216 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The atomic weights of certain members of the group have received 

 a singularly large amount of attention. Those of cerium and lanthanum, 

 for instance, have been investigated more times than that of iron. Yet 

 owing to the unusual difficulties in the preparation of pure material, as 

 well as to troublesome analytical problems, the determinations of the 

 atomic weights of the rare earths are probably of a lower order of ac- 

 curacy than those of the common elements. 



Among the standard methods which have heretofore been employed 

 in the determination of atomic weights of the rare earths, the following 

 are the most important : 



The Anali/sis of the Oxalate. (Stolba.)^ The dried oxalate is ana- 

 lyzed for the oxide by ignition, and for oxalic acid by titration with 

 permanganate. The complete analysis of the salt is necessary on ac- 

 count of the invariable presence of an uncertain amount of crystal water 

 even in the dried salt. Gibbs ^ has pointed out, on account of irregu- 

 larity in the water content, the necessity of grinding thoroughly the 

 whole specimen of oxalate which is to be used for analysis, in order to 

 insure homogeneity of material. Furthermore it has been shown by 

 Baxter and Griffin ^ and Baxter and Daudt ■* that the oxalate must be 

 precipitated in acid solution, since in neutral or alkaline solution the 

 oxalates of the alkalies and ammonium are carried down by the rare 

 earth oxalate in considerable quantities. Brauner ^ has found that in 

 some cases at any rate the oxalate hydrolyzes in contact with water, 

 although the magnitude of the latter error is doubtful. In spite of these 

 difficulties the oxalate method is as easy to carry out as any and prob- 

 ably the most satisfactory for approximate purposes. 



The Anahjsis of the Sulphate. The sulphates have been used for 

 atomic weight purposes in several ways. Bahr and Bunsen ^ first em- 

 ployed the method of dissolving weighed amounts of oxide in sulphuric 

 acid, and weighing the sulphate after expelling the excess of acid. By 

 the ignition of the oxalate it is doubtless possible to prepare the oxides 

 in a very pure state if great care is taken to protect them from access 

 to moisture or carbon dioxide, at any rate with elements which form 

 trivalent compounds only. To prepare the sulphate in a neutral dry 

 condition is a much more difficult proposition, owing to the retention 

 of water and sulphuric acid if the temperature of heating is low, and 



^ Sitzungsbcr. bohm. Ges., Dec. (1S7S) 



2 Proc. Amer. Acad., 28, 2G1 (1893). 



3 Jour. Amcr. Chem. Soc, 28, 16S4 (1906). 

 * Ibid., 30, .563 (1908). 



B Trans. Chcm.'Soc., 81, 1286 (1902). 

 6 Lieb. Ann., 137, 5 (1S66). 



