1908-9.] 
The Atomic Weight of Platinum. 
721 
XLII. — The Atomic Weight of Platinum. By E. H. Archibald. 
Communicated by Professor MacGregor. 
(MS. received March 10, 1909. Read June 7, 1909.) 
Twenty-two years have passed since the atomic weight of platinum was 
studied by Dittmar and M £ Arthur .* This is the last investigation recorded 
which has been concerned with the value of this constant. 
During the intervening period, as the result of many investigations 
carried on by Professor T. W. Richards and his students,]* the accuracy 
with which many of the manipulations incident to atomic weight investiga- 
tions can be executed has been greatly increased. Perhaps of still greater 
importance, methods for the preparation of many compounds of undoubted 
purity have been devised, and the principles underlying the preparation 
of pure substances have been clearly set forth.j 
In the case of the atomic weight of platinum, the different investigations, 
which are comparatively few in number, have seldom given results which 
show even fair agreement ; and even in these cases, as will be shown later, 
the agreement between the mean values obtained by different ratios 
leaves much to be desired. It seems unfortunate that many of the ratios 
used in calculating the value at present accepted were obtained by 
weighing a salt, which, after being precipitated from a water solution, 
was heated to a temperature of only 150 for the purpose of expelling 
the water. 
The observations of Precht § have shed a great deal of light upon the 
difficulties involved in the preparation of pure halogen compounds of 
platinum. 
In view of the above considerations, and particularly as the atomic 
weight of platinum is constantly being used in the estimation of potash, a 
* Trans. Roy. Soc. Edin., xxxiii. 561 (1887). 
+ Richards, Proc. Am. Acad., xxii. 342 (1887), xxiii. 177 (1888), xxv. 195 (1890), xxvi. 
240 (1891), xxviii. 1 (1893), xxix. 369 (1894) ; Richards and Rogers, ibid., xxviii. 200 (1893) ; 
Richards and Parker, ibid., xxxii. 55 (1896) ; Richards and Cushman, ibid., xxxiii. 97 (1897) ; 
Richards and Baxter, ibid., xxxiii. 115 (1897) ; Richards and Marigold, ibid., xxxvii. 365 
(1902) ; Richards and Archibald, ibid., xxxviii. 443 (1903) ; Richards and Wells, Jour. Am. 
Chem. Soc., xxvii. 475 (1905) ; Richards, Staehler, Forbes, Mueller, and Jones, Carnegie Inst, 
of Washington, Publication 69. 
I Richards, Proc. Amer. Phil. Soc., xlii. 28 (1903). 
§ Precht, Zeit. Anal. Chem., xviii. 509 (1879). 
VOL. XXIX. 
46 
