136 On the Combinations [Feb. 



sion, that this acid consists of phosphorus 72*75 parts, oxygen 

 27*25 parts : or phosphorus 100 parts, oxygen 37*44 parts. It 

 is, however, admitted, that the acid may contain hydrogen, in 

 which case its name must be changed to hydroxyphosphorous 

 acid. 



The next compound of phosphorus and oxygen is the phos- 

 phorous acid : this substance has usually been obtained by the 

 slow combustion of phophorus in the atmosphere ; but by this 

 process it is not procured in a pure state. The method that is 

 recommended is to form the compound of chlorine and phospho- 

 rus, and to decompose it by water : then by evaporating the 

 water, the acid in question is obtained, and hydrochloric acid is 

 disengaged. To ascertain the proportion of the constituents of 

 this acid, it is sufficient to discover the quantity of chlorine with 

 which the phosphorus is combined in the compound ; and this 

 is accomplished by precipitating a given weight of the chloruret, 

 after it has been acted upon by the water, by means of the 

 nitrate of silver. Proceeding upon this datum, we learn that 

 phosphorous acid is composed of phosphorus 57*18 parts, and 

 oxygen 42*82 parts ; or phosphorus 100 parts, and oxvgen 

 74*88 parts. 



The acid which is generated by the slow combustion of phos- 

 phorus in the atmosphere differs from the above acid both in 

 its composition and even in its nature. What have been described 

 by Fourcroy and Vauquelin as phosphites, are either phosphates 

 or mixtures of the phosphates and phosphites. This is illus- 

 trated by the products that we obtain from the neutral com- 

 pounds of this acid, and the alkalies and alkaline earths. We 

 are not, however, to consider the acid as a simple mixture of the 

 phosphoric and the phosphorous acids ; because the proportions 

 of the oxygen and the phosphorus seem to be constant, which 

 would scarcely be the case if it were an accidental compound, or 

 one formed of two substances, which have no necessary con- 

 nexion with each other. Considering it, therefore, as a sub- 

 stance of definite properties, M. Dulong proposes to give it the 

 name of phosphatic acid. In order to ascertain its composition, 

 the same process was employed as with respect to the h ypophos- 

 phorous acid, the result of which was, that it consists of 100 

 parts of phosphorus, and 109 of oxygen ; but as these numbers 

 do not agree with the relations of oxygen to phosphoric acid, 

 we may take the one which approaches the nearest to it, that of 

 nine to ten, which would give 1 12*4 parts of oxygen in phosphatic 

 acid. It is indeed admitted that it is the first example of com- 

 binations with fixed proportions that differ so little from each 

 other ; but, as we extend our researches, we shall probably find 

 the case not a singular one. Professor Berzelius, for example, 

 does not admit the existence of an intermediate oxide of 

 iron, the proportions of which are given by M. Gay-Lussac, 

 merely because it is very near the red oxide, and does not 



