the admixture; but since we know that the operation of these causes extends
but a little way below the surface, and that, far within the limits to which
animal life has heretofore been supposed to range, their action is altogether
arrested, it is evident that the absorption of air must he mainly dependent on some
constant property Of fluid and gaseous matter unconnected with these agencies.
Since the tendency of fluids to absorb gaseous bodies is constant under all
circumstances, although, as already stated, the quantity they are capable of
appropriating increases with the pressure, it follows that, the deeper the stratum
of water, the greater must be the amount of gaseous matter held in solution by
it. In illustration of the manner in which a column of sea-water becomes saturated
with its complement of gaseous matter, let us suppose atmosphere and ocean
to be brought into contact for the fii'st time, and the process of absorption to be
commencing. As soon as the superficial stratum of water received its fidl charge,
it would transfer a portion to the stratum immediately below it. This stratum,
when fully charged, would in like method transfer a portion to the succeeding
one, receiving a fresh supply from the stratum above, and so on through the
entire series; whilst in order to meet the demand thus continually increasing from
below, an uninterrupted absorption would be carried on at the surface, the lowest
stratum being the first saturated; whilst the highest would serve as a channel of
transmission for the whole of those below it, and be the last to arrive at the point
of saturation, notmthstanding that its gaseous contents are the smallest in quantity.
Were the pressure equable throughout the entire vertical range of the column,
the transmission of gaseous matter from above downwards would only proceed
until the quantity absorbed iu equal volumes of water was similar at the surface
and at the bottom. But, inasmuch as the pressure increases with the depth, and
the quantity of gaseous matter absorbed is in a direct ratio to the pressure, it is
evident that the operation must be protracted until each stratum of water has
received, through the entire series of strata above it, the quantity absorbable in
virtue of the pressure it is subject to. On this being accomplished, but not till then,
every portion of the column would be in equilibrium with the portions adjacent to
it, and, accordingly, aU further abstraction of gaseous matter from the atmosphere
would cease, in the absence of any expenditm-e taking place from below.
I t may, perhaps, be urged against this theory, that, instead of remaining in the
state of equilibrium indicated, the tendency of gaseous matter would be to rise
and escape at the surface ; but, inasmuch as the capacity to absorb a given quantity
of such matter, under a given degree of pressure, admits of no variation, and it is
clear that the affinity of the fluid for the gas is more powerful than any other
counteracting influence, since otherwise absorption could not take place even at
the immediate surface of the sea, there are no circumstances under which the
evolution of the gaseous matter could occur. The example of ascending thermal
springs charged with gases and salts is by no means a parallel one ; for in these
the column is circumscribed within solid walls, and each successive zone or
stratum of water, in ascending, carries along with it its gaseous contents ; and, as
a necessary consequence of removal from a greater to a smaUer degree of pressure,
the disengagement of gas takes place. In the ocean there are no ascending
currents of the kind ; but even if there were, the conditions would be dissimilar,
inasmuch as each portion of water ascending would instantly be replaced by fresh
portions poured in under the same degi-ee of pressure, and therefore charged with
the same amount of gaseous matter. Hence, unless it can be shown that increase
of pressure does not enable water to absorb a greater quantity of gaseous matter,
it follows that the latter must always bear a direct ratio to the former.
I t is far from my meaning to assert that the quantities of oxygen, nitrogen, and
carbonic-acid gases absorbed by sea-water at any given depth would equal the
quantity of one of these gases absorbed by the same volume of water under the
same pressure, supposing no other gaseous or saline constituent to be present in
it, but only that some increase of absorbent capacity must take place for every
increase of pressure, and, hence, that the increased quantity of one of these
gases absorbed at the base of the column of sea-water, or at any point intermediate
between the sea-bed and the surface, would bear a definite ratio to that of
the other gases absorbed along with it. I t is well known that, in order to ensure
the greatest amount of absorption by a liquid, it is essential to free it, in the first
place, by boiling or otherwise, of all other gaseous matter, and, moreover, that
the absorbent power is greatly diminished by the presence of saline substances in
solution. Now sea-water contains both other gases and as much as 3-527 of saline
substances in solution, and it is highly probable that the proportion of the latter
increases with the depth. Therefore we should naturally expect to find the
increase of absorption moderate in comparison with what would take place were
the water in a state of purity.