0.025 ..
" 169 " 0.025 ..
" 189 " 0.012 ..

During this time the alkalinity was reduced to the equivalent of 30 mg.
CaCO3 per litre, ocean water having an alkalinity equivalent to 50-55
mg. per litre. It has been suggested that the organic nitrogen becomes
oxidized into nitrous, then into nitric acid, which lowers the carbonate
values. A great deal of reduction of this nitrogenous contamination can
be effected by filtration, a method first introduced successfully at
Hamburg, where a most thriving aquarium has been maintained by the local
Zoological Society for many years on the circulation principle, new
water being added only to compensate for waste and evaporation. The
filters consist of open double boxes, the inner having a bottom of
perforated slate on which rests rough gravel; on the latter is fine
gravel, then coarse, and finally fine sand. Filtration may be either
upwards or downwards through the inner box to the outer. Such filters,
intercalated between tanks and reservoir, have been shown by analysis to
stop a very large proportion of nitrogenous matter. It is doubtful
whether aquarium water will not always show an excess of nitrogenous
compounds, but they must be kept down in every way possible. In small
tanks, well lighted, seaweeds can be got to flourish in a way that has
not been found practicable in large tanks with a circulation; these,
with Lamellibranchs and small Crustacea as scavengers, will be found
useful in this connexion. Slight or occasional circulation should be
employed here also, to remove the film of dust and other matters, which
otherwise covers the surface of the water and prevents due oxygenation.

In such small tanks for domestic use the fauna must be practically
limited to bottom-living animals, but for purposes of research it is
often desired to keep alive larval and other surface-swimming animals
(plankton). In this case a further difficulty is presented, that of
helping to suspend the animals in the water, and thus to avoid the
exhaustion and death which soon follow their unaided efforts to keep off
the bottom; this duty is effected in nature by specific gravity, tide
and surface current. In order to deal with this difficulty a simple but
efficient apparatus has been devised by Mr E.T. Browne; a "plunger,"
generally a glass plate or filter funnel, moves slowly up and down in a
bell-jar or o