BAXTER AND WILSON. — THE ATOMIC WEIGHT OF LEAD. 369 



dissolved and crystallized from dilute nitric acid once in glass and six 

 times in platinum vessels, with centrifugal drainage. Hydrochloric 

 acid was then distilled into a large quartz dish, and the solution of 

 the nitrate was slowly added with constant stirring with a quartz rod. 

 The chloride was freed from aqua regia as far as possible by washing 

 with cold water, and was once crystallized from aqueous solution in 

 quartz dishes to remove last traces of aqua regia. Finally the salt 

 was crystallized three times in platinum. 



It could reasonably be expected that both of these samples were of a 

 high degree of purity ; nevertheless, upon heating the salt in an atmos- 

 phere of hydrochloric acid, the salt itself turned somewhat dark, and 

 upon solution of the fused salt in water a slight dark residue remained. 

 Although in a few preliminary experiments attempts were made to 

 determine this residue by filtration and ignition, it was subsequently 

 found that even a small filter paper adsorbs appreciable amounts of 

 lead compounds from a solution of the chloride, which cannot be re- 

 moved by washing with water. From three to thirteen hundredths of 

 a milligram of residue were obtained in several blank experiments, by 

 ignition of filters through which half per cent solutions of lead chloride 

 had been passed, with subsequent very thorough washing. In order to 

 avoid the uncertainty of this correction, further attempts were made 

 to obtain a sample of the salt which would give a perfectly clear solu- 

 tion in water after fusion, and thus render filtration unnecessary. With 

 this end in view a considerable quantity of Sample A was fused in a 

 large platinum boat in a current of hydrochloric acid. The fused salt 

 was powdered in an agate mortar, dissolved in water in a platinum 

 vessel, and the solution was freed from the residue by filtration through 

 a tiny filter in a platinum funnel into a platinum dish, where it was 

 allowed to crystallize. This sample was then twice recrystallized with 

 centrifugal drainage. Notwithstanding the drastic treatment to which 

 it had been subjected, when a portion of this material was fused in hy- 

 drochloric acid, the same darkening as before was observed, and the 

 same residue was obtained. The suspicion that the difficulty was due 

 to dissolving of the filter paper by the solution of the salt 15 led to 

 a second more successful attempt by crystallization from hydrochloric 

 acid solution in platinum vessels. In this way it was found possible 

 to prepare salt which showed no tendency to darken upon heating, and 

 which, after fusion, left absolutely no residue upon solution in water. 

 Portions of Samples A and B were thus recrystallized three times 

 more. Since these two specimens of material gave identical results, 



15 Mr. P. B. Goode in this laboratory has recently found a similar difficulty 

 with the chlorides of the alkaline earths. 

 vol. xliii — 21 



