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of what are termed water-breathing creatures, the phenomena are essentially the
same, inasmuch as the supply of oxygen, though not derived immediately from
the atmosphere, is obtained from a certain quantity of atmospheric air present
under all circumstances in waters fitted to be the habitation of organized beings.
Any of the causes which materially diminish the supply of atmospheric air
tend to impair vital energy, and, if sufficiently protracted, to destroy life. Thus,
fishes and all other aquatic animals die almost immediately if placed in water
which has been deprived of its air by boiling.
Under these circumstances, the due aeration of the waters of the ocean constitutes
one of the most important conditions of marine life *; and it becomes a question
of the highest interest to trace the process whereby it is brought about in these
deep abysses which would seem, at first sight, to be completely beyond the pale
of atmospheric infiuence.
I t is still a disputed point whether the oxygen and nitrogen of the atmosphere
enter into chemical combination, or simply constitute a mechanical mixture.
Whichever view is correct, the union takes place in determinate proportions
—namely, one volume of oxygen to four volumes of nitrogen ; and these remain
constant under every degree of rarefaction and compression, and at all temperatures
short of combustion.
Although the above may be regarded as the normal composition of atmospheric
air, it is certain that the present order of life on the earth could not be maintained
were these the sole elements present. Even animal life, which, for anything
that we know to the contrary, depends on the respiration of air thus constituted,
would speedily become extinct, inasmuch as it is dependent on the previous
manifestation of plant-life, and plants require for their respiration and growth
carbonic acid, aqueous moisture, and ammonia. Now, by a wondrous system of
compensation, the gases evolved during the processes of animal respiration, combustion,
and fermentation, are exactly those which minister to the respiration
and nutrition of plants ; whilst plants, in their turn, exhale the gas which is
essential to the life of animals, and consume those elements which, if permitted
* I t is necessary to state th at aeration is made secondary to temperature, inasmuch as under the
existing order of natui’e it is impossible to imagine the proportion of air present in the waters of the
ocean so altered as to endanger the life of creatures inhabiting them ; but it is far from being so to
conceive the occurrence of changes of temperature which might not only endanger but destroy it.
to accumulate in the atmosphere to an indefinite extent, would render it unfit
for the support of all organized beings whatever.
I f we turn to the ocean, we may find the conditions modified but not changed.
For whilst atmospheric air, as already pointed out, is indispensable to the
respiration of water-breathing (or, in other word.s, marine) animals, carbonic acid
is equally indispensable for that of marine plants. I t has been customary
to regard carbonic acid as present in sea-water in comparatively minute quantity,
and, as if in apology for this minuteness, to maintain that it could not “ possibly
be wanting in a fluid in which a far greater number of animals breathe than on
land ” *. 1 shall hereafter endeavour to prove that the supply of this gas
demanded for the ocean is infinitely in excess of the quantity usually ascribed to
it, and that, whilst it is derived by absorption from the atmosphere, the actual
quantity poured forth into the atmosphere during the processes of animal respiration,
combustion, and fermentation could be reduced by no other means than
this absorption to the small proportional amount discoverable by analysis at any
single period.
In the same manner that the aqueous vapour present in the atmosphere is
merely held in suspension, so are the gaseous bodies present in sea-water merely
held in solution ; that is to say, iu neither instance does a chemical combination
occur under ordinary pressure. Water absorbs a certain quantity of all gases,
some, however, in much larger quantity than others. Thus, whilst it absorbs
nearly volume for volume of carbonic acid under ordinaiy pressure, it can only
take up about -ÿyth of its volume of oxygen and ^ijth of nitrogen. AVben
the two latter gases occur in the shape of atmospheric air, the quantities are
slightly modified—rather more oxygen, and rather less nitrogen, being absorbed.
But, under an increase of pressure, the proportion of gaseous matter taken up by
water is very greatly increased, as may readily be understood when it is borne
in mind that fluids are nearly incompressible even under extreme pressure,
whereas gases are easily compressible, and that the relative volumes of the gas
absorbed and the fluid absorbing are acted on in widely different degrees.
The mode in which the solution of atmospheric air in sea-water is brought
about is but imperfectly understood. I t is probable that the agitation of the
surface resulting from the action of winds and currents assists in effecting
* Bischof, vol. i. p. 116.
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