MERCURY, SILVER, GOLD, AND PLATINUM. 269 



vegetable fibre, it is much used in solution as an indelible 

 ink for marking linen and cotton. Mercury introduced 

 into a weak solution of it precipitates the metallic silver 

 in beautiful tree-like forms called arbor Diance. 



There have been cases in which nitrate of silver (lunar 

 caustic) has been swallowed in considerable quantity by 

 mistake. The sure antidote is common salt, producing two 

 harmless articles, chloride of silver and nitrate of sodium. 



381. The Silver Assay. The process by which the amount of al- 

 loy in silver is ascertained is called the silver assay. A sample of the sil- 

 ver to be examined is first dissolved in nitric acid. The assayer then in- 

 troduces salt (chloride of sodium) into this solution, and a curdlike sub- 

 stance is precipitated. This substance is chloride of silver, formed by the 

 union of the chlorine of the salt with the silver. He adds the salt slowly 

 till there ceases to be any precipitation, and then Stops, because he knows 

 that there is no more silver for the chlorine to unite with. Now observe 

 how he tests by this process the amount of silver in the specimen. Of 

 course, the more silver there is and the less alloy, the more salt is required 

 to precipitate all the silver. The assayer, therefore, judges of the purity 

 of the specimen by the amount of salt which he is obliged to use to com- 

 plete the process, and in order to ascertain this accurately he employs a so- 

 lution of a certain strength, which he pours from a graduated glass. He 

 knows beforehand just how much of this is required to precipitate a cer- 

 tain amount of pure silver for example, a gramme. If now he is obliged 

 to use only half as much for a gramme of any sample, he infers that it 

 is only half silver ; if three fourths, it is three fourths silver, etc. The 

 explanation of the process is this : The solution of the silver in nitric acid 

 is a solution, not of silver, but of the salt called nitrate of silver. This is 

 decomposed, as is also the chloride of sodium when the two solutions min- 

 gle, producing chloride of silver and nitrate of sodium, as indicated in the 

 equation : 



AgXO, + NaCl = AgCl + NaNO 3 . 



382. Gold. Gold is nearly always found in its metallic 

 state. It is usually, however, alloyed with silver. Some- 

 times it occurs in masses, but commonly in small round or 

 flattened grains. It is also found in veins in various rocks. 



