20 



REPORT 1877, 



of colour. At 2u0° decrepitation and incipient decomposition ensued. The 

 total loss at a temperature not exceeding 140° was only -067 per cent, of the 

 weight of the precipitate. This, in the experiments by method I., would only 

 cause a difference of -02 per cent, in the quantify of chloride of potassium 

 found. Hence it is clear that there is no advantage in drying the preci- 

 pitate at 130° rather than at 100°. On the other hand the occurrence of 

 decrepitation shows that the crystals contain cavities filled with water or 

 platinum choride solution, and therefore that the production of large crystals 

 should he avoided. It seems possible that the difference in the nature of 

 the liquid filling the cavities may he the cause of the greater error observed 

 when a large excess of platinum solution is employed than when little more 

 than the theoretical amount is used. 



In the foregoing Tables the results obtained by Frank's method were 

 calculated with the Committee's factor -3056 instead of that employed by 

 Prank and Berrand themselves (-30507)*. By the use of the latter factor 

 the results would come out about "17 per cent, lower than the figures given in 

 the Tables. Some of the results by this process are exceedingly good, but in 

 other cases they are seriously in excess of the truth (see Experiments 3, -1, 

 59 and 60). 



One very considerable advantage attaches in practice to Tatlock's method 

 which is not shared by the others. In consequence of employing an aqueous 

 liquid at first, any sulphates present can be readily washed out, and therefore 

 there is no occasion to separate any moderate amount beforehand. The 

 influence of sulphates is well shown by the following results by Tatlock's 

 method : — 



Table VIII.— 82 per cent, KC1 + 18 per cent. Ha. SO.. 



Table IX. — K.,80, with sufficient NaCl (-5grm.) to ensure the reaction 

 K 2 S0 4 -f- 2NaCl + PtCl 4 =K 2 TtCl c + Xa 2 S0 4 . 



* The factor employed by Frank and Berrand is based on Andrews's determination of 



the atomic weight of platinum. This observer states that potassium chloroplatinatc 

 retains -55 per cent, of water even when dried at temperatures considerably above 100° C. 

 If this be true, the low factor employed by Frank and Berrand would partly compensate 

 the error thus introduced. In the experiments detailed in the text only' '25 per cent, 

 was lost at a temperature of 200°, but decrepitation occurred on raising the temperature 

 still higher. 



t This precipitate, after drying at 130° G, gave 9972 per cent, of K,S0 4 . 



