This communication will consist of a detail of an experimental investigation which has been carried on for nearly the last twelve months, and which appears to illustrate, in a marked degree, that beautiful and wonderful provision which we see every where displayed throughout the animal and vegetable kingdoms, whereby their continued existence and stability are so admirably sustained, and by which they are made mutually to subserve each for the other's nutriment, and even for its indispensable wants and vital existence. The experiment has reference to the healthy life of fish preserved in a limited and confined portion of water. It was commenced in May 1849, and the subjects chosen were two small gold-fish. These were placed in a large glass receiver of about twelve gallons capacity, having a cover of thin muslin, stretched over a stout copper wire bent into a circle, placed over its mouth, so as to exclude as much as possible the sooty dust of the London atmosphere, without, at the same time, impeding the free passage of the atmospheric air.

This receiver was about half filled with ordinary spring water, and supplied at the bottom with sand and mud, together with loose stones of larger size of limestone tufa, from the neighbourhood of Matlock, and sandstone; these were arranged so that the fish could get below them, if they wished so to do. At the same time that the fish were placed in this miniature pond - if I may so term it - a small plant of the Vallisneria spiralis was introduced, its roots being inserted in the mud and sand, and covered by one of the loose stones, so as to retain the plant in its position. The Vallisneria spiralis is one of those delicate aquatic plants generally selected by the microscopist for the exhibition of the circulation of the sap in plants; it throws out an abundance of long wiry strap-like leaves, of about ^ inch in breadth, and from 1 to 3 feet in length: these leaves, when the sun shines on them, evolve a continued stream of oxygen gas, which rises in a current of minute bubbles, particularly from any part of the leaf which may have received an injury.

The materials being thus arranged, all appeared to go on well for a short time, until circumstances occurred which indicated that another and very material agent was required to perfect the adjustment, and which, from my not having thought of it at the time of commencing the experiment, had not been provided against. The circumstances I allude to arose from the internal decay of the leaves of the Vallisneria, which became yellow from having lost their vitality, and began to decompose; this, by accumulation, rendered the water turbid, and caused a growth of mucus, or green slimy matter, on the surface of the water and on the sides of the receiver. If this had been allowed to increase, I conceive that the healthy life of the fish must have suffered, and probably their vital functions have been destroyed. The removal of these decaying leaves from the water, therefore, became a point of paramount importance to the success of the experiment. To effect this, I had recourse to a very useful little scavenger, whose beneficial functions have been too much overlooked in the economy of animal life - I mean the water-snail, whose natural food is the very green slimy growth, or mucus, and decaying vegetable matter, which threatened to destroy the object which was wished to be obtained.

Five or six of these creatures - the Limnsea stagnalis - were consequently introduced, and, by their continued and rapid locomotion and extraordinary voracity, soon removed the cause of interference, and restored the whole to a healthy state; thus perfecting the balance between the animal and vegetable inhabitants, and enabling both to perform their vital functions with health and energy.

So luxuriant was the growth of the Vallisneria under these circumstances, that, by the autumn, the one solitary plant that had been originally introduced had thrown out myriads of offshoots and suckers, thus multiplying to the extent of upwards of thirty-five strong plants; and these threw up their long spiral flowering stems in all directions, so that at one time more than forty blossoms were counted lying on the surface of the water.

The fish have been lively, bright in colour, and appear very healthy; and the snails also - judging from the enormous quantity of gelatinous masses of eggs which they have deposited on all parts of the receiver, as well as on the fragments of stone - appear to thrive wonderfully, and, besides their functions in sustaining the perfect adjustment of the series, afford a large quantity of food to the fish in the form of the young snails, which are devoured as soon as they exhibit signs of vitality and locomotion, and before their shell has become hardened.

Thus we have that admirable balance sustained between the animal and vegetable kingdoms, and that in a liquid element. The fish, in its respiration, consumes the oxygen held in solution by the water as atmospheric air, furnishes carbonic acid, feeds on the insects and young snails, and excretes material well adapted as a rich food to the plant, and well fitted for its luxuriant growth.

The plant, by its respiration, consumes the carbonic acid produced by the fish, appropriating the carbon to the construction of its tissues and fibre, and liberates the oxygen in its gaseous state to sustain the healthy functions of the animal life, at the same time that it feeds on the rejected matter which has fulfilled its purposes in the nourishment of the fish and snail, and preserves the water constantly in a clear and healthy condition. While the slimy snail, finding its proper nutriment in the decomposing vegetable matter and minute confervoid growth, prevents their accumulation by removing them from the field, and, by its vital powers, converts what would otherwise act as a poison into a rich and fruitful nutriment, again to constitute a pabulum for the vegetable growth, while it also acts the important part of a purveyor to its finny neighbours.