In the tank, or aquarium, were two gold fish, six water-snails, and two or three specimens of that elegant aquatic plant _Valisperia sporalis_, which, before the introduction of the water-snails, by its decayed leaves caused a growth of slimy mucus, and made the water turbid and likely to destroy both plants and fish. But under the improved arrangement the slime, as fast as it was engendered, was consumed by the water-snails, which reproduced it in the shape of young snails, which furnished a succulent food to the fish. Meanwhile the _Valisperia_ plants absorbed the carbonic acid exhaled by the respiration of their companions, fixing the carbon in their growing stems and luxuriant blossoms, and refreshing the oxygen (during sunshine in visible little streams) for the respiration of the snails and the fish. The spectacle of perfect equilibrium thus simply maintained between animal, vegetable, and inorganic activity, was strikingly beautiful; and such means might possibly hereafter be made available on a large scale for keeping tanked water sweet and clean.--_Quarterly Review_, 1850. HOW TO IMITATE SEA-WATER. The demand for Sea-water to supply the Marine Aquarium--now to be seen in so many houses--induced Mr. Gosse to attempt the manufacture of Sea-water, more especially as the constituents are well known. He accordingly took Scheveitzer’s analysis of Sea-water for his guide. In one thousand grains of sea-water taken off Brighton, it gave: water, 964·744; chloride of sodium, 27·059; chloride of magnesium, 3·666; chloride of potassium, 9·755; bromide of magnesium, 0·29; sulphate of magnesia, 2·295; sulphate of lime, 1·407; carbonate of lime, 0·033: total, 999·998. Omitting the bromide of magnesium, the carbonate of lime, and the sulphate of lime, as being very small quantities, the component parts were reduced to common salt, 3½ oz.; Epsom salts, ¼ oz.; chloride of magnesium, 200 grains troy; chloride of potassium, 40 grains troy; and four quarts of water. Next day the mixture was filtered through a sponge into a glass jar, the bottom covered with shore-pebbles and fragments of stone and fronds of green sea-weed. A coating of green spores was soon deposited on the sides of the glass, and bubbles of oxygen were copiously thrown off every day under the excitement of the sun’s light. In a week Mr. Gosse put in species of _Actinia Bowerbankia_, _Cellularia_, _Serpula_, &c. with some red sea-weeds; and the whole throve well. VELOCITY OF IMPRESSIONS TRANSMITTED TO THE BRAIN.