482 
areas throughout all the known portion of the 
earth, forming the substance of many mesas, 
plateaus, and mountain systems, in which val- 
leys and valley systems are carved. The pour- 
ing-out of this volcanic matter is not confined 
to the present time, or to late geologic time. 
Nor can the geologist assert that the rate of 
extravasation has increased or diminished from 
the earliest known geologic time to the present. 
It seems to have been paroxysmal by districts, 
but uniform, considering the whole extent of 
the surface of the earth. The magnitude of 
the volcanic formations exposed at the surface 
is such that the origin of the material cannot 
be attributed to local and trivial causes: it 
must be explained by laws of universal appli- 
cation. Extravasation is always associated, so 
far as the phenomena have been studied, with 
displacement ; and this association is of such a 
nature that they must have a common expla- 
nation. ‘This common explanation, as postu- 
lated by geologists, is a fluid interior. 
ITI, The argument from internal temperature. 
The hypothesis of a fluid interior is reached 
by snother inductive method, —through the 
facts relating to increase of temperature from 
the surface downward. ‘The rate of increase is 
not well known; it seems to be greatly vari- 
able. In general, it may be roughly stated, as 
itis by Thomson and Tait, as about one degree 
for each fifty feet; but in many cases the rate 
is much higher. Such an increase, known to 
extend so far down as observation and experi- 
ment have reached, if continued at the same 
rate, would soon give a temperature at which 
all known rocks would be melted; and the hy- 
pothesis of a fluid condition is thereby strength- 
ened. 
IV. The argument from the‘ flow of solids.’ 
It is an hypothesis worthy of consideration, 
that pressure itself would reduce the interior 
of the earth toa fluid condition. That rigidity, 
which is the characteristic of the solid state, 
is due to molecular cohesion; but geologists 
everywhere in their researches discover that 
this molecular cohesion, or rigidity, may be 
overcome by pressure: for everywhere they 
find that rocks may be squeezed into new 
forms, bent, contorted, and implicated; that 
is, the force of compression existing in many 
thousands of feet of superincumbent rock over- 
comes molecular cohesion to such an extent 
as to cause rocks to yield (the molecular 
cohesion is broken down). Doubtless the 
element of time is involved, to some extent, 
SCIENCE. 
as a rock may be bent with a small force, if 
sufficient time be allowed. But with increase 
of force there may be decrease of time; and 
the force engaged in compression, being the 
weight of miles of superincumbent rock, must 
be sufficient to greatly reduce the element of 
time, and perhaps to cause it to disappear. 
The last few years of experiment have added 
to the argument derived from geologic obser- 
vation. Many solids have already been found 
to flow under pressure. 
stitution of solids is found to undergo a change 
by reason of pressure, so that new compounds 
may be formed thereby; and in pressure we 
have conditions for chemical change analogous 
to the conditions produced by melting. It is 
therefore an inductive hypothesis of the high- 
est value, that all rocks may be reduced to a 
fluid condition —i.e., be caused to behave as 
bodies of minute parts, without rigidity of 
structure — by pressure alone. 
The facts of observation and experiment 
characterized above are vastly multifarious 
and cumulative, and the conclusions in each 
case are strictly inductive. ‘The theory reached 
by the consilience of these four inductive 
processes is so strong, that structural geolo- 
gists are compelled to accept it, and contra- 
dictory conclusions are rejected. It there- 
fore behooves the physicist to re-examine his 
data and his methods of logical procedure ; for, 
perchance, he may discover that an error lurks 
therein. J. W. Powe Lt. 
INERTIA. 
Recent conversations with teachers of phys- 
ics have shown me that there exists, in this 
country at least, great diversity of opinion as 
to the proper definition and use of the term 
‘inertia.’ 
Elementary text-books usually speak of in- 
ertia as a mere inability, —the inability of a 
body to set itself in motion, or to stop itself 
when once in motion. This is an old use of 
the term, but certainly not the best use. Max- 
well states, that at the revival of science, 
‘¢ while the men of science understood by this 
term [the inertia of matter | the tendency of the 
body to persevere in its state of motion (or 
rest), and considered it a measurable quantity 
[the Italics are mine], those philosophers who 4 
were unacquainted with science understood in- 
ertia in its literal sense as a quality — mere 
want of activity, or laziness.” 
Maxwell suggests certain simple experiments 
1 Theory of heat, p. 86. 
. 2! Aen 
[Vou. III., No. 63. 
The molecular con- 
a 
