SILVER ASSAYING 827 



Temperature of the Solution. After having described the manner of measuring by 

 volume the normal solution of the salt, we shall indicate the most convenient means 

 of taking the temperature. The thermometer is placed in a tube of glass, T, fig. 1847, 

 which the solution traverses to arrive at the pipette. It is suspended in it by a piece of 

 cork, grooved on the four sides to afford passage to the liquid. The scale is engraved 

 upon the tube itself, and is repeated at the opposite side, to fix the eye by the coinci- 

 dence of this double division at the level of the thermometric column. The tube is 

 joined below to another narrower one, through which it is attached by means of a cork 

 stopper B, in the socket of the stopcock of the pipette. At its upper part it is cemented 

 into a brass socket, screw-tapped in the inside, which is connected in its turn by a 

 cock, with the extremity, also tapped, of the tube above T, belonging to the reservoir 

 of the normal solution. The corks employed here as connecting links between the 

 parts of the apparatus, give them a certain flexibility, and allow of their being dis- 

 mounted and remounted in a very short time ; but it is indispensable to make them be 

 traversed by a hollow tube of glass or metal, which will hinder them from being 

 crushed by the pressure they are exposed to. If tho precaution be taken to grease 

 them with a little suet, and to fill their pores, they will suffer no leakage. 



For tho preservation of the normal solution of salt, M. Gay-Lussac uses a cylindrical 

 vessel or drum of copper, of a capacity of about 110 litres, having its inside covered 

 with a resin-and-wax cement. If the drum contains 110 litres, we should only put 

 105 into it, in order that sufficient space may be left for agitating the liquor without 

 throwing it out. According to the principle that 100 cubic centimeters, or -j^th of a 

 litre of the solution should contain enough of salt to precipitate a gramme of pure 

 silver; and, admitting moreover, 13'516 for the equivalent of silver, and 7'335 for 

 that of salt, we shall find the quantity of pure salt that should be dissolved in the 105 

 litres of water, and which corresponds to 105 x 10 = 1050 grammes of silver, to be, by 

 the following proportion : 



13-516 : 7'335::1050 grammes : # = 569'83 grammes. 



And as the solution of the salt of commerce, formerly mentioned, contains approxima- 

 tively 250 grammes per kilogramme, we must make 2279'3 grammes of this solution 

 to have 669'83 grammes of salt. The mixture being perfectly made, the tubes and the 

 pipette must be several times washed by running the solution through them, and 

 putting it into the drum. The standard of the solution must be determined after it 

 has been well agitated, supposing the temperature to remain uniform. 



To arrive more conveniently at this result, wo begin by preparing two decimal solu- 

 tions ; one of silver, and another of salt. 



The decimal solution of silver is obtained by dissolving 1 gramme of silver in nitric 

 acid, and diluting the solution with water till its volume becomes a litre. 



The decimal solution of salt may bo obtained by dissolving - 543 gramme of pure 

 salt in water, so that the solution shall occupy a, litre ; but we shall prepare it even 

 with the normal solution which we wish to test, by mixing a measure of it with 9 

 measures of water ; it being understood that this solution is not rigorously equivalent 

 to that of silver, and that it will become so only when the normal solution employed 

 for its preparation shall be finally of the true standard. Lastly, we prepare before- 

 hand several stoppered bottles, in each of which we dissolve 1 gramme of silver in 8 

 or 10 grammes of nitric acid. For brevity's sake, we shall call these ' tests.' 



Now, to investigate the standard of the normal solution, we must transfer a pipette 

 of it into one of these test-bottles ; and we must agitate the liquors briskly to clarify 

 them. After some instants of repose, we pour in 2 thousandths of the decimal solution 

 of salt, which we suppose will produce a precipitate. The normal liquor is conse- 

 quently too feeble; and we should expect this, since the salt employed was not 

 perfectly pure. We agitate and add 2 fresh thousandths, which will also produce a 

 precipitate. "We continue thus, by successive additions of 2 thousandths till the last 

 produces no precipitation. Suppose that we have added 1 6 thousandths : the last 

 two should not be reckoned, as they produced no precipitate ; the preceding two were 

 necessary, but only in part ; that is to say, the useful thousandths added are above 1 2 

 and below 14, or otherwise they are on an average equal to 13. 



Thus, in the condition of the normal solution, we require 1,013 parts of it to 

 precipitate 1 gramme of silver, while we should require only 1 ,000. We shall find 

 the quantity of concentrated solution of salt that we should add, by noting that the 

 quantity of solution of salt, at first employed, viz. 2279'3 grammes, produced a 

 standard of only 987 thousandths = 1000 13 ; and by using the following proportion : 



987 : 2279-3::13 : x = 30-02 grammes. 



This quantity of the strong solution of salt, mixed with the normal solution in the 

 drum, will correct its standard, and we shall see now by how much. 



