BAXTER. — ATOMIC WEIGHT OF PHOSPHORUS. 593 



The train of bulbs 1 to 8 was next sealed off with a blowpipe at the 

 capillaries, and the individual bulbs were separated from the connect- 

 ing tubes by sealing the capillary tubes. Great care was taken in 

 sealing the capillaries to avoid decomposing the tribromide, by first 

 heating them gently to evaporate any tribromide with which they 

 might be wet. There never was any sign of decomposition of the tri- 

 bromide during this process. 



The final samples of the tribromide were almost colorless when cold, 

 but the very slight yellowish tint of the cold substance was materially 

 increased by a rise in temperature. Since no difference in color could 

 be detected between the first and last samples collected, it seems prob- 

 able that phosphorus tribromide really possesses a slight yellowish tint. 



Two series of samples of tribromide were distilled and sealed in bulbs 

 fi-om two different preparations of material. Ten fractions were col- 

 lected in the first series and eight in the second. 



In a third preparation the removal of the bromine was accelerated 

 by distillation from metallic silver. The bulbs J and K were partly 

 filled with fine electrolytic crystals of the purest silver which had been 

 freed from moisture by ignition in a vacuum. After the tribromide 

 had been distilled into J and K, and the excess of bromine had been 

 distilled into M and N, the capillary tube L was sealed as described 

 before. Then the bulbs J and K with the residual tribromide were 

 warmed for some time with constant shaking before the tribromide was 

 finally distilled into the collecting bulbs. 



The Determination oe the Weight of the Tribromide and 



ITS Analysis. 



In the analysis of the tribromide it was first decomposed with water, 

 and then the bromine was precipitated as silver bromide. The reduc- 

 ing effect of phosphorous acid upon silver salts is well known, so that 

 the necessity was obvious of oxidizing the phosphorous acid. This 

 operation was a somewhat delicate one, however, since the hydrobromic 

 acid also is easily oxidized, with consequent loss of bromine. Hydrogen 

 dioxide was eventually found to be the safest oxidizing agent, although 

 its action on the phosphorous acid is not particularly rapid. In pre- 

 liminary experiments the bulb of tribromide was broken under water 

 and the oxidation was then effected by hydrogen peroxide in dilute 

 nitric acid solution. In the final experiments the method of breaking 

 the bulb under an ammoniacal solution of hydrogen dioxide was em- 

 ployed, since in this way the greater part of the oxidation was almost 

 instantaneously effected. Then after some time the solution was anidi- 



VOL. XLVII. — 38 



