that the lessive may not absorb the carbonic acid contained in the atmosphere, the vessel should be almost closed, so as to prevent the circulation of the external

air.

It is also necessary to keep the mixture always covered with water, and to collect it only until it comes insipid from the tube.

The liquors obtained are nearly of the same strength to the end, when they suddenly become so weak as to be useless.

To evaporate the waters, an iron kettle may be used. Begin with those last obtained, which are rather the weakest, in order to avoid keeping the strongest for a long time in contact with the air; and employ a strong ebullition.

When it is concentrated to a certain point, the sul, phate of potash crystallizes and precipitates: it may be easily collected, by placing at the bottom of the vessel an iron skimmer, into which the salt will fall. The strongest ebullition is necessary to keep out the atmospheric air, and it assists the precipitation of the sulphate of potash into the skimmer.

If the object be to obtain common caustic, the concentrated liquor is to be poured into a smaller iron vessel, and afterwards evaporated to such a point, that when poured upon a plate of iron or marble it congeals *.

If, for nice chemical experiments it be desirable to obtain this alkali more pure, instead of using potash, supertartarite of potash, or cream of tartar calcined, may be

* We think it necessary to observe, that melted potash, thus prepared, is much more caustic than that which is prepared in pharmacy under the name of common caustic; and we caution those who may use it to be very prudent in employing this medicament.

employed;

employed; or the melted potash of which we have been speaking may be still farther purified by alcohol, according to Berthollet's method. Experiment has satisfied us that it is possible by this means to obtain a very pure alkali *. In this case the ley is evaporated to the consistence of a thick syrup in a silver basin, and it is best for the vessels to be closed: then the matter is dissolved in alcohol; the potash alone combines with it; the sulphate and muriate of potash, the portions of earth, and even the carbonic acid which it obstinately retains, or imbibes from the air during the evaporation, remain at the bottom of the solution. If alcohol be poured on this matter while warm, and if this re-agent be not employed in greater quantity than is necessary to dissolve the potash, it crystallizes as it cools in white plates, which are sometimes several inches long. If you wish to separate the potash from the alcohol, and to obtain it in a state of dryness, the solution must be evaporated in a silver basin, and not in a glass vessel; for the potash frequently dissolves a portion of the silex, which injures its purity.

By this operation, the details of which should be seen in Berthollet's memoir, caustic potash is deprived of silex, of carbonic acid, of all the foreign salts, and of the small quantity of iron which it may have taken from the vessel in which the liquor has been evaporated.

What we here relate is extracted from a memoir of the celebrated Berthollet; and may save some trouble to those who have not his work at hand.

On

On a fulminating Combination of Silver of a White Colour and crystalline Appearance. By M. DESCOTILS.

THERE

From the ANNALES DE CHIMIE.

HERE has been lately sold at Paris, as an object of amusement, a detonating powder inclosed between the folds of cards cut in two lengthways. This powder is placed at one end of the card, and the other end is marked to distinguish it with facility. If they are held by the marked end, and the other is applied to the flame of a candle, the detonation soon takes place, with a sharp explosion, and a violet-coloured flame; the card is torn and reddened, and the part which touched the composition is covered with a slight metallic coat, of a greyishwhite.

Having been consulted on the nature of this material, which is sent ready prepared to Paris, I am convinced, from various experiments, which it would be useless to relate, that it is a combination of oxyd of silver, ammoniac, and a vegetable matter; a combination analogous, as we see, to that which constitutes the fulminating mercury of M. Howard.

We may obtain this combination, which I shall call detonating silver, to distinguish it from the fulminating silver of M. Berthollet, by dissolving some silver in pure nitric acid, and by pouring into it, while the solution is taking place, a sufficient quantity of rectified alcohol, or by pouring alcohol into a nitric solution of silver with considerable excess of acid.

In the first case, the nitric acid in which the silver is put must be slightly heated until the solution begins to VOL. XI.-SECOND SERIES.

Nan

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