588 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Hydrogen Peroxide. — Merck's " Perhydrol," containing 30 per 

 cent hydrogen peroxide, was found to be free from sulphuric acid and 

 halogens and to leave no residue upon evaporation. Hence it was 

 employed without further treatment. 



Nitrogen. — Atmospheric nitrogen free from oxygen was obtained 

 by Wanklyn's well-known method of passing air through very con- 

 centrated ammonia solution, and then over hot copper gauze in a hard 

 glass tube. The excess of ammonia was removed by scrubbing with a 

 dilute solution of sulphuric acid in three large gas-washing bottles. 

 Since hydrogen is formed by the catalytic decomposition of a portion 

 of the excess of ammonia, this impurity was oxidized in a hard glass 

 tube filled with red hot copper oxide, and the gas was then dried in 

 six towers filled with beads drenched with concentrated sulphuric acid 

 and a long tube filled with resublimed phosphorus pentoxide. Finally 

 the gas was passed through a third hard glass tube filled with hot 

 copper to combine with any last trace of oxygen. The apparatus in 

 which these operations were carried out was constructed entirely of 

 glass with ground glass connections throughout. 



Preparation of Phosphorus Tribromide. 



Much time was consumed in finding a satisfactory method of pre- 

 paring pure phosphorus tribromide. It was obvious at the outset that 

 the substance must be rigorously protected from access of moisture, in 

 order to avoid the formation of hydrobromic and phosphorous acids. 

 This object was most simply gained by synthesizing the tribromide 

 in a vacuum from bromine and phosphorus which were initially dry. 

 But since phosphorus dissolves in its tribromide, and can not be readily 

 separated from the latter substance by distillation, an excess of bro- 

 mine was added at the beginning. Fortunately it was necessary to 

 add only a slight excess of bromine, since a surprisingly small amount 

 of phosphorus pentabromide can be detected in the tribromide by its 

 darker color, especially at high temperatures. This dark color is un- 

 doubtedly due in large part to dissociation of the pentabromide into 

 tribromide and bromine. The comparatively easy elimination of the 

 pentabromide from the tribromide also depends upon this dissocia- 

 tion, for the free bromine can be removed by distillation in the first 

 fractions. 



At first the attempt was made to remove the excess of bromine by 

 distillation from red phosphorus. But either the red phosphorus still 

 contained traces of the yellow modification, or else the red modification 

 dissolves in the tribromide and vaporizes vvith it when distilled (pos- 



