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 across the window; and the tank, both top
and sides, wrapped in folds of paper. A week of darkness changed the
deep green to a dingy olive. But the experiment could not be continued.
The nightly admission of air by lifting the paper covering was
insufficient to maintain the imprisoned creatures. They were happy,
though captive, while in a mimic ocean, but miserable in a dark dungeon.
Languid and spiritless, they lay supine, or crawled listlessly and
aimlessly about. This would not do, and so light was again admitted
freely to all but one side of the tank; there, a screen of yellow paper
intercepted the direct rays of the sun, while upon the top they fell
through the foliage of a Clematis vine.

Three weeks more wrought a slight change for the better; but it was too
slight and too slow for my patience, or that of curious friends waiting
to see my Aquarium.

The second experiment had failed, and so once more the tank was emptied.
Two or three animals only had died; all the others gave evidence of
health. Again they were removed to other vessels, and again I began
anew.

Clean sand, clean stones, water drawn at high tide and carefully