Halogens mth Phosphorus. S^? 



that the pentachloride of phosphorus is reduced to the ter- 

 chloride by the action of phosphuretted hydrogen ; and this 

 again is completely decomposed, if the current of gas be con- 

 tinued. If the yellow crystals of pentabromide be subjected 

 to the same treatment, a liquid compound is instantly pro- 

 duced, whilst acid fumes are evolved ; and the long-continued 

 action of the phosphuretted hydrogen gas causes the entire 

 disappearance of the liquid, yellow phosphorus being left in 

 its place. The reaction is evidently the same as in the case 

 of the chloride, and may be expressed by the two formulae, — 



SPBr^-f PH3 = 'iPBr3+3HBr, 

 and 



PBr3 + PH3=2P + 3HBr. 



Although phosphuretted hydrogen is thus capable of re- 

 ducing these combinations of phosphorus with the halogens, 

 pure hydrogen gas has not the slightest effect upon the penta- 

 chloride of phosphorus even at the subliming point of that 

 substance; nor does it produce any alteration in the penta- 

 bromide, — at least not at the ordinary temperature. Heat 

 alone, however, will effect the reduction of this latter com- 

 pound, provided a stream of dry air be passed over it : the 

 experiment is conveniently made in a water-bath at 100°C., 

 when bromine passes off, and the liquid terbromide re- 

 mains. The peculiar reddening of the pentabromide during 

 sublimation, first observed by Balard, is in all probability 

 owing to this cause. When the penta-compound fuses, it is 

 partially decomposed into terbromide of phosphorus and free 

 bromine, which colours the liquid red; the two rise in vapour 

 together, thus producing a red gas; and when they reach a 

 sufficiently cool portion of the vessel, recombine to form the 

 lemon-yellow crystals of the penta-compound. It is not in- 

 tended by this explanation to deny that pentabromide of phos- 

 phorus may be capable of volatilization without being decom- 

 posed. In a similar manner, the higher compound of iodine 

 and phosphorus may be reduced by the aid of heat alone. 

 The pentachloride, on the contrary, resists the decomposing 

 action of heat. 



The relative force of affinity between phosphorus and the 

 three halogens to which my attention has been more espe- 

 cially directed, is in the order, — Chlorine, Bromine, Iodine. 

 Thus, teriodide of phosphorus is decomposed by both chlorine 

 and bromine; the terbromide by chlorine, but not by .iodine; 

 whilst the terchloride is not decomposed by either of the other 

 elements. The same is indicated by the fact that terbromide 

 of phosphorus distilled with chloride of mercury yields ter-» 



