第20章

Chloride of Silver.--(Oxide of Silver.)--Take any quantity of silver coin or other silver, roll or hammer it thin; cut in small pieces.

This in order to save time. Put the silver in a glass or earthen vessel (Florence flask is best); pour in nitric acid and water, about three parts of the former to one of the latter. The operation of cutting up the silver may be facilitated by applying a gentle heat.

This blue solution consists of oxide of silver and oxide of copper, both combined with nitric acid. Should the operator wish a pure solution of silver, which, however, is not always used, he may obtain it in the following manner:

To separate the two metals contained in the above solution from each other, put some bright copper coins into the solution and set it aside in a warm place for three or four days, occasionally giving it a circular motion.

The separated laminae are pure silver, which is to be digested with ammonia until it ceases to be colored blue. The silver, after being washed and dried, is again dissolved in nitric acid, and the liquid, diluted with water, is kept as solution, of silver.

Either of the above solutions (the one of oxide of silver and copper, and the pure silver solution) may be prepared for use by putting them in a bottle, with a quantity of water, and adding common fine salt, you obtain a white curdy precipitate of chloride of silver.

No matter how much salt is used, provided enough be added to throw down all the chloride of silver. This solution should be well agitated and then allowed to stand for a few minutes; thus the white precipitate is in the bottom of the bottle. When the water has become clear, pour it off with care, leaving the sediment behind, then add a fresh quantity of clean water, shake, let settle, and pour off as before.

Repeat the same for several times, and the excess of salt will disappear, leaving the white precipitate, which may be drained of the water and dried in the dark, and kept free from light and air.

CYANIDE OF POTASSIUM.

Cyanide of Potassium.--This important article is worthy the undivided attention of every Daguerreotypist.

I here give Mr. Smee's process for its preparation.

This is from that author's work entitled, "Electro Metallurgy,"American edition:

"The cyanide of potassium, so often alluded to while treating of the metallo-cyanides, may be formed in several ways.

It may be obtained by heating to a dull redness the yellow ferrocyanate of potash, in a covered vessel, filtering and rapidly evaporating it.

The objection to this method, however, is that without great care the whole of the ferrocyanate is not decomposed, a circumstance which much reduces its value for electro-metallurgy. By boiling, however, the ignited residue with spirits of wine this difficulty is said to be overcome, as the ferrocyanate is absolutely insoluble in that menstruum, while the cyanuret, at that heat, freely dissolves, and is as easily re-deposited on cooling.

"There is, however, a much better process by which this salt may be formed, viz. by simply transmitting hydrocyanic acid through potassium.

Although the modes of making this acid are very numerous, there is but one which is likely to be employed on a very large scale, and that is its formation from the yellow ferrocyanate by means of sulphuric acid.

This process is performed as follows: any given weight of the yellow salt is taken and dissolved in about five times its weight of water;this is placed in a retort, or some such analogous vessel, to which is then added a quantity of strong sulphuric acid, twice the weight of the salt, and diluted with three or four times its quantity of water.

A pipe is carried from the neck of the retort to the receiving bottle, which should be kept as cool as possible.

"For small operations, those invaluable vessels, Florence flasks, answer well: a bent tube being connected at one end to its month, the other passing into the second vessel; heat should be cautiously applied by means of an Argand lamp, a little vessel of sand being placed under the flask, which helps the acid to decompose the salt.

Prussic acid is then generated and passes through the tube to the recipient vessel, which is to be charged with liquor potassae.

"When the potash is saturated, the operation is completed.

The Germans recommend a strong, alcoholic solution of potassa to be used in the second vessel, for in this case, the hydrocyanic or prussic acid combines with the potassa, forming a hydrocyanate of potassa, or, the water being abstracted, the cyanuret of potassium, which spontaneously precipitates, on the saturation of the fluid, the cyanuret, being insoluble in strong alcohol.

The ferrocyanate of potash may be considered as containing three equivalents of hydrocyanic acid, two of potash and one of iron; but, unfortunately, we can only obtain half the acid from the salt, owing to the formation of a compound during its decomposition which resists the action of the acid.