188 PROCEEDINGS OF THE AMERICAN ACADEMY. 



drying and fusing the salt in a platinum tube instead of in a quartz 

 tube, but even under these conditions the insoluble residue was 

 formed. We are still somewhat uncertain as to the cause of the for- 

 mation of the basic salt during prolonged fusion, although incomplete 

 preliminary drying is invariably the cause when the fusion period is 

 short, but we are inclined to the opinion that the difficulty is due to 

 a trace of air in the hydrochloric acid gas. 



Since we were unable to avoid the formation of the insoluble material 

 in every case, in two experiments in which the salt was dried as for 

 analysis and then fused for a short period, the insoluble residue was 

 collected and weighed. The results of these experiments follow. As 

 before, about 6 grams of anhydrous salt were used in each experiment. 



Period of Fusion 0.5 min. 4.0 min. 



Wt. of Insoluble Residue 0.0001 gm. 0.0010 gm. 



In order to discover whether the basic salt dissolves to an appreci- 

 able extent, in experiments with three separate portions of material 

 which had been collected, dried and weighed upon a platinum-sponge 

 crucible, about a liter of water was allowed to flow slowly through the 

 crucible during the course of an hour. The crucible and contents 

 were then dried and reweighed. The losses in weight per liter of water 

 were 0.9, 0.5 and 0.3 mg. While these figures are undoubtedly some- 

 what less than the real solubility in water, they are probably far 

 greater than the solubility in dilute praseodymium chloride solution. 

 This explains the extreme slowness with which a mere trace of the 

 basic salt dissolves in the solution of the neutral salt and supports the 

 conclusion that the quantity of basic salt present in the material fused 

 for a short time is very small. 



In preparing the salt for the actual analyses the period of fusion was 

 less than one minute, so that the weight of residue certainly must have 

 been considerably less than 1 mg. But since even as large a propor- 

 tion as 1 mg. of oxychloride in 5 gm. of salt, if it dissolves eventually, 

 would raise the apparent atomic weight of praseodymium by only 

 0.028, we feel that little danger is introduced by not attempting to 

 apply any correction for the residue. 



In a research upon the atomic weight of neodymium carried out 

 immediately at the conclusion of this research, similar evidence was 

 obtained as to the nature of the insoluble neodymium compound which 

 forms under essentially the same conditions. 



