LIV. POTASSIUM.

Potassium was obtained by Davy in the first of those experiments by which the existence of the alkaligenous metals was made known. It may be derived from the alkali by voltaic action, or by heat and chemical affinities, in the same manner as the other metals of the class to which it belongs. It is best procured by distillation from a mixture of hydrate of potassa, charcoal, and iron filings.

The metal when newly cut resembles mercury in lustre and nearly in colour. At 32° F. it is brittle, and when broken exhibits a crystalline texture. By an increase of temperature it becomes malleable, and at 50° is so soft that it may be moulded by the pressure of the fingers like wax. By a further rise of temperature it becomes viscid, but is not entirely liquefied until the temperature reaches 150°. Atmospheric air being excluded, it volatilizes at a low red heat. It is the lightest of known metals, its density at the temperature of 60° being .865. It is, like sodium, a good conductor of electricity. It is rapidly oxidated in the air, becoming covered with a crust of potassa in a few minutes, on which account it is preserved in vessels from which air is excluded, or under fluids, such as naphtha, which do not yield oxygen. When heated in the open air it takes fire, and burns with a brilliant purple flame. It decomposes water the instant it touches it, and is itself set on fire by the heat evolved, burning brightly on the surface, while the hydrogen evolved is likewise set on fire. If plunged under water, a violent action in like manner takes place.

Its combining weight has been estimated at 39.26. It combines with oxygen, for which it has the greatest affi

nity of any known substance; with chlorine, sulphur, and other bodies, in the manner of sodium, forming analogous compounds.

Its protoxide, potassa, has been known and used in the arts from ancient times, and was familiar to the alchymists in their fruitless labours. But the alchymists mistook the hydrate for the pure alkali, and when they termed it the vegetable alkali, were ignorant of its existence in the mineral kingdom. It is found, like soda, in all the great rock formations, and forms the basis of numerous mineral species, chiefly along with silica, alumina, and other earthy bodies. In the anhydrous state it is a white solid, intensely caustic, and fusible at a temperature a little above that of redness, but capable of bearing the strongest heat without being decomposed or volatilized. It has a powerful affinity for water, and great heat is evolved during their combination. Water dissolves it in any proportion, but with the single equivalent, it forms the true hydrate: the excess may be driven off by heat, but the single equivalent which constitutes the hydrate remains so strongly combined, that it cannot be thus separated. This hydrate is solid at common temperatures, but fuses at a heat somewhat below redness, and in cooling assumes a crystalline texture. It has the essential characters of the anhydrous oxide, possessing the properties termed alkaline in the highest degree, and neutralizing the most powerful acids.

The chloride of potassium is similar to the chloride of sodium, and so it may be said of all the analogous combinations of the two metals which have been compared together. And not only are sodium and potassium thus closely allied in their properties, but they follow in a natural sequence the bodies which precede them. From magnesium, the gradations consist in an increasing affinity for oxygen, and a corresponding increase in the alkaline characters

of the compounds. Their oxides are all soluble in water, caustic and bitter, and they all act similarly upon vegetable colours, constituting a natural group as well defined as any in the mineral kingdom.

But these oxides are each composed of a metal combined with oxygen. There is, however, another body possessing the very same characters, and known from the earliest periods of chemical inquiry as an alkali; but composed, not of a metal combined with oxygen, but of two other bodies, hitherto regarded as simple, combined with one another, hydrogen and nitrogen. This body is ammonia, the known composition of which proves, beyond dispute, the compound nature of the alkaligenous metals, and, consequently, of all the metals.

AMMONIA.

Ammonia is obtained by a well-known process from salammoniac, which is regarded as a compound of hydrochloric acid and ammonia. The substance obtained from this salt has been proved, by synthesis and analysis, to be a compound of hydrogen and nitrogen, H3 N. It is a colourless aëriform fluid, having a peculiar and pungent odour. It is altogether irrespirable in the pure state, and can only be received into the lungs when largely mixed with air. It extinguishes the flame of burning bodies, and yet is itself slightly inflammable, for a small jet of it will burn in an atmosphere of oxygen gas, and when a lighted taper is immersed in it, the flame enlarges before being extinguished. When mixed with oxygen gas, it detonates by means of the electric spark, water being formed and nitrogen set free. At the temperature of 50°, and under a pressure of 63 atmospheres, it becomes liquid, and is then colourless and transparent, and of a density of about 6.76.

This substance possesses the properties termed alkaline in a high degree. It affects in the same manner as the other alkalies the vegetable colours; it has the acrid taste which characterizes them, and, like them, it combines with acids, and neutralizes them.

Ammonia has a powerful affinity for water. According to Sir Humphry Davy, water takes up 670 times its bulk of the gas, under a pressure of 29.8 of an inch, at 50° F.; and under strong compression, or at a higher temperature, it absorbs a larger quantity. During this absorption heat is evolved, and a great expansion of the liquid takes place. In saturating with gas 6 cubic inches of water, Dr Thomson found that the water increased in bulk to 10 cubic inches.

The aqueous solution of ammonia possesses the taste, the pungent odour, and the alkaline properties of the gas. It enters into ebullition at the temperature of about 130°, and it freezes at nearly the same temperature as mercury.

The salts of ammonia are similar in their essential characters to the other alkaline salts, distinguished only by the results produced by the base of the one being volatile, and the other fixed. Thus none of the salts of ammonia can bear a red heat, without being dissipated in vapour, or decomposed. When the acid as well as the base is volatile, they both pass off together unchanged; but when the acid is fixed at a low red heat, then the ammonia alone is volatilized.

Now, here is a substance alkaline, in every sense of the word, associated with the other alkalies by characters and properties which connect it as closely to them as the species of any genus are related to one another. Can we believe that these substances, though represented by different chemical formulæ, are really different in their essential constitution? Are we not rather compelled to believe, that our chemical formulæ, whatever they be, must represent substances of a common nature? But ammonia is represented by H3N, and the alkalies by MO, M representing the alkaligenous metal. We must therefore find a common expression for H3N and MO; and this we can only do by reducing them to a common root.

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Now, H3N, on the hypothesis = H3+CO=H3 + H2 C2= H5 C2. By the table, potassium=H11+ C2 O2, and potassa accordingly, H1 C2 02+0=H" C203=H11C2+H6C3=H17 C5; and upon the same principle sodium, H11 C2, and soda,= H11 C2 + H2 C=H13 C3. We have therefore, as expressions for the three substances respectively,

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