1884.] Electro-chemical Equivalent of Silver, Sfc. 



449 



as many as five, have been included in the circuit of the measured 

 current. In order to render the deposit more compact, and thus to 

 diminish the danger of loss in the subsequent manipulations, acetate of 

 silver was added in the earlier experiments to the standard solution of 

 nitrate. Experience, however, has shown that the principal risk is 

 not in the loss of metal, but in the obstinate retention of salt within 

 the fine pores of the deposit, leading to an over-estimate of the 

 amount. When the texture is very compact this danger increases, 

 and deposits from a solution containing acetate are often decidedly 

 too heavy, even after the most careful and protracted washings. On 

 heating to low redness a portion, at any rate, of the retained salt is 

 decomposed, NX) 3 is driven off, and a loss of weight ensues. With 

 pure nitrate, to which we finally recurred, the risk is much less. 



The actual weights of deposited silver were usually from 2 to 

 3 grms.,and, so far as the mere weighings are concerned, should have 

 been correct to r owo- Discrepancies three or fonr times as great as 

 this are, however, actually met with, whether due to retention of salt 

 or to loss of metal it is difficult to say. The final number, expressing 

 in C.G-.S. measure the electro-chemical equivalent of silver, is a little 

 lower than that (1'119 X 10 -2 ) given on a previous occasion ("Cam- 

 bridge Proceedings" for November 26, 1883). It approximates 

 closely to ril8xl0~ 3 , and is thus in precise agreement with the 

 number announced within the last few weeks by Kohlrausch, viz., 

 1*1183 X 10~ 2 . Its substantial correctness can therefore hardly be 

 doubted, more especially as it does not differ very much from the 

 number (T124) obtained by Mascart. In terms of practical units, we 

 may say that the ampere current deposits per hour 4'025 grms. of 

 silver. 



When we are provided with means for the absolute measurement of 

 currents, the determination of electromotive force is a very simple 

 matter if we assume a knowledge of absolute resistance. A galvanic 

 cell is balanced against the known difference of potentials generated 

 by a known current in traversing a known resistance. The difficulty 

 relates entirely to the preparation and definition of the standard cells. 

 A considerable number of Clark cells have been set up and tested at 

 intervals during the last six months, and their behaviour has been 

 satisfactory, the extreme range (after the first ten days) not much 

 exceeding 10 L 00 . A modified form of cell in which the solid zinc is 

 replaced by an amalgam, is at present under trial. 



In Mr. Latimer Clark's own determination the B.A. unit is assumed 

 to be correct, and the E.M.F. of the cell at 15° C. was found to be 

 1/457 volt. On the same assumption, we obtain the not greatly 

 differing value 1*453 volt. If we take the true value of the B.A. unit 

 as -9867 ohm, 1-453 will be replaced by 1*434. 



Experiments are also in progress to determine in absolute measure 



