atmosphere as a universal thing, and I call it a world, needing no aid for its continuance and the perfect adjustment of its balance of power from external things. I take a vessel of glass, a few pebbles, a few pieces of sandstone rock, and a sufficiency of water, and to that I commit my fishes and insects, and say, "There is your world; the order of nature is such, that you may henceforth live and die without human interference." I say nothing here of the details of management; I am looking for instruction in the laws of life and death.

The two requisites of animal life, food and air, must be generated in this world, or it ceases to instruct me; yet the water contains but little of each, and whence is its supply to come? God has ordained such a wealth of organic forms, that wherever the conditions of life are found, life takes possession of the spot, whether it be the bottom of the ocean, the dripping roof of a cave, the expanse of the viewless air, or the mimic lake I call an aquarium. Forthwith the dead stones become alive with greenness, the glass walls assume the semblance of a meadow, the milky hue of the water disappears as the earthy particles it held in solution subside, and the light that streams through it takes a tint of greenness. There is an order of vegetation appointed to occupy such sites, and almost every non-metallic, and some metallic substances too, become speedily coated with confervæ, when their surfaces are kept moist a sufficient length of time. Were it not so, the inhabitants of my world must perish; and to prove the fact I try an experiment. I place some fishes in a clean vessel of water, without pebbles

and without rock; the moment the first dim bronzy speck appears, I rub it off the glass, and so thwart the course of Nature. The fishes soon exhaust the water of its oxygen, and though the water attempts to renew its supplies by absorption from the atmosphere, the compensation is too slow, the fishes come gasping to the surface, and in a short while perish.

Even then I learn something from their death, if I leave the vessel in the hands of Nature. Death has no sooner spread his black banner over my household gods than life of another kind arises to confound him, and the microscope reveals to me myriads of animals and plants, and organisms that seemingly occupy an intermediate place between the two great kingdoms, rioting upon the wreck that death has made. My half-dozen dead fishes have given birth to existences numerous as the stars in heaven, or as the sand upon the sea-shore, innumerable. While these devour the banquet death has spread for them, while forests of confervoid threads rise in silken tufts like microscopic savannahs, Nature is passing portions of the ichthyic débris through her laboratory, and the very source of life for which they pined and perished-oxygen-is poured in in large measure, and the corruption is quickly changed to sweet

ness.

Of the once sportive fishes some portions have become air, other portions have become water, but the chief of their bulk lives already in the vegetation which hides their grave, and the moving throng with which that vegetation is peopled. God's purpose in the working of the laws in obedience to which these changes have taken place, is manifestly to keep ever true that balance

of life and death of which He holds the beam in His own hands.

But my aquarium which has not thus been interfered with, presents already a similar scene of life and bustle. When first supplied, the milky-looking water was abundantly full of gaseous matters, and every part of the rough rockwork was, for a time, studded with silvery globules. The fishes consumed all that in the process of breathing. As the water passed through their gills the oxygen was absorbed; that oxygen, by a process of refined chemistry, and perhaps by the help of iron also, gave their gills a bright red colour, gave their blood its red colour too, and by other processes not less refined, sustained the balance of life's functions within them, for without it they must perish. We believe that not the airiest particle of earth, atmosphere, or water, nor the most minute globule of condensed moisture, or the most infinitesimal point of meteoric dust, can ever be lost, at least during Time, from the fabric of the universe. My fishes tell me that the oxygen they absorb from the water, they again return to it, but in another form. They inspire oxygen and expire carbonic acid, just as a man does, and every other living creature that moveth upon the face of all the earth. Is it within the reach of human power, even when reason, imagination, and fancy combine together as a bold triad to look direct upon a fact, to appreciate that principle of terrestrial life by which animal and vegetable organisms reciprocally labour to maintain the balance of atmospheric purity? The carbonic acid given off by the animal is poison to it, if it accumulate while the supply of oxygen is cut short.

It was carbonic acid as much as absence of oxygen that killed our fishes just now, for though inhabitants of water they were not the less suffocated. Therefore I see why, in the tank that has been left alone, plants have cast anchor on the glass walls, the brown pebbles, and the gray blocks of sandstone rock. My fishes breathe, and breathe. If their numbers are properly proportioned to the area they occupy, they will never exhaust the water of oxygen, never render it foetid with carbonic acid, so long as one necessity of vegetable life-lightis allowed to use its active influence to paint the plants green, even as oxygen gives a sanguine hue to the gills or lungs of the fishes. To those plants the carbonic acid which the fishes expire day and night, is as essential as oxygen is to the animal economy; and thus, without introducing a single scrap of any living plant, the balance is sustained, and death seems to be kept at a distance. If at first I threw in a tuft of callitriche or anacharis, or any other true aquatic vegetable, oxygen would be supplied abundantly; and in practice it might be well to begin so, because some little time elapses ere the seeds of the microscopic forest, the tops of whose trees present to the eye but a felt-like coating of superficial greenness, are developed into true plants; though with a fair amount of indirect daylight, and at certain seasons of the year, a few hours suffice to set the vegetative process, with all its proper consequences, in full action. Many of the readers of this paper will call to mind the aquarium that stands in my entrance hall. It contains twenty fishes large and small, and not a single scrap of vegetation except what has been developed in situ by

spontaneous generation. It is five years since that was fitted and stocked, and committed to the management of Nature, with the sole exception of the external aid afforded by regular supplies of food for its inmates, which need not be taken account of, now that we are considering it as a world in which the balance of life and death is sustained by the operation of principles ordained by the Creator.

It is when we leave the principles, and attempt to classify the details of the scheme, that we become bewildered. The smooth revolution of the flywheel and the noiseless oscillation of the piston, convince the unprofessional observer of a great engine, that mechanical motions are possessed of poetry; but if he would analyze the relations of the cog-wheels, the indications of the "governor," the "gauge," and the pressure-valve, he must descend to hard facts, and forget for a while the sublime suggestions of a system of mechanism that throbs like a living creature. Admit a full glare of summer sun to the aquarium, and forthwith the water loses its pellucid fluidity, and becomes deeply tinged throughout of a dull green, as if some pigment had been dissolved in it. Instead of plants attached to stones and glass only, and animals that float unseen, the whole of the water is occupied by visible masses of animal and vegetable life; and if the fishes suffer, it will be from undue heat, not from the addition to the element in which they live of this new mass of being. Shut out the sunshine, let the fresh air play over the surface of the water, let moderate daylight stream through it as before, and speedily the green fog clears away, the water again becomes trans