Taking the maximum of pressure as a fixed quantity, or, as not being
subject to increase, and assuming the degree to be not less than
equal to the tenacity of steel, there must of necessity be means of
mitigating the maximum, so that in the scale of descent every degree
of force should be attainable; and more, to keep the equilibrium in
a state of disturbance, without which all things must be, and remain
in the rest of death. Were there no minus-pressure means, the solid,
or perhaps aëriform state of matter would exist everywhere, and of
motion there could be none. Such means for promoting motion are amply
supplied, and without any addition of matter to the measured quantity
sufficient for the formation of bodies and service of nature generally,
in the elements themselves, of matter.
As the body which is involved in a medium of air is under less pressure
than in a medium of water, and still less within a medium of elementary
matter, so is elementary matter, and the elements generally, the
natural means of mitigating the maximum of pressure on and within
bodies. All bodies within and on the surface of the earth, possess
removable elementary matter, which prevents superficial contact, and
excludes medium of space proportionally from their interior; and
because the medium of space is the cause of pressure, in being thus
rendered discontinuous, so is its force, as it were, intercepted
or lessened. For instance, a polished needle floats on water, but
when wetted or smoked is precipitated, from having its electric or
minus-pressure atmosphere removed; from which it is obvious that with
the minus-pressure atmosphere, the needle is under less pressure than
when without it; and the same atmosphere it is which makes the bed in
the water so much larger every way than the needle.
The minus-pressure principle is well exemplified in the rise of water
within a tube over which fire is situated. When the fire is removed,
the water falls. The fire must be in the state of combustion--mere
ignition does not answer. The elements forced out of the combustible,
as combustion proceeds, cover the orifice of the tube, and intercept
the general pressure, notwithstanding they are under the general
pressure. By such minus-pressure means is the equilibrium destroyed,
and by the unaltered pressure on the water outside the lower orifice
of the tube, the water is forced upwards. So is it that the water of
the sea is raised to the minus-pressure, elementary matter descending
from a cloud in the shape of an inverted cone, and known as the
water-spout. Astronomers can best say whether the sun and moon be not
minus-pressure means in promoting the rise of the ocean, productive of
the tides; a miniature representation of which is effected by holding
a charged jar over a surface of water, to which the water rises in a
small cone,--which cone follows every motion of the jar, and falls when
the jar is discharged. Capillary ascent is promoted by the interposed
minus-pressure electric matter which fills the caliber of the tube: the
same matter prevents the horizontal flow of water through such tubes;
but when the tubes are de-electrised, the flow is free and constant:
boiling water, or fire de-electrises all such tubes. The electric
matter on a bar of iron is a hinderance to water running down, but
when removed by means of fire, the water runs down the bar freely. The
atmosphere is a minus-pressure medium to the earth, and on the general
principle that _interposed elementary matter renders discontinuous_ the
medium of pressure, which is the medium of space.
Minus-pressure means exist in other than the elementary form,
as in blotting-paper, candle-wick, pledgets of lint. Within the
cupping-glass, which is empty of air only, it is the minus-pressure
matter obtained from flame which promotes the rise of blister.
Within the vessels of the vascular system, as mucilaginous lining,
minus-pressure matter assists the circulation of fluids, on the
foregoing capillary principle. The slime on deep-water fish, seems
provided to lessen the pressure of the water on the inhabitants of
those seas. Minus-pressure matter on one side only of a body, destroys
the equilibrium, and promotes the motion of the body; and generally,
the partial action, implied by motion, of the medium of space on bodies
or their parts, is promoted by interposed minus-pressure matter in
every instance of physical change. Only in minus-pressure means, which
serve as a partial vacuum in some cases, to disturb the equilibrium
of pressure, is motion, or change of place of the elements of bodies,
or of bodies themselves promoted: without such means there is nothing
to promote the blowing of a wind, or to put the medium of space into
action. Cause being given, the _General Pressure_ in the production
of every physical effect, the sole province of philosophy consists in
tracing out the minus-pressure means which promote the occasional and
partial action of the medium of pressure.