BAXTER. — ATOMIC WEIGHT OF PHOSPHORUS. 591 



acid, and outside this, to prevent the absorption of moisture, soft paraf- 

 fine was smeared. 



Next the calculated amount of pure dry bromine was measured from 

 a burette into the separatory funnel B and the funnel was stoppered. 

 After cooling the flask containing the phosphorus with ice water to re- 

 duce the violence of the reaction, bromine was slowly admitted. The 

 rapid evaporation of a portion of the bromine produced the solidification 

 of the remainder. This solid bromine, upon falling into the flask, reacted 

 violently with the phosphorus until a considerable amount of tribromide 

 had been formed. As soon, however, as sufiicient tribromide had been 

 produced to dissolve the unchanged phosphorus the action became much 

 less violent so that the bromine could be introduced more rapidly. 



When nearly the theoretical amount of bromine had been used, the 

 flask D was immersed in hot water in order to decompose a small 

 amount of solid pentabromide which formed in the upper part of the 

 flask. The bromine thus liberated diffused to the bottom of the flask 

 where it combined with the residual phosphorus dissolved in the tri- 

 bromide. More bromine was then admitted in very small portions 

 until the warm tribromide acquired the reddish color which indicated 

 an excess of bromine. The end of the reaction can be determined so 

 sharply by the color that the excess of the bromine probably never 

 amounted to more than a few hundredths of a gram, whereas nearly 

 one hundred and twenty-five grams of bromine in all were used in 

 each preparation. 



When it was certain that an excess of bromine had been added, the 

 flask D with the tribromide was surrounded with ice water and nitro- 

 gen was allowed to enter through A, until the pressure inside the flask 

 was greater than atmospheric. The end of the tube F was then broken 

 off, and the remainder of the apparatus, which had previously been 

 filled with dry nitrogen, was attached by the gound joint E. The flasks 

 D, G, J, K, M, and N were next exhausted through 0, while the stop- 

 cocks A, R, S, and T were closed. In order to prevent access of bro- 

 mine to the mercury of the Topler pump, a U-tube V filled with fused 

 potassium hydroxide was located between the pump and the remainder 

 of the apparatus, and this U-tube was cooled with concentrated cal- 

 cium chloride solution and ice. 



During the fractional distillation of the tribromide to remove the 

 excess of bromine it was highly desirable to protect the bulbs 1 to 8, in 

 which the tribromide was eventually to be collected, from contact with 

 the first fractions. This was effected by passing the first fractions 

 through the side tube L into the bulbs M and N, while the bulbs 1 to 8 

 were cut off" from the rest of the apparatus by the valve P. This valve 



