SILVER-METER. This is an instrument for testing accurately the quantity of nitrate of silver in an old nitrate bath containing impur ities which affect the S. G. of the liquid, and therefore render the hydrometer useless as a means of testing its strength in silver.
The principle on which this meter acts consists in ascertaining how much of a solution of pure chloride of sodium of definite strength is necessary to precipitate the whole of the silver contained in a definite quantity of the nitrate bath. The following is the mode of pro ceeding : Make a solution of pure chloride of sodium by adding pure hydro chloric acid to a solution of carbonate of soda until effervescence ceases, and the acid is a little in excess. Evaporate the solution to dryness, and fuse the pure chloride of sodium in a porcelain capsule over a spirit lamp. Then make a solution of it in distilled water of the strength of 17 grains of the salt to 12 fluid ounces of water. This is the standard test solution ; which must be made very accurately. A
fluid drachm of it will exactly precipitate half a grain of nitrate of silver.
In order to test the strength of the bath, put a small measured quantity, say a drachm, into a clean stoppered bottle, and add to it a little distilled water. Then add the test solution from a graduated measure, a little at a time, shaking after -each addition and allowing the white chloride of silver to settle each time, until the salt ceases to produce any more cloudiness in the clear liquid. The quantity of the test solution employed, estimated at the rate of one drachm to half a grain of nitrate of silver, will then determine the exact quan tity of nitrate of silver contained in a fluid drachm of the nitrate bath.
Mr. G. Wood, of 117, Cheapside, manufactures a neat form of apparatus on the above principle, by which the operator is saved all the trouble of calculation. We have one of these instruments, and find it of great use, and very accurate.