THE HALOGEN FAMILY 



201 



carbonate of soda solution, and covered thickly with vaseline, or a 

 salve of boric acid in lanoline, to protect them from infection. 



Chemical Properties. A jet of burning hydrogen will con- 

 tinue to burn in bromine vapor, giving hydrogen bromide, a gas 

 which fumes in moist air like hydrogen chloride : 



H 2 + Br 2 -> 2HBr. 



Many of the metals, when thrown in the form of powder, leaf, 

 or foil, into bromine vapor, combine directly, giving bromides. 

 The action is similar to that with chlorine, but less vigorous. 



Hydrogen Bromide HBr, Preparation. Hydrogen and 

 bromine vapor unite much less readily than hydrogen and chlo- 

 rine. A stream of pure hydrogen bromide is most easily made 

 by moistening red phosphorus with water, and allowing bromine 

 to fall drop by drop into the paste (Fig. 61). To absorb the 

 bromine vapor, carried by the gas, the latter is passed through 

 a U-tube containing dry red phos- 

 phorus mixed with broken glass or 

 beads : 



2P + 3Br 2 - 2PBr 3 . 

 PBr 3 + 3H 2 - 3HBr | + HaPOa. 



The bromine forms phosphorus tribro- 

 mide, which is immediately decom- 

 posed by the water. The phosphor- 

 ous acid H 3 P0 3 remains, dissolved in 

 the water, in the flask. The gas can 

 be collected by upward displacement FIG. 61 



of air. 



It might seem that a simpler action would be that of sulphuric 

 acid upon a bromide (compare p. 126) : 



H 2 S0 4 + KBr r* KHS0 4 + HBr t . 



