the form rectangular. As the weight of the aquarium, when filled with
water, is enormous,--far more than we should at first imagine,--it
follows that it must be capable of resisting pressure both from above
and from within. The floor and stand, the frame and joints must be
strong and compact, and the walls of plate or thick crown glass. The
bottom should be of slate; and if it is designed to attach arches of
rock-work inside to the ends, they, too, must be of slate, as cement
will not stick to glass. The frame should be iron, zinc, or well-turned
wood; the joints closed with white-lead putty; the front and back of
glass. There is one objection to having the side which faces the light
of transparent glass, and that is that it transmits too much glare of
sunlight for the health of the animals. In Nature's aquarium the light
enters only from above; and the fish and delicate creatures have always,
even then, the shady fronds of aquatic plants or the shelter of the
rocks,--as well as the power of seeking greater depths of water, where
the light is less,--to protect themselves from too intense a sunshine.
It is, therefore, sometimes advisable to have the window side of the
aquarium made of glass stained of a green color. It is desirable that
all aquarial tanks should have a movable glass cover to protect them
from dust, impure gases, and smoke.

When we speak of an aquarium, we mean a vessel holding from eight to
thirty gallons of water. Mr. Gosse describes his larger tank as being
two feet long by eighteen inches wide and eighteen inches deep, and
holding some twenty gallons. Smaller and very pretty tanks may be
made fifteen inches long by twelve inches wide and twelve deep. Great
varieties in form and elegance may be adapted to various situations.

There are two kinds of aquaria, the fresh- and the salt-water: the one
fitted for the plants and animals of ponds and rivers; the other for the