828 SILVER ASSAYING 



After having washed the tubes and the pipette with the new solution, we must 

 repeat the experiment upon a fresh gramme of silver. We shall find, for example, in 

 proceeding only by a thousandth at a time, that the first causes a precipitate, but not 

 the second. The standard of the solution is still too weak, and is comprised between 

 1000 and 1001 ; that is to say, it may be equal to 1000, but we must make a closer 

 approximation. 



We pour into the test-bottle 2 thousandths of the decimal solution of silver, which 

 will destroy 2 thousandths of salt, and the operation will have retrograded by 

 2 thousandths ; that is to say, it will be brought back to the point at which it was 

 first of all. If, after having cleared up the liquor, we add half a thousandth of the 

 decimal solution, there will necessarily bo a precipitate, as we knew beforehand, but a 

 second will cause no turbidity. The standard of the normal liquor will be consequently 

 comprehended between 1000 and 1000J, or equal to 1000|. 



We should rest content with this standard ; but if we wish to correct it, we may 

 remark that the two quantities of solution of salt added, viz. 2279'3 gr. + 30'02 gr. = 

 2309-32 gr., have produced only 99975 thousandths, and that we must add a new 

 quantity of it corresponding to of a thousandth. We make, therefore, the proprtion 



99975 : 2309-32:: 0-25 : x. 



But since the first term differs very little from 1000, we may content ourselves to 

 have x by taking the ^L of 2309'32, and we shall find 0'577 gr. for the quantity of 

 solution of salt to be added to the normal solution. 



It is not convenient to take exactly so small a quantity of solution of salt by 

 the balance, but we shall succeed easily by the following process. We weigh 60 

 grammes of this solution, and we dilute it with water, so that it occupies exactly half 

 a litre, or 500 centimeters cube. A pipette of this solution, one centimeter cube in 

 volume, will give a decigramme of the primitive solution, and as such a small pipette 

 is divided into twenty drops, each drop, for example, will present 5 milligrammes of 

 the solution. We should arrive at quantities smaller still by diluting the solution with 

 a proper quantity of water ; but greater precision would be entirely needless. 



The testing of the normal liquor just described is, in reality, less tedious than might 

 be supposed. It deserves also to be remarked, that liquor has been prepared for more 

 than 1,000 assays ; and that, in preparing a fresh quantity, we shall obtain directly its 

 true standard, or nearly so, if we bear in mind the quantities of water and solution of 

 salt which have been employed. 



Correction of the Standard of the Normal Solution of Salt, when the Temperature 

 changes. We have supposed, in deter mining, the standard of the normal solution of 

 salt, that the temperature remained uniform. The assays made in such circum- 

 stances have no need of correction ; but if the temperature should change, the same 

 measure of the solution will not contain the same quantity of salt. Supposing that 

 we have tested the solution of the salt at the temperature of 15 C. ; if, at the time of 

 making the experiment, the temperature is 18 C., for example, the solution will be 

 too weak on account of its expansion, and the pipette will contain less of it by weight ; 

 if, on the contrary, the temperature has fallen to 12, the solution will be thereby 

 concentrated, and will prove too strong. It is therefore proper to determine the cor- 

 rection necessary to be made for any variation of temperature. 



To ascertain this point, the temperature of the solution of salt was made successively, 

 to be 0, 5, 10, 15, 20, 25, and 30 C., and three pipettes of the solution were 

 weighed exactly at each of these temperatures. The third of these weighings gave the 

 mean weight of a pipette. The corresponding weights of a pipette of the solution 

 were afterwards graphically interpolated from degree to degree. These weights form 

 the second column of the following Table. They enable us to correct any temperature 

 between and 30 C. (32 and 86 F.) when the solution of salt has been prepared 

 in the same limits. 



Let us suppose, for example, that the solution has been made standard at 15, and 

 that at the time of using it, the temperature has become 18. We see by the second 

 column of the Table, that a weight of a measure of the solution is 100-099 gr. at 15, 

 and 100 - 065 at 18 ; the difference, 0-034 gr., is the quantity of solution less which 

 has been really taken ; and of course we must add it to the normal measure, in order 

 to make it equal to one thousand mUliemes. If the temperature of the solution had 

 fallen to 10 the difference of the weight of a measure from 10 to 15 would be 

 0'019 pr., which we must, on the contrary, deduct from the measure, since it had 

 been taken too large. These differences of weight of a measure of solution at 15, 

 from that of a measure at any other temperature, form the column 15 of the table 

 where they are expressed in thousandths ; they are inscribed on the same horizontal 

 lines as to the temperatures to which each of them relates with the sign + plus, when 

 they must be added, and with the sign minus, when they must be subtracted. 



