The Aquarium furnishes a very interesting example of the mutual dependence of the three natural kingdoms. Here, in a box holding a few gallons of water and a little atmospheric air, is a miniature world, secluded, and supplying its own wants. Its success depends on the number and character of the animals and plants being so adapted as to secure just the requisite amount of active growth to each to sustain the life of the other: that the plants should be sufficient to support, by the superfluities of their growth, the vegetarians among the animated tribes that surround them; and that all the animal tribes of the aquarium, whether subsisting upon the vegetables or on their smaller and weaker fellow-creatures, should restore to the water in excrements the mineral substances which will enable the plants to make good the daily loss occasioned by the depredations of the sea-rovers that live upon them. Thus an aquarium, its constituents once correctly adjusted, has all the requisites for perpetuity; or rather, the only obstacle to its unlimited continuance is, that it is a mortal, and not a Divine hand, that controls its light and heat. In the examination of the materials appropriated by plants from the soil, we find that mineral substances are sometimes taken up in solution in larger amount than the growth of the plant and the maturation of its fruit require, and the excess is deposited again, in crystalline form in the substance of the plant. If we cut across a stalk of the garden rhubarb, we can see, with the aid of a microscope, the fine needle-shaped crystals of oxalate of potash lying among the fibres of the plant,--a provision for an extra supply of the oxalic acid which is the source of the intense sourness of this vegetable. When the sap of the sugar-maple is boiled down to the consistence of syrup and allowed to stand, it sometimes deposits a considerable amount of sand; indeed, this is probably always present in some degree, and justifies, perhaps, the occasional complaint of the grittiness of maple-sugar. But it is a native grit, and not chargeable upon the sugar-makers. It is nothing less than flint, which the roots of the maple absorbed, while it was dissolved in water in the soil. The sap, still holding the flint in solution, flows out, clear as water, when the tree is tapped; but when it is concentrated by boiling, the silicious mineral is deposited in little crystals, so that the bottom of the pan appears to be covered with sand. We could not select a more interesting example of the very wide diffusion of some compound substances than this one of silicic acid. It is found in the mineral and vegetable kingdoms. Being a mineral, it cannot be appropriated to animal uses, without being decomposed and transformed into an organic condition; but in the numerous species of plants whose stalks require stiffening against the winds,--in the grasses and canes, including all our grains, the