidity, and therefore producing very great resistances to contractile 
crushing, and hence great elevation of temperature at local points during cooling. 
And this appears supported by the Iiillen of Madler, which to the writer’s eye, with 
Mr. Nasmyth’s best telescope, and also to that gentleman, appear as cracks or deep 
sharp-edged fissures in a rigid surface. The prevailing direction of these Iiillen is 
about at right angles to the lines of elevation, which is just what we should expect if 
the elevating force were such as here indicated, viz. a tangential compression, for in that 
the lines of tension must he orthogonal to the lines of elevating pressure, and therefore 
the resulting cracks orthogonal to the lines of elevation. 
226. It would be foreign to our direct purpose, and lead too far, to go into the pro- 
bable causes that have led to the great relative diameters of the lunar craters. 
Thus far we discern that volcanic activity generally is dependent upon the solid and 
liquid materials (elements) constituting any planet, upon their conductivity (rate at which 
they can part with heat), upon the mass of the planet, upon which both its original 
temperature (when all in fusion) and its rate of cooling are dependent, and upon its 
distance from the sun ; possibly also upon whether or not it traverses in space regions 
of variable temperature. 
227. In the sun itself we but behold vulcanicity in its earliest and most potent stage, 
that of the condensation and chemical exhaustion of a primordial world of vast dimen- 
sions, with its vulcanicity exhibited in (to the imagination) terrific grandeur. In our own 
globe we see it developed by quite the like train of causation acting differently at suc- 
cessive epochs, extending over time that we cannot measure, and now dwindled down to 
its present point, when it is but part of the beneficent machinery of our earth, to make it 
the safe abode of plants, animals, and man himself. In the moon we see it, after having 
passed through all its stages, died out and gone. 
228. If, then, in what has been here advanced, and, so far as the writer’s knowledge 
extends, for the first time, the cause assigned for volcanic heat, viz. the crushing 
of the earth’s solid crust, accounts for all the phenomena, leaves none unexplained or 
