68 RESEARCHES UPON ATOMIC WEIGHTS. 



experiments attempts were made to determine this residue by tiitration and 

 ignition, it was subsequently found that even a small filter paper adsorbs ap- 

 preciable amounts of lead compounds from a solution of the chloride, which can 

 not be removed by washing with water. From 0.03 mg. to 0.13 mg. of residue 

 were obtained in several blank experiments, by ignition of filters through 

 which 0.5 per cent solutions of lead chloride had been passed, with subsequent 

 very thorough washing. In order to avoid the uncertainty of this correc- 

 tion, further attempts were made to obtain a sample of the salt which would 

 give a perfectly clear solution in water after fusion, and thus render filtra- 

 tion 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. Not- 

 withstanding the drastic treatment to which it had been subjected, when a 

 portion of this material was fused in hydrochloric 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 ' led to a second more successful attempt by crystallization from hydro- 

 chloric 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 recrystalHzed three times more. Since these two 

 specimens of material gave identical results, for two final experiments, portions 

 from each of these samples were mixed and then subjected to three additional 

 crystallizations. This last sample was designated Sample C. 



DRYING OF LEAD CHLORIDE AND METHOD OF ANALYSIS. 



The method of analysis did not differ materially from that used in the analy- 

 sis of cadmium and manganese chlorides (pages 7 and 45). The lead chloride, 

 contained in a weighed platinum boat, was first fused in a current of hydro- 

 chloric-acid gas, and the boat was transferred to its weighing-bottle and 

 weighed. On account of the small solubility of lead chloride it was a somewhat 

 troublesome matter to dissolve the fused material. This was done in most of 

 the analyses by prolonged contact with water nearly at the boihng point in a 

 Jena glass flask. In the last two analyses, in order to show that no error was 

 introduced through the solubility of the glass, the solution was prepared in 

 a large platinum retort and was transferred to the precipitating flask only 

 when cold. 



' Mr. P. B. Goode in this laboratory has recently found a similar difficulty v/ith the chlo- 

 rides of the alkaline earths. 



