By "floor" is meant the surface of the sand, gravel, or earth, at the bottom of the tank. In the common fish-globes, this is frequently merely the glass itself, though sometimes a handful of gravel is used to cover the bottom. But in a properly constructed aquarium, where a considerable variety of animals and plants are to be kept, great attention should be paid to the floor and to the soil beneath it. Some animals are very fond of burrowing, and some of the plants require soil in which to grow, and the needs of both should be provided for.

The great difficulty will be found in regard to the soil. If merely placed on the bottom of the tank and covered with sand or small gravel, the crayfish, etc., will probably dig down to it, disturb it, and muddy the water. They like no better fun, but it spoils the aquarium. We have found that the best plan is to cover the bottom with rich soil, and then pave it over closely with thin, flat stones. The roots of the plants will find no difficulty in getting down, but the crayfish can not follow. As it is not well to have the soil packed too closely by the weight of the sand, gravel, and stones above, we generally mix it with small stones, and upon these the pavement rests. The pavement of thin stones is then covered to the depth of one or two inches with fine gravel or coarse sand. For the marine aquarium, well-washed sea-sand is the best: for the fresh-water aquarium, the best material will be found in the bed of some clear and rapid stream. This, when freed from mud by washing, will be sure to answer well. Where this can not be had, use good building-sand or gravel, well washed. The amount of washing required is something enormous; and unless this operation is thoroughly performed, the tank will never prove satisfactory. The sand should be so clean that when a handful of it is poured into a tall jar it will sink to the bottom in less than one minute and leave the water perfectly clear.

The beautiful white gravel used for roofing makes a very pleasing floor; but as it is brought from the sea it requires not only washing, but thorough soaking for some time before it can be used in the fresh-water aquarium.

Avoid limy and ferruginous sand; that is to say, sand that is impregnated with lime or iron. Such sand may, in general, be known by its peculiar reddish color.

The soil at the bottom of the tank may be any rich garden mould. Some plants - such as hornwort, anacharis, etc. - grow freely while simply floating in the water: these need no soil. Others do better when securely anchored; but for them sand or gravel is sufficient. There are some, however, like valisne-ria, beccabunga, cress, etc., that do not thrive well unless rooted in soil. There are two ways of supplying their needs: One is to cover the entire bottom of the aquarium with soil, and plant the specimens in this, covering it over with sand or fine gravel, as previously described. Another way - and the one which we confess we like best - is to set out the plants in small pots, which are sunk to the very bottom of the tank and concealed by a heap of rock. The pots for this purpose should be shallow: common flower-pots cut down answer very well. This can almost always be done very readily by means of an old saw. A height of 2 1/2 inches is quite sufficient. A cocoanut-shell makes a good pot for such purposes. It does not decay readily; it is easily cut with a saw; and the bottom may be drilled full of fine holes, which is a great advantage. But the neatest and best pot may be made out of a piece of soft sandstone, cut with a chisel to the proper shape, and hollowed out for the reception of the soil. We have often wondered that the dealers in aquarium stock do not manufacture pots specially for this purpose. They should be of a shape the reverse of the common flower-pots, - that is, widest at the base.