And now, having got the aquarium in order, comes the point how to keep it in order,--how to keep the creatures alive, and how to prevent the water from growing cloudy and thick. The main rule is to secure sunlight,--hot enough to raise the water to a temperature above that of the outer air,-- to remove all dirt and floating scum, and to furnish the tank on every cloudy day with a supply of air and with motion by means of a syringe. The creatures should never be fed in warm weather with any animal substance, its decay being certain to corrupt the water. A little meal or a few crumbs of bread may now and then be given; but even this is not necessary; for Nature furnishes all the food that is needed, in the spores thrown off by the seaweed, in the seaweed itself, whose growth is generally sufficiently rapid to make up for the ravages committed upon it, and in the host of infusoria constantly produced in the water. If any of the creatures die, their bodies should be immediately removed,--though sometimes the omnivorous crabs will do this work rapidly enough. As the water evaporates, it should be filled up to its original level with fresh spring-water,--the salts in it undergoing no diminution by evaporation. If, suddenly, the water should grow thick, it should be taken from the tank, a portion at a time, and filtered back into it slowly through pounded charcoal, the process being repeated till the purity seems to be returning, and at the same time the rocks and seaweed should be removed and carefully washed in fresh water. If, however, the water should by any ill chance grow tainted and emit a bad odor, nothing can be done to restore it, and, unless it is at once changed, the creatures will die. To meet such an emergency, which is of rare occurrence, it is well to have a double quantity of the salts sent with the tank to secure a new supply of water. But we have known aquariums that have kept in order for more than a year with no change of the water, a supply of spring-water being put in from time to time as we have directed; and at this moment, as we write, there is an aquarium at our side which has been in active operation for six months, and the water is as clear as it was the day it was put in. If, spite of everything, the seawater fail, then try a fresh-water aquarium. Use your tank for the pond instead of the ocean; and in the spotted newt, the tortoise, the tadpole, the caddis-worm, and the thousand other inhabitants of our inland ponds and brooks, with the weeds among which they live, you will find as much entertainment as in watching the wonders of the great sea. A camel's-hair brush, a bent spoon on a long handle, a sponge tied to a stick, and one or two other instruments which use will suggest, are all that are needed for keeping the sides of the tank free from growth or removing obnoxious substances from its bottom.