604 PROCEEDINGS OF THE AMERICAN ACADEMY. 



one, 0.574453, the difference, which is less than 0.002 per cent, is too 

 small to be significant. 



In examining critically the results recorded in the foregoing tables 

 it should first of all be noted that a given percentage error in the 

 experimental work is multiplied nine times in the calculation of the 

 atomic weight of phosphorus. That is, an experimental error of one 

 one-hundredth of a per cent affects the atomic weight of phosphorus by 

 0.027 unit. The highest value for the atomic weight of phosphorus in 

 these tables is 31.040, the lowest 31.013, a difference corresponding 

 exactly to one one-hundredth of a per cent in the experimental work. 

 On the whole, however, the agreement of the results is better than 

 this, since, of the thirty-six results, twenty-seven fall between the limits 

 31.035 and 31.021, a difference only half as large. In other words, the 

 different specimens of material seem to be identical as far as the 

 method is capable of testing this point. 



In each series, if the first iraction, that of the highest number, 8 or 

 10, contained an excess of bromine, it would have yielded too low a 

 result, while if decomposition occurred during the distillation, with the 

 possible production of lower bromides of phosphorus, the residual frac- 

 tion, 1, would have yielded too high a result. The only differences in 

 each series, however, seem to be purely accidental. Furthermore, the 

 three series yield average results in practical unanimity. 



In the following table the final average of this research is compared 

 with that of Baxter and Jones with silver phosphate: 



Summary of Results. 



1. A method is described for the preparation of pure phosphorus 

 tribromide. 



2. It is shown that the precipitation of the halogen of phosphorus 

 halides after decomposition with water, can safely be done only after 

 the oxidation of the greater portion of the phosphorous acid produced. 



3. A method is described for the determination of the bromine in 

 phosphorus tribromide by comparison with silver and as silver bromide. 



4. The molecular weight of phosphorus tribromide referred to silver 

 107.88 is found to be 270.775, whence phosphorus has the atomic 

 weight 31.027. If silver is taken at 107.87, the atomic weight of 



