210 ANNUAL OF SCIENTIFIC DISCOVERY. 



ON THE PREPARATION OF PURE SILVER FROM CHLORIDE OF 



SILVER. BY C. BRANNER. 



IT has long been known that pure silver for chemical purposes is 

 best prepared by the decomposition of chloride of silver. This decom- 

 position can be performed in various ways : Poggendorff, several years 

 ago, described a process in which it was effected by galvanism ; this 

 appears to me to be preferable to all others hitherto known, and the 

 one here described can only be regarded as a modification of it. 

 Well-washed precipitated chloride of silver is to be put into a cup of 

 silver, platina or copper the outer surface of which is covered with 

 wax in such a manner that only a round space of one or two inches 

 in diameter, according to the size of the cup, remains uncovered. On 

 the bottom of a larger earthen cup a disc of amalgamated zinc is to be 

 laid, on the middle of which the cup containing the chloride of silver 

 is placed, in such a manner that the portion not covered with wax 

 may come in contact with the zinc. Water slightly acidulated with 

 sulphuric acid is now poured into the apparatus, until it rises above 

 the margin of the inner cup, so that this will be completely sunk in 

 the water. The decomposition of the chloride of silver immediately 

 commences at the edge of the cup containing it and proceeds inwards 

 to the middle ; this is readily known by the dark gray color assumed 

 by the silver as it separates ; the decomposition will be completed in 

 from 24 to 48 hours ; its completion may be known by there being no 

 longer any chloride of silver visible on stirring the precipitate. The 

 silver thus procured is to be washed with water, and any small residue 

 of chloride of silver which it sometimes retains may be got rid of by 

 diluted ammonia. The silver thus prepared is perfectly pure. It is 

 readily seen that any foreign metals that may be contained in the 

 zinc, can never mix with it, as the disc of -zinc lies during the whole 

 operation below the cup containing the silver and never comes in 

 contact with it. 



CRYSTALLIZATION OF GLASS. 



SOME interesting experiments on this subject have been made by 

 M. Leydolt, in the course of his investigations upon the crystallization 

 of the silicates. He had examined agate by subjecting it to the 

 dissolving action of fluohydric acid, and obtained a surface vriih pro- 

 jecting crystals of quartz, that were left untouched by the acid. On 

 subjecting glass in the same manner, he was surprised to see that it 

 was far from homogeneous in its texture. All the kinds of glass 

 examined contain more or less perfectly distinct crystals, regular and 

 transparent encased in an amorphous base. The crystals were 

 brought out by exposing it to the vapors of fluohydric acid and vapor 

 of water, and arresting it when the crystals appear ; the amorphous 

 part is a little the most soluble in the acid. M. Leydolt observes also 

 that some natural crystals pure and transparent and apparently 



