NICKEL AND COBALT. 



307 



On examination of the silver it was found that traces of cobalt were 

 retained — less than 0.5 mg. in the first determination and less than 0.2 

 mg. in the second. Taking these amounts as corrections, the two experi- 

 ments give for the ratios Ag._, : Co : : 100 : x the subjoined values : 



27.706 

 27.687 



These figures confirm those previously found, and as they fall within 

 the limits of the preceding series, they ma}^ fairly be included in it, when 

 all eight values give a mean of 27.705, rb .0050. 



Still another method, radically different from all of the foregoing pro- 

 cesses, was adopted by Winkler in 1894.* The metals were thrown down 

 electrolytically upon platinum, and so weighed. Then the}' were treated 

 with a known excess of a decinormal solution of iodine in potassium 

 iodide, which redissolved them as iodides. The excess of free iodine was 

 then determined by titration with sodium thiosulphate. and in that way 

 the direct ratio between metal and haloid was ascertained. The results 

 were as follows, with the metal proportional to 100 parts of iodine given 

 in the third column : 



Cobalt. 



First series 



Second series. 



•I 



IVl. Co. 



.4999 

 .5084 

 .5290 

 .6822 

 .6715 



.5185 

 .5267 



• 5319 



m. I. 



2.128837 

 2.166750 



2.254335 

 2.908399 

 2.861617 

 2.209694 

 2.246037 

 2.268736 



Ratio. 



23.482 



23-463 

 23.466 



23.456 

 23.466 



23.465 

 23-450 

 23-445 



Mean, 23.255, ± .0091 



In these experiments, a« well as in some previous series, a possible 

 source of error is to be considered in the occlusion of hydrogen by the 



* Zeitsch. Anorg. Chem., S, i. 1894. 



