382 RECENT PROGRESS IN PHYSICS. 



formerly called the neutral, while the first was designated a« the hasic, 

 and the last as the acid salt. 



In No. 2, HO, and in No. 3, 2H0 are essential constituents of the 

 salt, HO takes tlie place of one equivalent of base, and thus in all 

 three cases one equivalent of acid is combined with three equivalents 

 of base. 



If a solution of acetate of lead is added to that of No. 2, a precipi- 

 tate of phosphate of lead is produced, which also contains three equiva- 

 lents of base to one of acid, and therefore consists of 3 PCO, PO5. 



A solution of nitrate of silver precipitates from a solution of No. 2 

 the tri-basic phosphate of silver 3 AgO, PO^, which is distinguished 

 by its yellow color. 



This modification of phosphoric acid, which is characterized by 

 always saturating three equivalents of bases, is therefore called tribasic 

 phosphoric acid, and is also designated as c phosphoric acid. If three 

 equivalents of bases are replaced by three equivalents of water, this 

 combination gives the c hydrate, the ter-hydrate, or the tri-basic hy- 

 drate of phosphoric acid. 



When the common phosphate of soda 2 N^OHO, PO5 is ignited its 

 nature is entirely changed. After being re- dissolved it gives crystals of 

 an entirely different salt, which was formerly denominated pyro-phos- 

 phate of soda. By ignition, the equivalent of basic water has been 

 expelled, and thereby a salt obtained which has only two equivalents 

 of soda for one equivalent of PO5, or is 2 N„0, PO5. 



On adding acetate of lead bi-basic phosphate of lead is precipita.ted 

 2 PCO, PO5, vv'hich, when decomposed by sulphuretted hydrogen, gives 

 the solution of the bi-basic or h phosphoric aci^. 



Graham considers this dilute acid as the solution of the hydrate 

 2 HO, PO5, which he denominates deuto-hydrate of phosphoric acid. 



When this acid is saturated with bases two equivalents of the base 

 always combine with one eq^^iivalent of acid. The h phosphate of soda 

 gives by the addition of nitrate of silver a white precipitate of h phos- 

 phate of silver, while the tri-basic silver salt has, as already men- 

 tioned, a yellow color. 



By igniting the salt, formerly known by the name of acid phosphate 

 of soda, N^0.2H0, PO5, the two equivalents of water are expelled, 

 and the mono-basic phosphate or a phosphate of soda remains N^O, 

 PO5. By adding acetate of lead or nitrate of silver the corresponding 

 lead and silver salts are obtained PCO, PO5, and A^O, POr;. These 

 salts are distinguished by their gelatinous character. 



The hydrate of the mono-basic, or a phosphoric acid, or the proto- 

 hydrate of phosphoric acid is also distinguished from the other hydrates 

 of the same acid by producing a white precipitate in a solution of 

 albumen. 



The a and h hydrates of phosphoric acid are converted into common 

 phosphoric acid by heating their solutions. 



We will now proceed to Daniell's and Miller's experiments on the 

 electrolysis of the phosphates. A strong solution of tri-basic phos- 

 phate of soda and water 2 N,^ OHO, PO^ was poured into the negative 

 side of the double cell, and a dilute solution of soda into the other. 

 On passing the current from a constant battery of 20 cups, oxygen was 



