1818.] of Phosphorus and Oxygen. 137 



agree with the laws which he had discovered in many other 

 bodies. 



Of all the compounds of oxygen and phosphorus, the phos- 

 phoric acid is the only one which can be subjected without 

 alteration to an elevated temperature, and which forms salts, 

 that can be obtained in the dry state. It becomes, therefore, 

 essential to reduce all the estimates to the proportions of the 

 phosphoric acid, and, consequently, to ascertain this with the 

 greatest accuracy. Besides, the great differences which there 

 are in the estimates of different chemists, rendered it necessary 

 to examine the subject in detail. When we endeavour to deter- 

 mine the proportions of phosphoric acid by the acidification of 

 phosphorus by means of nitric acid, there are many causes of 

 error which it is not easy to obviate. There is also a great liabi- 

 lity to error in the estimates that are made of the weight of the 

 acid from the variableness in the proportions of the insoluble 

 phosphate that is employed in the process. The phosphates of 

 lead, barytes, and lime, which are commonly employed, are not 

 uniform in their composition ; besides that the insoluble phos- 

 phates often combine with a portion of different soluble salts, 

 which affects the result. On this principle we are to explain the 

 different conclusions at which M. Rose and Dr. Thomson 

 arrived, although they employed the same process. 



If we unite the phosphorus to a metal, which is easily acted 

 upon by nitric acid, we may use this substance in estimating the 

 composition of phosphoric acid, without being liable to those 

 sources of error which interfere with the results when we employ 

 the acid in a separate state. The author employed copper for 

 this purpose ; having carefully formed a quantity of phosphuret 

 of copper, it was dissolved in nitric acid, by which means all the 

 phosphorus was converted into phosphoric acid. If we precipi- 

 tate the copper by caustic potash, we may learn by the weight 

 of the oxide whether any phosphoric acid remains attached to 

 it; nitrate of either barytes, lime, or lead, is then added to the 

 fluid, and a phosphate is formed, from which we learn the quan- 

 tity of phosphoric acid. This method does not appear, how- 

 ever, to admit of very great accuracy ; in six experiments that 

 were performed, the quantity of oxygen varied from 117 to 125 

 parts, to 100 parts of phosphorus. It appears that we can 

 arrive at greater accuracy by dissolving the phosphuret of copper 

 in nitric acid, and evaporating and calcining the residuum in a 

 platma crucible ; by this means a phosphate of copper is formed, 

 from which we are able to calculate the quantity of oxide of 

 copper; and the amount of the phosphorus being known, we 

 ascertain that of the oxygen. By this means numbers were 

 obtained much more nearly coinciding than in the former case ; 

 the highest was 123-3, and the lowest 122. 



Although there seemed every reason to trust to the correct- 

 ness of this, estimate, yet it still remained to determine, whether 



