396 Original Articles. [July, 
perhaps, conveniently accessible; and even when it is reached, the 
traveller may be surrounded by circumstances which very seriously 
interfere with his personal comfort, or disturb that tranquillity which 
is so requisite a condition for close and accurate observation, and thus 
lead him to hasty conclusions, which he has no future opportunity to 
rectify. In strong contrast with such circumstances is the position 
of the astronomer, comfortably placed beside his telescope, in the 
silence and tranquillity of a fine clear night, with all distracting 
objects excluded from his view. The whole of his attention is thus 
brought to focus, as it were, on the point under investigation there 
and then presented to his scrutiny, and ready to yield perfectly truth- 
ful replies to his questions; nothing being requisite for a correct 
interpretation of facts, other than a quick eye backed by a sound 
and unbiassed judgment. 
It is from circumstances such as these that we have acquired, by 
a long course of assiduous observation and reflection, an amount of 
intimate acquaintance with the physical structure of the Moon’s 
exterior, in many important respects far more accurate than is our 
knowledge of that portion of the earth. 
Tn order rightly to interpret the details of the Moon’s surface, as 
revealed to us by the aid of the telescope, we ought, in the first place, 
to bear in mind the true nature of volcanic action, namely, that while 
it has reference to the existence of intense temperature and molten 
matter, it does not derive its origin from combustion, considered as 
such in a strictly chemical sense, but proceeds from an incandescent 
condition, induced in matter by the action of that great cosmical law 
which caused an intense heat to result from the gravitation of particles 
of matter towards a common centre. These particles, originally exist- 
ing in a diffused condition, were, by the action of gravitation, made 
to coalesce, and so to forma planet. Volcanic action, then, has in 
all probability for its source the heat consequent upon the collapse of 
such diffused matter, resulting in that molten condition through 
which there is strong reason to believe all planetary bodies to have 
passed in their primitive state, and of which condition the geological 
history of our earth furnishes abundant evidence. Thus the molten 
lava which we see issuing from an active volcano on the earth, is 
really and truly a residual portion of that molten matter of which 
the entire globe once consisted. 
In reference to the nature and origin of that eruptive force which 
had, again and again in the early periods of the Moon’s history, 
caused the remaining molten matter of her interior to be ejected from 
beneath her solidified crust, and so to assume nearly every variety of 
volcanic formation in its most characteristic aspect, the key to these may 
be found in the action of that law which pervades almost all matter in 
a molten condition, namely, that “molten matter occupies less bulk, 
weight for weight, than the same material when it has ceased from 
the molten state ;” or, in other words, that “ matter in a molten state 
is specifically more dense than the same material in a solidified con- 
dition.” Thus it is that in passing from the molten to the solid 
state the normal law is resumed, and expansion of bulk either just 
