[An abstract of a Lecture on “Potassium and its Compounds,” delivered before the Mechanics' Institute, at Cincinnati, Ohio, by Prof. Chas. W. Wright] Potassium is obtained by heating a mixture of carbonate of potash and charcoal to whiteness in an iron retort. The potassium is condensed in a copper receiver, kept cool by being surrounded by a freezing mixture, and which contains naphtha orrock-oil,into which the melted potassium drops, and is preserved Irom the action of the oxygen of the air. Potassium is a brilliant white metal, with the lustre of polished steel; and is so soft that it can be moulded by the lingers, and can be readily cut with a knife. It melts at 150, and is so light that it floats upon the surface water. On exposure to the air it is instantly tarnished, being converted into oxide of potassium, or what is chemically termed " Po-tassa." When thrown upon water it bursts into a beautiful purple flame, forming a solution of potassa, which has an alkaline re-action. In this experiment the combustion is due, in part, to the potassium combining with the oxygen of the water, it having the power to decompose that liquid; hence it is kept under the surface of naphtha, a liquid which contains no oxygen. The equivalent, or combining number, of potassium 01 " kalium " is 39 19, and its symbol, It. Hydrate of Potassamdash;water combined with the oxyde of potassium, K.O H.O.mdash;This substance is obtained by boiling 10 parts of carbonate of potash in 100 parts of water, and adding little by little, 8 parts of recently slacked lime. Alter boiling a short time it is allowed to cool, when the carbonate oi lime or chalk subsides to the bottom of the vessel. When properly prepared, the clear liquid does nafc effervesce on the addition of an acid.JThe whole operation must be conducted in a covered scessel, * * to exclude the air. When evaporated to dryness, and melted, and run into moulds, it constitutes the" potassa fusa" of the drugstores; when in solution it is termed " liquor potassse." Potassa, or caustic potash, is deliquescent, and rapidly absorbs carbonic acid from the atmosphere, and must be preserved in well stoppered bottles. It readily attacks aid dissolves the skin, and is highly poisonma. The antidote is vinegar or sweet oil. With the fixed oils, potassa combines and forms a class of salts commonly called " soaps." Moat of the tats and oils consist of oleic, mar-garic, and stearic acids, comamp;ined with an organic base, " glycerine ;" potassa being the stronger base, combines with the fatty acids, forming salts or soaps, which, when potassa is the base, are soft, but if soda is the base they are hard. Carbonate of Potassa, K.O.CO.mdash;This compound is always obtained by lixiviating or leaching wood ashes. Carbonate of potash, however, never exists as such in plants, the potassa being in combination with some vegetable acid, as oxalic, citric, tartaric, amp;c. When the plants are burned the vegetable acids are converted into carbonic acid, which, combining with the potassa, forms the carbonate of patassa,or what is commonly termed "potashes." The branches of trees yield more than their trunks, shrubs next, and herbs and leaves still more, on incineration. This distribution is probably due to the saline matter existing chiefly in the juices of the plant. Certain plants, as wormwood, for example, yield more carbonate of potassa, when burned, than others. Organic acids, when combined with potassa, undergo the same change when taken into the systems of animals, that they do when burned in the air, being converted into carbonic acid, which, uniting with the potassa, is thrown from the system as carboaate of potassa. Nitrate of Potassa, Nitre, Saltpetre, K.O.N. O5.mdash;In the decomposition of animal matter containing nitrogen, in contact with alkaline bases, nitric acid is always lomied, which combining with the bases, generates that class of salts called "nitres," or the "nitrates." Nitre acid is probably tormed in all cases by the oxydation of ammonia, which is the nitrogenous compound evolved in all cases of putrefactive decomposition. Thus, eight equivalents oi oxygen and one of ammonia yield one equivalent of nitric acid and three equivalents ol water. This change will be more intelligible when expressed in the form of a rationale, thus :mdash;N.Hs.+L.O.mdash;N.O5.+3H.O. In certain districts in India, nitrate of potassa appears to be formed in the soil in this way, and is obtained by leaching the earth taken from such localities. In some countries this natural process is imitated by keeping decomposing animal matter and lime together, whereby the nitrate of lime is generated, and decomposed by being lixiviated with carbonate ol potassa, by which the nitrate of potassa and carbonate of lime are formed. Thus :mdash; Ca.ON.OH-K.O.C.O2=K.O.N.O5.+Ca.O.C. O2. Nitrate of lime exists naturally in certain caves, and is converted into nitrate of potassa when treated in the manner above mentioned. At the Mammoth Cave, in Kentucky nitrate of potassa was once obtained in this way. This cave is the resort of innumerable bats, which, by their death and decay, will account, in part at least, for the presence of nitrate of lime. Nitrate of potassa readily parts with its oxygen at an elevated temperature, and. from its containing a large quantity of that element, is extensively used in the manufacture of deflagrating mixtures, as gunpowder amp;c. The ingredients of gunpowder are sulphur, charcoal, and nitre. The sulphur accelerates the combustion and generates most of the heat, i-hile the combustion of the charcoal furnishes carbonic acid gas, which, by occupying much greater space than its constituents, before combustion, produces much of the force of the explosion. The following rationale will give an idea of the re-action of the constituents of gunpowder produced by their explosion :mdash;3C.+K.O.N.O5+S.=3C.O2.+K.S. +N ; three equivalents of carbonic acid, one of sulphide of potassium, and one of nitrogen gas, being the result of the deflagration. Gunpowder, when ignited, does not explode instantaneously, but the combustion is communicated from one particle to the next until it is entirely consumed. Bodies which explode instantaneously are not adapted for the movement of projectiles, their action not being productive of a sustained effort, is local, and would tend as much to shatter the fire-arms as to project the ball. Gunpowder, when struck violently, sometimes explodes, and tatal accidents are occasionally the result of inattention to this fact. Fire-arms are sometimes bust by being discharged when the ball is not in contact with the charge of gunpowdermdash;when it is not " rammed horre," as the phrase is. This is cause by the great expansion of the air which is contained between the ball and the charge, and which undeigoes so great dilatation, in connection with the deflagration of the powder, as to burst the piece. Chlorate of Potassa,K.O.Cl.O5.mdash;This salt is obtained by transmitting a stream of chlorine gas into a solution of potassa. Six equivalents of chlorine, and six equivalents of potassa, yield one equivalent of chlorate of potassa, and five equivalents of chloride of potassium, one equivalent of the chlorine being converted into chloric acid, by abstracting five equivalents of oxygen from five-sixths of the potassa, used in the experiment, the chloric acid combining with the remaining equivalent of potassa, forms the chlorate of potassa, and the five remaining equivalents of chlorine and potassium, combining from five equivalents ot chloride of potassium. These salts are separated by crystallization. The following is a rationale of the process :mdash;6K.O.-|-6C1.=K. O.C1.O5.+5K.C1. When mixed with combustibles, this salt deflagrates with.much more violence than saltpetre. It was at one time used in the manufacture of percussion caps, as a mixture of it and sulphur detonates violently when struck by a hammer. Chlorate of potassa is used as an oxydizina; agent in calico printing, and for the preparation of pure oxygen gas. Iodide of Potassium, K.I.mdash;This substance is obtained by decomposing the iodide of zinc with an equivalent quantity of carbonate of potassa ; carbonate of zinc and iodide of potassium being the result ot the decomposition, thus:mdash;Zn.I.-f-K.O.C.Os.=K I.+Zn.O.C.O!. Iodide of potassium is extensively used in medicine, and has been recently employed in the preservation of butter; but as it is no way superior in this respect, to common salt, and and possesses a poisonous action, when slowly introduced into the system, for a considerable length of time, by producing glandular absorption, its employment for this purpose should be staictly avoided.