ay, it was again drawn off from the
sediment, (at this time, small in quantity,) and poured into the tank.
Several newly obtained plants of well-growing Enteromorpha and Corallina
were arranged among the stones, and the Aquarium was left at rest.
Gradually the water became nearly clear, but not perfectly so until
after the introduction of animals.

Eight days after it was filled, the Actinias were put in; on the ninth,
several small Mollusks; on the tenth, Crustacea; and on the eleventh and
twelfth, other varieties of the same types; but not until the fourteenth
day were fishes ventured upon.

Day by day the water grew clearer and clearer, until, at the end of
three weeks, it was beautifully translucent. Three more weeks passed,
during which the beauty of the Aquarium was much heightened by a
luxuriant growth of Confervae mingled with Enteromorpha, which together
covered all those parts of the stones which received a direct light.
The mimic rocks seemed draped in green velvet, and in the sunlight were
studded with pearly bubbles. There was, however, one blemish: the hungry
crabs had so nibbled the larger plants that it was deemed necessary to
renew them, in order to secure a sufficient supply of food and oxygen.
Accordingly, a fine specimen of Enteromorpha was added. It consisted of
five or six delicate fronds about five inches in length, and these soon
increased to treble their original number and twice their original size.
At the end of about two weeks, they suddenly became covered with a dull
bluish mould, at the same time ceasing to give out bubbles; and the
whole plant, instead of rising to the surface of the water as hitherto,
hung limp from the fissure where it was placed, and trailed upon the
sand. Coincidently, (was it consequently?) a greenish tinge pervaded the
water, speedily increasing in depth and opacity. In five days, no object
could be discerned six inches from the glass, and my beautiful Aquarium
was transformed to an unsightly ditch.

Yet the water was apparently pure, and the activity of its inhabitants
was in no wise lessened. What was this vexatious greenness? Was it
animal or vegetable? Was it the diffused spores of the perfected
Enteromorpha or of the rank Confervae upon the stones? If neither, what
was its cause?

Excess of light was the most obvious suggestion; and so it was supposed
that its exclusion might be a potent remedy. Therefore a double curtain
of glazed muslin was stretched acros