3*8 Dr. Gladstone on the Compounds of the 



chloride of phosphorus, and bromide of mercury, which re- 

 mains in the retort. None of these three ter-compounds is 

 directly acted upon by oxygen gas; the terchloride of phos- 

 phorus is unaffected by sulphur at any temperature; the ter- 

 bromide also appears to enter into no definite combination ; 

 yet, if the oxygen or sulphur be united with hydrogen, double 

 decomposition ensues. Water, as is well known, resolves 

 these ter-compounds into the hydracids and phosphorous acid: 

 hydrosulphuric acid, as observed by Serullas, resolves ter- 

 chloride of phosphorus into l)ydrochloric acid and tersulphide 

 of phosphorus: a similar reaction takes place with the ter- 

 bromide, which may be expressed by the formula — 



PBr3+3HS=PS3 + 3HBr. 



When we consider the action of these non-metallic elements, 

 and their hydracids, upon the penta-compounds, we are again 

 reminded of the comparative feebleness with which the two 

 additional atoms are combined. Pentachloride of phosphorus, 

 it is true, is unaffected by bromine, and the action of iodine 

 upon it did not appear to me well-defined ; but pentabromide 

 of phosphorus, carefully prepared so as to avoid excess of 

 bromine, when brought into contact with iodine at the ordi- 

 nary temperature, quickly ran down into a red liquid, and 

 terbromide was found in the vessel. Very little iodine is re- 

 quired to effect this reaction ; and since the compound IBr- 

 is known to be readily formed, the decomposition in all pro- 

 bability may be thus expressed : — 



5PBr5 + 2l = 5PBr3 + 2lBr5. 



The action of sulphur upon these penta-compounds is remark- 

 able. If it be mixed with the pentachloride of phosphorus, 

 and the whole fused together, combination ensues ; but this 

 is not a case of reduction ; and the new products — a liquid 

 and a crystalline body — require a more lengthy consideration 

 than can be given them in this paper. Again, if pentabro- 

 mide of phosphorus be mixed with sulphur, no action takes 

 place in the cold ; but upon gently raising the temperature, 

 combination is instituted; the odour of bromide of sulphur is 

 perceptible; and a liquid is produced, which appears to con- 

 sist, partially at least, of a compound that will be presently 

 described. 



We have now to consider the manner in which these penta- 

 compounds of the halogens with phosphorus I'requently part 

 with the two additional atoms when treated with hydrogen 

 compounds. Hydriodic acid reduces the pentachloride of 

 phosphorus to the terchloride, a fact first remarked by Se- 



