, a King-Crab, though of
most unregal aspect) was allowed to make his burrow in the sand. On
the eighth day, four Hermit and Soldier Crabs and two Sand-Crabs were
invited to choose their several retreats. On the ninth, three fine
Sticklebacks and three Minnows were made free of the mimic ocean; and on
the tenth, an Eel and two Prawns.

All seemed well until the evening of the twelfth day, when a small white
cloud was seen rising from the bottom. The spot was searched for some
dead member of the new colony; but none was found, either there, or in
any other part of the tank.

Supposing that the impure gas might be generated by the decay of minute
creatures congregated in the cloudy corner, a lump of charcoal was tied
to a stone and sunk upon the spot. Next morning, the cloud had cleared
from around the charcoal, but slender wreaths of similar appearance were
rapidly rising from the sand in every other part of the Aquarium. The
fishes came oftener to the surface than they were wont, and all the
animals had lost vigor.

Aeration was resorted to, which was performed by dipping up the water,
and pouring it back in a thin stream from a height of several feet,
continuing the operation for ten minutes. This was repeated four or five
times during the day, and at night more charcoal was added. Some of the
pieces were sunk to the bottom, and others were suspended at different
depths in the water.

Two or three days passed in this way,--the putrescence kept in check by
the means used, but not entirely overcome. Meantime, though none of the
stock had died, there was less vitality than at first; especially each
morning, after seven or eight hours unaided by aeration.

Tired of what seemed an ineffectual struggle, I determined to leave the
Aquarium untouched for a day, and await the result. Accordingly, the
charcoal was withdrawn and aeration discontinued. The milky cloud
increased in density, and the whole mass of water became turbid. The
fishes kept constantly near the surface, swam languidly, and snatched
mouthfuls of atmospheric air. The Eel became bloodshot about the gills,
and, writhing, gasped for breath. The Soldier-Crabs hung listlessly
from their shells, and no longer went about in quest of food. Even the
Actinise shrunk to half their former size; and the Buccina, crawling
above the water, ranged themselves in a row upon the dry glass.

Disappointed, but not discouraged, I filled several shallow pans with
pure sea-wate