832 



SILVER, CHLORIDE OF 



of which contain 1 gr. of silver) is added, and the assay completed as before with the 

 decimal salt-solution. The measure of decimal solution of salt corresponding to the 

 measure of the decimal solution of silver added, is deducted, and the remainder equals 

 the quantity of salt solution required to precipitate the silver in the metal operated on. 

 6. By weighing the chloride of silver. This process is used for the Indian Mint 

 assays, special apparatus being employed to carry on a number of assays at one 

 time. The process may be conducted as follows : A portion of the silver, or alloy, is 

 accurately weighed, transferred to a stoppered bottle, dilute nitric acid added, and 

 solution effected by heating the bottle in a water-bath or otherwise. When decom- 

 posed, the solution is diluted with water, hydrochloric acid added in excess, and the 

 bottle well shaken till the precipitated chloride of silver collects, and the solution is 

 clear. The bottle is now filled to the neck with water, allowed to settle, and the super- 

 natant liquor carefully removed by means of a glass syphon. The bottle is again filled 

 with water, the chloride of silver allowed to settle, and the solution syphoned off as 

 before. Two or more washings are made, according to the purity of the silver or 

 alloy operated on. The bottle is now inverted over a small Wedgwood crucible, and 

 manipulated until the whole of the chloride of silver is collected therein. The 

 chloride of silver is now broken up and gently stirred, by means of a glass rod, until 

 it lies evenly at the bottom of the crucible. The water is carefully drained off, and 

 the crucible heated at first at a low temperature, and afterwards for some time at a 

 temperature of about 300 F. When the chloride of silver is thoroughly dried, it is 

 allowed to cool, and then carefully transferred to the skiff of the assay balance, and 

 weighed. The amount of chloride of silver obtained from a known weight of pure 

 silver by working under similar conditions is ascertained, and the calculations made 

 from this data. Special weights are used in India, to facilitate calculations. The 

 assay weight indicating from the amount of chloride of silver the actual quantity of 

 silver present in 1,000 parts. 



SILVER COINAGE IK 1873. 



SILVER, BROMIDE OF (AgBr), is occasionally found native. If a soluble 

 bromide is added to a solution of nitrate of silver, a precipitate of bromide of silver is 

 formed of a very pale yellow colour. This salt changes readily under the action of 

 the solar rays, and for photographic purposes possesses many very important pro- 

 perties, of which photographers have not availed 'themselves. This is mainly owing 

 to the neglect of scientific investigation amongst the body of photographic artists, 

 which is exceedingly to be regretted. 



SILVER, CHlioRIDE OP, (AgCl) is obtained by adding hydrochloric acid, or 

 any soluble chloride, to a solution of nitrate of silver. A curdy precipitate falls, quite 

 insoluble in water, which being dried and heated to dull redness, fuses into a semi- 

 transparent grey mass, called, from its appearance, horn-silver. Chloride of silver 

 dissolves readily in water of ammonia, and crystallises in proportion as the ammonia 

 evaporates. It is not decomposed by a red heat, even when mixed with calcined 

 charcoal ; but when hydrogen or steam is passed over the fused chloride, hydrochloric 

 acid exhales, and silver remains. When fused along with potash (or its carbonate), 

 the silver is also revived ; while oxygen (or also carbonic acid) gas is liberated, and 

 chloride of potassium is : formed. Alkaline solutions do not decompose chloride of 

 silver. When this compound is exposed to light, it suffers a partial decomposition, 

 hydrochloric acid being disengaged. 



The best way of reducing the chloride of silver, says Mohr, is to mix it with one- 

 third of its weight of colophony (black resin), and to heat the mixture moderately in 

 a crucible till the flame ceases to have a greenish-blue colour ; then suddenly to in- 

 crease the fire, so as to melt the metal into an ingot. 



The subchloride may be directly formed by pouring a solution of proto- chloride 

 of copper or iron upon silver-leaf. 



