244 



RJiPORTS ON INVESTIGATIONS AND PROJECTS. 



In order to confirm the value 31.03 (Ag^ 107.870) previously found 

 for the atomic weight of phosphorus by the analysis of silver phosphate and 

 phosphorus tribromide (see Year Books Nos. 8 and 10), the preparation of 

 pure phosphorus trichloride and its analysis were undertaken by Dr. C. J. 

 Moore. 



The trichloride was synthesized by allowing pure dry phosphorus to unite 

 with a slight excess of pure dry chlorine in a vacuum to avoid contact with 

 air and moisture. The small amount of pentachloride and possible traces 

 of hydrochloric acid were eliminated by fractional distillation in a vacuum, 

 and the remaining material was distilled into a series of small glass bulbs 

 which were individually sealed off while exhausted. 



The weight of each specimen of trichloride was found and the analysis, 

 by comparison with metallic silver and by weighing the silver chloride pro- 

 duced, was carried out almost exactly as described in the case of phosphorus 

 tribromide. 



Two series of samples were collected by fractional distillation of two 

 different preparations. 



In the following table of results a higher number indicates a more volatile 

 fraction, while the lowest number indicates essentially residual material. 

 The atomic weight of silver is assumed to be 107.870. 



The close agreement of the results of the analyses in each series and of 

 the averages of the different series is the best evidence of the purity of the 

 trichloride. Unless a constant boiling mixture of trichloride and penta- 

 chloride exists, the uniformity of the material indicates a high degree of 

 purity. 



The three researches upon the atomic weight of phosphorus yield the 

 following values : 



