f>56 PROF. LETTS AND MR R. F. BLAKE ON 



2. Very little difference exists in the percentage amount of carbon and hydrogen in 

 dibenzyl phosphine and oxide of tribenzyl phosphine, as the following numbers show :• — ■ 





(C 7 H 7 ) 2 H1> 



(C 7 H 7 ) s PO 



Carbon, 



78-50 



78-75 



Hydrogen. . 



701 



6-56 



Consequently it would not be possible to decide with absolute precision between the 

 two substances by a mere combustion. On the other hand, there is a considerable 

 difference between the two bodies in their percentage of phosphorus — 



(C ; H 7 ) 2 HP (C 7 H 7 ) 3 PO 



Phosphorus, . . . 1448 969 



Unfortunately, however, as we have again and again found, the processes for phosphorus 

 determinations in ordinary organic substances are absolutely untrustworthy when 

 applied to phosphines. A new method was therefore necessary, and after many trials 

 we believe we have found one which is perfectly accurate, trustworthy, and capable of 

 general application. It is extremely simple, though somewhat tedious in carrying out. 

 It consists in making an ordinary combustion of the substance with pure oxide of 

 copper, and afterwards dissolving the contents of the combustion tube in nitric acid, 

 and determining the phosphorus with molybdate of ammonia, &c. Applying this 

 method to the analysis of the supposed dibenzyl phosphine, we obtained the following 

 results (IV. and V.). 



We give at the same time the determinations of phosphorus made both by Hofmann 

 (1.) (by a method not described) and by one of us and W. Wheeler (II. and III.), by 

 burning the substance with lime in a stream of oxygen, dissolving and titrating with 

 .standard uranium solution, 



Obtained. 



I. II. III. IV. V. . 



Phosphorus, . . 136 1435 1500 986 998 



As we have before stated, many of the compounds of the supposed dibenzyl phosphine 

 were prepared and analysed, and although their composition appeared remarkable for 

 the compounds of a secondary phosphine, the analytical results agreed fairly well in 

 most cases with formulae of derivatives of dibenzyl phosphine. We even concluded, 

 from the existence of these compounds, that the exceptional properties of the phosphine 

 were due to its consisting of a double molecule, viz.: — 



(C 7 H 7 ) 2 H S P = P^C 7 H 7 ) 2 H. 



It is a remarkable coincidence that the analytical results agree equally well for com- 

 pounds of the oxide of tribenzyl phosphine. This fact has added considerably to the 

 difficulties and uncertainty of the research, but at the same time it has caused us to 

 exercise great caution in the inferences to be drawn from the analyses. 



Subjoined is a brief account of the chief compounds we have examined of Hofmann's 

 dibenzyl phosphine, and we give at the same time the results obtained (in some cases) 



