THE MARINE AQUARIUM. 97 
the water, then place the finger on again, and lift the 
whole away. The advantage of this plan is, that no dis- 
turbance is occasioned. A pewter spoon bent to a right 
angle, and attached to a stick is very useful for lifting up 
objects that are too large for the dipping tube. 
Density of the Water.—Evaporation causes the pure 
water to escape from the Aquarium, and hence there is a 
constant tendency to increase of density—in fact, if left 
alone, the water in the tank soon gets too salt. To pre- 
vent this, additions of pure spring or distilled water must 
be made from time to time, and the amount regulated by 
a specific gravity bubble, which it would be as well to 
leave at all times floating in the tank. This instrument 
registers the density much more accurately than the 
hydrometer, for it is more delicate in its determination of 
a balance. I find it expedient to use two bubbles of dif- 
ferent specific gravities—one which just floats when the 
water is sufficiently fluid, and another which just sinks 
when it is sufficiently saline. The movement of either 
indicates at once the exact cause, and enables the aquarian 
to regulate the density to perfection. 
Green Stain.—I have never been troubled with con- 
fervoid growths in tanks filled with artificial water, though 
the same water has been in use during periods of from a 
few months to two years. When it occurs, the mollusks 
are the natural remedy, the sponge the artificial one. 
But a strange affection—which I think is most common 
in spring—is the sudden occurrence of a green turbidity, 
which destroys the translucence, and for which neither 
sponge nor winkle can afford a remedy. It is not a 
H 
