[An abstract of a Lecture on Water, delivered before the Mechanics' Institute, at Cincinnati, Ohio, by Prof. Chas. W. Wright.] Water is composed of oxygen and hydrogen combined, in the proportions of eight parts of the former to one of the latter, by weight, and by volume, of two of hydrogen and one of oxygen, or in 100 parts of water there are 11-1 of hydrogen united to 88'9 01 oxygen. Water plays the part of a base towards acids, and of an acid towards bases: thus, when it combines with sulphuric acid it is termed the sulphate of water, and when it unites with lime the compound takes the name of the hydrate of lime. By most chemists, water is looked upon'as a metallic oxide or rust, hydrogen being considered a metal; and Dr. Kane has shown that there is the closest similarity between the oxydes of zinc and copper, and water, which is the oxyde of the metal hy drogen. In some quantities, water is transparent and colorless, but in large volumes it is blue by reflected, -and green by transmitted light. The s0lvent powers of water exceed that of any other liquid. As a generating, this solvent power is increased by heat, as regards solids, but the reverse is the case as regards gases. Under great pressure, water will dissolve bodies, as glass, which are insoluble in it at a moderate heat and pressure. Walter, like most 0ther bodies, contracts on the abstraction of heat ; but when its temperature is reduced to 39", the 1o8s cf heat, instead of causing contraction, increases its volume, and hence, at '390, water is at its point of maximum density, and the addition or abstraction of heat will augment its volume. From 39° to many degrees below the freezing point o f water, it expands, and hence ice is ofless specific gravity than water, and floats upon it. The impor- tan®e of this, in preventing the consolidation 0f large bodies of water, is evident. At 32°, if water be agitated, it freezes, but ifit be under great pressure, which resists its expansion, it will not consolidate at that degree of temperature, and the same is true in regard to other substances that expand in congealing. Water, in the act of freezing, parts with all matter which it holds in solution and suspension, and hence the water of the ocean is rendered sweet and drinkable by being frozen. On the same principle, ice, which is cut from pools that are stagnant in summer, is fit for d0mestic use. Ice which contains air and 0ther impurities, mechanically suspended in it, thaws much sooner than that which is free from such contamination. In passing into the condition of water, ice conceals, or renders latent, 140” of heat, and water, in the act of freezing, imparts the same quantity of heat to surrounding bodies. Hence, freezing is a, heating process, and thawing a cooling one; for the same reason laige masses of snow and ice are never melted suddenly, and inundations are less frequent than they otherwise would be. When the mercury in the barometer stands at 30 inches, water boils at 212° in a metallic vessel, but in a smooth glass vessel it requires 214° to make water boil. Water, free from air and other impurities, does not boil at 212", but when it reaches a temperature of '270° it explodes violently. This can best be shown by melting ice under the surface ol oil, which prevents the absorption of air, when at 270° it explodes. A diminution of atmospheric pressure reduces the boiling point, and if it be increased the reverse is the case. Water evaporates at all temperatures, even in the condition of ice it assumes the gaseous form, with- 0ut passing into the intermediate condition of a liquid. This is seen when articles of clothing are hung out to dry in winter, when they freeze and dry without thawing, and the same thing is seen when the mud in the streets freezes and dries, although the temperature may remain below 32° In boiling and passing into the condition of steam, water takes up and conceals 1000'” of heat; and when this fluid is boiled violently, it is no hotter than when it boils . slowly, lrom the fact of the ad. ditional heat being carried offby.the steam,and the temperature of the water, under ordinary atmospheric pressure never rises higher than 212". The knowledge of this fact is of great importance in domestic economy; tor water, boiling gently, will accomplish as much in culinary operations as when the ebullition is violent. The latent heat carried off by steam has much to do with regulating the temperature of the earth, and also that of the bodies of animals, for the same law holds at low as well as at high temperatures. At 212° the elastic force of steam is equal to a pressure of one atmosphere ; at 250'” the pressure is doubled; and at a temperature of 500° we have a pressure equal to 50 atmospheres. Thus we sea, contrary to' the common impression, it does not require twice the amount ot heat to double the pressure. The action of water on metallic vessels is not a little singuiar: thus, it the water be pure, its action is more corrosive to lead aijd zinc than if it have mineral matter in solution. The Water of the oeean, at great depths, has a different action upon me talsfrom what it has at the surface. Ali the surface, according to the experiments t;f Dr. A. A. Hayes, metals are converted into 6xydes and chlorides, but at great depths the same metals are converted into the sulphurets.