618 PROFESSOR DITTMAR AND MR JOHN M'ARTHUR ON 



potassium be accepted as pure KCl. Yet the sal-ammoniac form of the 

 Finkener method is invaluable, being the only method for extracting a small 

 proportion of potassium from a mass of, e.g., soda-salts ; only it must be 

 wrought with great circumspection, and in its original form be only used as a 

 mode of extracting the potassium, not as a method for its determination. 



Analysis (3) — TatlocKs Method. 



Chloride of potassium used = A = 14*22 mgs. 



(We could not have used as much as in (1) or (2) without wasting an 

 unreasonable amount of platinum solution.) C =41*0; C x 030627 = 1256 = T. 

 T— A = — 1-66 mgs., or 117 per cent, of A. 



Platinum from C = 169 mgs. = 12*86 mgs. of KCl. Chloride of potassium 

 recovered from the washings, and weighed, ultimately as PtCl 6 K 2 = 2*92 mgs.; 

 hence A = 11-30. T — A = + 126 mgs., or 11*1 per cent, of A . 



With salt-mixtures like sea- water salts the Tatlock method obviously loses 

 its applicability. Nor was it ever intended for such mixtures. 



IV. Experiments on Chloroplatinate of Eubidium. 



These experiments were planned at a very early stage of our investigation. 

 They were suggested by the obvious consideration that for the synthetical 

 determination of the weight-ratio Pt : 2C1 between the platinum and fixed 

 chlorine in chloroplatinates, chloride of rubidium should be better adapted 

 than the potassium-salt, because, while itself soluble in alcohol, its chloroplat- 

 inate is less soluble in water than chloroplatinate of potassium. For a similar 

 reason, chloride of caesium should be preferable to chloride of rubidium ; 

 but we shrank from the great expense which would have been involved in 

 procuring the necessary supply of the rarer of the two rare alkalies. 



Our raw material for the preparation of chloride of rubidum was a supply of 

 " rubidium alum " from Trommsdorff in Erfurt. The alum was dissolved in hot 

 water, and its rubidium precipitated by addition of chloroplatinic acid, the 

 precipitate washed, reduced in the dry way with hydrogen, and the chloride of 

 rubidium extracted with water. As it turned out to contain a very appreciable 

 quantity of sulphate, it was redissolved, reconverted into chloroplatinate, and 

 recovered from the latter by means of hydrogen. The salt thus obtained was 

 contaminated with sulphuric acid, and two or three repetitions of the cycle of 

 operations failed to eliminate this impurity quite completely. Going by the 

 aspect of the chloride of barium precipitate, the last precipitation, indeed, 

 seemed to have effected no improvement ; we therefore evaporated the whole 

 of our (last) chloride of rubidium to dryness, and thus obtained about 32 grms. 



