150 
BRAMBLES AND BAY LEAVES. 
It was carbonic acid as much as absence of oxygen that 
killed our fishes just now, for though inhabitants of 
water they were not the less suffocated. Therefore I see 
why, in the tank that has been left alone, plants have 
cast anchor on the glass walls, the brown pebbles, and 
the gray blocks of sandstone rock. My fishes breathe, 
and breathe. If their numbers are properly propor¬ 
tioned to the area they occupy, they will never exhaust 
the water of oxygen, never render it foetid with carbonic 
acid, so long as one necessity of vegetable life—light— 
is allowed to use its active influence to paint the plants 
green, even as oxygen gives a sanguine hue to the gills 
or lungs of the fishes. To those plants the carbonic 
acid which the fishes expire day and night, is as essential 
as oxygen is to the animal economy; and thus, without 
introducing a single scrap of any living plant, the balance 
is sustained, and death seems to be kept at a distance. 
If at first I threw in a tuft of callitriche or anacharis, or 
any other true aquatic vegetable, oxygen would be 
supplied abundantly; and in practice it might be well 
to begin so, because some little time elapses ere che 
seeds of the microscopic forest, the tops of whose trees 
present to the eye but a felt-like coating of superficial 
greenness, are developed into true plants; though with a 
fair amount of indirect daylight, and at certain seasons of 
the year, a few hours suffice to set the vegetative process, 
with all its proper consequences, in full action. Many 
of the readers of this paper will call to mind the aqua¬ 
rium that stands in my entrance hall. It contains 
twenty fishes large and small, and not a single scrap of 
vegetation except what has been developed in situ by 
