424 ASSAY OF GOLD AND SILVER COINS. 



vert into chloride of silver just 184.25 parts of pure silver; consequently the 

 quantity of salt necessary to convert into chloride 1000 parts of silver is 542.74, 

 as found by the proportion — 



184.25 : 100 : : 1000 : 542.74. 



A standard solution of salt is accordingly so prepared as that a given measure 

 (the French decilitre) shall contain 542.74 thousandths of a gram of salt. 

 The normal weight employed for silver assays is the gram, (equal to about 

 15.4 troy grains,) which is marked 1000, and has its subdivisions, in practical 

 weighings, to the half or quarter thousandth. 



Besides this standard solution, which effects the main precipitation of chloride 

 of silver, there is a decimal solntion, of one-tenth the proportion of salt, which 

 it is expedient to use for the lesser and final precipitations. 



In the mode of assay under consideration, it is necessary that the portion of 

 alloyed silver used shall contain as nearly as may be 1000 parts of pure silver. 

 The rigid standard requires that of 1000 parts by weight 900 shall be of pure 

 silver ; but the law allows a variation from this ratio, provided that it do not 

 exceed three thousandths. The fineness may, therclbre, be as low as 897, and 

 as high as 903. In the practice of the assay, it is found most convenient to 

 assume the lower extreme. Now, the weight of metal of the fineness 897, which 

 would contain 1000 parts of silver, is 1114.83, as found by the proportion — 



897 : 1000 : : 1000 : 1114.83. 



The nearest integer to this number is employed, and the weight of metal 

 taken for the assay is 1115. 



PROCLSS or ASSAY. 



The reserved silver coins are melted together in a black-lead crucible, with 

 the addition of fine charcoal within the pot, to prevent oxidation, and to allow 

 of dipping out. After stirring, a small portion of the fluid metal is poured 

 quickly into water, producing a gramdation, from which the portion for assay is 

 taken. As this differs from the mode pursued with gold, it must be specially 

 noted that in the case of silver alloyed with copper there is a separation, to a 

 greater or less degree, between the two metals in the act of gradual solidifica- 

 tion. Thus an ingot cooled in a mould, or any single coin cut out of such ingot, 

 though really 900 thousandths fine on the average, will show such variations, 

 according to the place of cutting, as might even exceed the limits allowed by 

 law. This fact has been established by many experiments, both in this mint 

 and the mint of Paris, since the enactment of our mint law ; and it possesses the 

 stubbornness of a law of chemistry. But the sudden chill produced by throwing 

 the liquid metal into water yields a granulation of entirely homogeneous mix- 

 tTu-e, and it can be proved that the same fineness results, whether by assaying a 

 single granule, or part of one, or a number together. 



From this sample the weight of 11 15 thousandths is taken, which is dissolved 

 in a glass bottle with nitric acid. 



Into this solution the large pipette-full of standard solution of salt is intro- 

 duced, and it produces immediately a white precipitate, which is chloride of silver, 

 and which contains, of the metallic silver, 1000 parts. 



To make this chloride subside to the bottom of the vessel and leave the liquid 

 clear, it is necessary that it be Aaolently shaken in the bottle ; and this is accord- 

 ingly done by a mechanical arrangement, for the necessary time. 



Unless the coins have chanced to be below the allowable limit of standard, 

 the liquid will still contain silver in solution, and accordingly a portion of the 

 decimal solution is introduced from the small pipette, capable of precipitating a 



