lower portion, or false bottom, that the flasks do not rise above its edge, or at least but little. This disposition is to

protect the chloride of silver from the light, for it decomposes in contact with water, and a little hydrochloric acid is produced, which requires for its precipitation a certain quantity of nitrate of silver, and so lowers the standard of the alloy. This cause of error is however not very great, at least when the light does not fall directly on the chloride; but it is easy to avoid, and should not be neglected. The disposition already pointed out does not at all complicate the process, and is moreover useful, as it prevents the fracture or upsetting of the bottles. When but one bottle is operated on, it is placed for agitation in a japanned tin-plate cylinder, which is held as shown at fig. 116. On placing the bottles in their respective places on the table, a brisk circular movement is given to them, so as to remove any chloride of silver adhering to the sides; their stoppers are removed and suspended by spring pincers, a a. These are formed of sheet-iron wire (see fig. 117). A thousandth

FIG. 117.

of the decime solution is then poured into each bottle, and before this has been completed there will have formed in the first bottles where there is any precipitate, a well-marked

nebular layer about a centimetre in thickness. At the back of the table is a black board, P P, divided into compartments numbered from 1 to 10, on each of which is marked with chalk the number of thousandths of decime liquid added to the contents of the corresponding bottle. The thousandths of salt announcing augmentation of standard are preceded by the sign +, those of nitrate of silver by the sign

Lastly, the black board carries a small shelf pierced with holes, tt, and these receive the funnels or drain the bottles; on this shelf also are fastened the pincers for sustaining the stoppers.

Cleansing the Bottles.-The assays terminated the liquid from each flask is poured into a large vessel in which there is always a slight excess of common salt, and when it is full the clear supernatant fluid is removed by means of a syphon. Immediately will be given the means of reducing the chloride of silver so collected to the metallic state. The bottles, to the number of ten, are first rinsed with the same water passed from one to the other, then a second, and then a third time with fresh water. They are then placed to drain on the board just mentioned, and the stoppers are placed in a stand by series of tens (see figs. 118 and 107). It is important to remark, that when a glass has been rinsed with distilled water, care must be taken not to rub it with the fingers, for water poured in such a vessel would always be clouded on the addition of nitrate of silver. This effect is due to the chlorides contained in the perspiration, and is of course more to be guarded against in summer.

FIG. 118.

Reduction of Chloride of Silver, obtained in the Assay of Alloys by the Humid Method.

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Chloride of silver can be reduced without sensible loss, after having been well washed, by plunging into it scraps iron or zinc, and adding dilute sulphuric acid in sufficient quantity to set up a slight disengagement of hydrogen gas.

The whole can be left to itself, and in the course of a few days the silver is completely reduced. This point can be easily determined by the colour and nature of the product, but better still by treating a small quantity by ammonia, which, if the chloride is perfectly reduced, will give no precipitate or cloudiness on treatment with an acid. The chlorine remains in solution in the water combined with zinc or iron. The residue must now be washed; the first washings are made with acidulated water, to dissolve oxide of iron which might have formed, and the following with ordinary water: after having completed the washing as much water as may be left is decanted, the mass dried, and a little powdered borax added. Nothing now remains but to fuse it. The powdered silver being voluminous, it is placed by separate portions into the crucible, in proportion as it sinks. The heat should be at first moderate, but towards the end of the operation should be sufficiently high to reduce the silver and slag to a state of complete liquidity. If it be found that not quite all the chloride was reduced by the iron or zinc, a little carbonate of potash or soda may be added to the powdered silver. The standard of silver thus obtained is from 999 to 1000 thousandths.

Preparation of Pure Silver.

Take the silver prepared as above, dissolve it in nitric acid, and leave the solution some time in perfect rest in water, to deposit any gold it might contain. Decant the solution, and precipitate with common salt, well wash the precipitate, and reduce it, when the resulting silver will be pure.

M. Gay Lussac here gives a description of a process for the precipitation of chlorine from nitric acid for use in the mode of assay already described; but as that acid in a state of purity forms an ordinary article of commerce, and can be obtained at most operative chemists, the process will not be here reproduced.

Modifications required in the Assay of Silver Alloys
containing Mercury.

Whenever mercury is present in solution with silver, it is thrown down as insoluble chloride, and the assay rendered inaccurate. The presence of mercury in silver can be readily detected by the remarkable change which occurs in chloride of silver on exposure to light (viz. blackening) when free from mercury; but if the smallest quantity of the latter metal be present, no blackening will ensue. This source of error was removed by M. Levol in the following manner :-The sample being dissolved, as usual, in nitric acid, it was supersaturated with 25 cubic centimetres of caustic ammonia; then add the pipetteful of normal solution, and supersaturate the excess of ammonia with 20 cubic centimetres of acetic acid, and the operation continued in the usual way.

It may not be superfluous to state, that it is very easy to obtain an excellent result of an assay of silver containing mercury, made in the ordinary way, and in which the presence of the mercury is rendered manifest by the noncolouration of the precipitate under the influence of light. It suffices for this purpose to dissolve the precipitate in concentrated ammonia, and to supersaturate with acetic acid.

The ordinary acetic acid of commerce is employed, and the ammonia diluted with its volume of water, to avoid the too violent reaction. Both agents must be free from chlorides.

Some little time after the publication of this, M. Gay Lussac examined the above process himself, and very considerably simplified it. He says: After having confirmed by several experiments the accuracy of M. Levol's process, I thought it might be simplified by adding to the nitric solution of silver the ammonia and acetic acid at one and the same time, but in sufficient quantity to saturate the whole of the nitric acid, both that in combination with the silver and that in the free state. Ten grammes of acetate of ammonia were added, with a little water, to the silver dissolved in nitric acid, and the assay finished in the ordinary

manner. The quantity indicated by synthesis was found very accurately, although 100 thousandths of mercury had been added.' Finally, M. Gay Lussac found that 10 grammes of acetate of soda, in crystals, also fully answered the purpose; and as that is a very cheap commercial salt, it is the best adapted for overcoming the difficulty in this class assay, as regards the presence of mercury.

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APPENDIX.

Here, how

In the foregone description of the method of assay by the humid method, it has been the object of the writer not to distract the attention by too numerous details. ever, will be given the processes to which personal experience has given the preference.

Apparatus for Weighing the Normal
Solution of Salt.

The apparatus about to be described enables the operator to weigh the normal solution of salt more rapidly than by means of the burette (fig. 82). It is a pipette, P (fig. 119), capable of furnishing in a continuous jet very nearly 100 grammes of solution, when filled up to the mark a b, at the ordinary temperature. As this weight changes its volume with the temperature, some marks are traced on the neck of the pipette, so as to regulate approximatively the volume to be taken. The pipette is terminated below by a three-footed stopcock, R, having a narrow outlet, p (about two millimetres).

FIG. 119.

It is filled with solution by means of a small silver funnel (fig. 120), or better still by the suction tube, T, of the apparatus fig. 103, making an addition similar to that represented by fig. 118. The pipette is adjusted by ab