BAXTER AND JONES. — ATOMIC WEIGHT OF PHOSPHORUS. 139 



method of limiting densities is the most reliable of all the methods for 

 applying the correction to the densities made necessary by deviations 

 from the laws of a perfect gas. 



The other methods are burdened with arbitrary assumptions and 

 empirical constants, and furthermore Baume 12 has shown that both 

 the method of molecular volumes and the method of critical constants 



T 



give correct results only with gases for which the ratio —^ is nearly 1, 



c 



whereas for phosphine this ratio is 1.26. 



If the molecular weight of phosphine be assumed to be 33.93, the 

 atomic weight of phosphorus is 30.91. In the light of this low result 

 it is unfortunate that Gazarian prepared phosphine by only one 

 method, and that he did not determine the purity of the gas, i. e. by 

 absorption. Gazarian used the method of Matignon and Trannoy l3 

 which consists in heating calcium phosphate and aluminum together 

 until they react, and then treating the product of this reaction without 

 further purification with water in a gas generator. Matignon and 

 Trannoy show that the gas prepared in this way by them contained 

 about three per cent of hydrogen, probably derived from calcium con- 

 tained by the phosphide. In this case some calcium nitride would be 

 formed, since the phosphide was made in air ; and this would produce 

 ammonia as an impurity in the phosphine. Although the gas was 

 purified by fractional distillation, according to Gazarian's statements 

 hydrogen is difficult to eliminate, and a proportion of only four-tenths 

 of one per cent would be sufficient to lower the atomic weight of phos- 

 phorus one-tenth of a unit. Ammonia would be even more difficult to 

 remove, since its boiling point is only 50° higher than that of phos- 

 phine- The effect of a given percentage of impurity is, however, much 

 less with ammonia than with hydrogen, although in the same direction. 



From the preceding brief summary it is evident that the uncertainty 

 in the atomic weight of phosphorus is as great as one tenth of a unit, 

 and that, as Brauner remarks at the conclusion of his review of the 

 subject, "a revision of the atomic weight of phosphorus with modern 

 means is urgently necessary." 



The analysis of silver phosphate was selected as one of the most 

 promising methods of attacking the problem, since the percent of silver 

 can be determined exactly by a method which has been carefully 

 studied, especially in this laboratory. The accuracy of the result will 

 therefore depend primarily upon the success attained in preparing 



12 Baume, J. Chim. Phys. 1908, 6, 76 and 86. 



13 C. R., 1909, 148, 167. 



