and of the simplest kinds; and not until they are well settled, and the
water beaded with oxygen bubbles, should we think of introducing living
creatures,--and even then only the hardier kinds of actinias, mollusks,
and crabs. All delicate animals must be intrusted one by one to their
new home, and carefully watched for deaths and decay, which, whether
arising from dead plants or animals, ruin everything very quickly,
unless they be promptly removed. For sulphuretted hydrogen, even in very
minute quantities, is sure death to all these little creatures.

The emanations from paint and putty are often fatal in new tanks.
Several weeks' exposure to water, air, and sunlight is necessary to
season the new-made aquarium. Of equal consequence is it that the water
be absolutely pure; and if brought from the sea, care must be exercised
about the vessel containing it. Salt acts upon the glazing of earthen
ware of some kinds. Stone or glass jars are safest. New oak casks are
fatal from the tannin which soaks out; fir casks are safe and good. So
delicate and sensitive are the minute creatures which people the sea,
that they have been found dead on opening a cask in which a new oak
bung was the only source of poison. And no wonder; for a very slight
proportion of tannic acid in the water corrugates and stiffens the thin,
smooth skin of the anemone, like the tanning of leather.

A certain natural density of the sea-water must also be preserved,
ranging between no wider limits than 1026 and 1028. And in the open tank
evaporation is constantly deranging this, and must be met by a supply
from without. As the pure water alone evaporates, and the salts and
earthy or mineral constituents are left behind, two things result: the
water remaining becomes constantly more dense; and this can be remedied
only by pure fresh water poured in to restore the equilibrium. Hence the
marine aquarium must be replenished with _fresh water_, until the proper