148 THE ATOMIC WEIGHTS. 



permangauate needed to produce distinct reddening, and 

 for the iron contained in the zinc. 11.956 grammes of the 

 latter metal contained iron corresponding to 0.6 cc. of the 

 standard solution. The permanganate was standardized 

 by comparison with pure ammonium-ferrous sulphate, 

 Am2Fe(S04)2. 6 H2O, so that, in point of fact, Zettnow es- 

 tablishes directly only the ratio between that salt and the 

 ferrous tungstate. From Zettnow's four experiments in 

 standardizing I find that 1 cc. of his solution corresponds to 

 0.0365457 grammes of the double sulphate, with a probable 

 error of ±: .0000012. 



Three sets of titrations were made. In the first a quantity 

 of ferrous tungstate was treated according to the process 

 given above ; the iron solution was diluted to 500 cc, and 

 four titrations made upon 100 cc. at a time. The second 

 set was like the first, except that three titrations were made 

 with 100 cc. each, and a fourth upon 150 cc. In the third 

 set the iron solution was diluted to 300 cc, and only two 

 titrations upon 100 cc each were made. In sets one and 

 two thirty grammes of zinc were used for the reduction of 

 each, while in number three but twenty grammes were taken. 

 Zettnow's figures, as given by him, are quite complicated ; 

 therefore I have reduced them to a common standard. After 

 applying all corrections the following quantities of tung- 

 state, in grammes, correspond to 1 cc. of permanganate 

 solution : 



.028301 I 



.028291 

 .028311 



.028'50I 



First set. 



.028367 ~] 



.028368 I 



.028367 I" Second set. 



.028367 J 



.028438 1 Third set. 

 .028438 i 



Mean, .0283549, dz .0000115 



With the silver tungstate, AgaWO^, Zettnow employed 

 two methods. In two experiments the substance was de- 



