ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 429 
liable to as great a disagreement as the numbers derived from experiments on 
different, days. 
§ 17. At this stage the question presented itself as to whether the deposits were 
really pure silver. Two or three gravimetric analyses by conversion into chloride, 
conducted both by ourselves and by Mr. Scott, to whose advice and assistance we 
have been constantly indebted throughout these investigations, having favoured the 
idea that the deposits were not quite pure, we arranged for a systematic volumetric 
analysis of all the deposits. The bulk of the metal after solution in pure nitric acid 
having been thrown down with a known quantity of chloride of sodium in strong 
solution, the titration was completed with weak (toVo) sa ^ solution from a burette in 
the usual manner. The bottle containing the solution was enclosed in a dark box 
and lighted in the manner recommended by Stas, with a convergent beam of yellow 
light which had passed through a flask containing chromate of potash. Towards the 
close of the operation the effect of the addition of two drops of solution (containing x^ 
milligrm. of salt) becomes difficult of observation unless the liquid be very thoroughly 
cleared. At this stage we found it convenient to filter off about half the liquid into 
another bottle, through a funnel plugged with (purified) cotton wool. As soon as the 
pores are penetrated by the chloride of silver the filtration is effective, and yet so 
rapid that but little time is lost by the adoption of this procedure. The two drops 
of chloride solution are added to the liquid thus filtered, and shaken up so as to effect 
a complete mixture, and the bottle is then placed so that the cone of light traverses 
the body of the liquid. After an interval varying from a few seconds to several 
minutes the cloudiness develops itself, and the delay gives an indication of how nearly 
the point is approached. Before each test the filtrate is of course returned to the 
stock bottle and thoroughly shaken up. The operation is complete when the last 
addition of two drops gives no effect after a quarter of an hour. There is no diffi¬ 
culty in determining in this way the necessary quantity of salt to xy milligrm., and 
the point may be recovered any number of times after addition of small known 
quantities of silver. 
In the interpretation of the results we placed no trust in the purity of the NaCl, 
nor depended upon any assumption as to the ratio of NaCl to Ag, but made com¬ 
parison with the numbers obtained from precisely similar determinations with substi¬ 
tution for the electro-deposits of equal weights of silver of the highest quality, supplied 
by Messrs. Johnson and Matthey. A large number of such comparisons showed 
that there was no difference that could be depended upon between the two kinds of 
silver; there was, indeed, a slight indication of inferiority in the deposits, to the 
extent of perhaps xoVij, but not more than might plausibly be attributed to the greater 
risk of loss in dissolving the deposits from off the platinum basins. The standard 
silver was dissolved without transference in the bottle used for the subsequent 
analysis, and thus under more advantageous conditions than were possible in the 
manipulation of the deposits. 
