180 PROCEEDINGS OF THE AMERICAN ACADEMY. 



grams there were noticeable at certain concentrations very faint 

 minima of absorption in the middle of all three bands in the blue. 

 These minima could be seen over a considerable range of concentration 

 in all three cases but the particular concentration at which they were 

 most marked was different for the three bands. Measurements were 

 made of the position of these minima with a comparator and the 

 cadmium spark spectrum as a standard, and the minima were found to 

 coincide with the centres of the absorption bands observed with the 

 most dilute solutions both visually and in the spectrograms. 



The ultraviolet absorption bands at 354 and 353 reported by 

 Forsling 12 and the one at 346 given by Exner 13 correspond to strong 

 absorption bands of neodymium. As we could not find the least trace 

 of these bands in the spectrograms, it seems probable that they were 

 produced by neodymium impurity. 



The Preparation of Praseodymium Chloride. 



Each fraction of double nitrate investigated was converted to 

 chloride as follows: The salt was dissolved, and the solution, after 

 dilution to about two liters, was filtered. A considerable quantity 

 of nitric acid was added, the solution was heated to boiling and prase- 

 odymium oxalate was precipitated by an excess of oxalic acid. After 

 the precipitate had been thoroughly washed by decantation, it was 

 collected upon a disk of filter paper in a large porcelain Gooch crucible, 

 and dried in an electric oven at 105°. In order to change the oxalate 

 to oxide it was heated to dull redness in a platinum boat in an elec- 

 trically heated porcelain muffle. Care was taken that the temperature 

 should not be high enough to vaporize platinum from the boat into the 

 oxide. 14 The black oxide was next dissolved in a quartz dish in a 

 large excess of nitric acid which had been distilled through a quartz 

 condenser, and the oxalate was reprecipitated from dilute solution by 

 adding a solution of twice recrystallized ammonium oxalate. After 

 thorough washing, the oxalate was collected, dried and ignited as be- 

 fore. The chloride was now prepared by dissolving the oxide in a 

 quartz dish in hydrochloric acid which had been distilled through 

 quartz. Free chlorine was expelled by heating on an electric stove, 



12 Kayser, loc. cit. 



13 Ibid. 



14 See Baxter and Chapin, Jour. Amer. Chem. Soc, 33, 16 (1911). 



