METAPHOSPHATE OF SODA. 477 



which is less soluble than the original phosphate, and crystal- 

 lizes in prismatic crystals, which are permanent in air, and con- 

 tain ten atoms of water. Its solution is essentially alkaline. 

 This salt is precipitated white, by nitrate of silver. It is to be 

 remarked that insoluble pyrophosphates, including pyrophos- 

 phate of silver, are soluble to a considerable degree in the solu- 

 tion of pyrophosphate of soda. The pyrophosphates of potash 

 and of ammonia can exist in solution, but pass into tribasic salts 

 when they crystallize. 



A bipyrophosphate of soda (HO, NaO, PO 5 ) exists, obtained 

 by the application of a graduated heat to the biphosphate of 

 soda, but it does not crystallize. Its solution has an acid re- 

 action. 



Metaphosphate of soda ; NaO, PQ 5 ; 1283.2 or 102.82. The 

 three phosphates last described, all contain but one equivalent 

 of fixed base, and afford the metaphosphate of soda, when 

 heated to redness ; microcosmic salt being readily procured, may 

 be recommended for that purpose. The metaphosphate of soda 

 fuses at a heat, which does not exceed low redness, and on cool- 

 ing forms a transparent glass, which is deliquescent in damp air, 

 and very soluble in water, but insoluble in alcohol ; its solution 

 has a feeble acid reaction, which can be negatived by the addi- 

 tion of 4 per cent of carbonate of soda. When evaporated, this 

 solution does not give crystals, but dries into a transparent 

 pellicle, like gum, which retains at the temperature of the air 

 somewhat more than a single equivalent of water. Added to 

 neutral, and not very dilute solutions of earthy and metallic 

 salts, metaphosphate of soda throws down insoluble hydrated 

 metaphosphates, of which the physical condition is remarkable. 

 They are all soft solids, or semifluid bodies ; the metaphosphate 

 of lime having the degree of fluidity of Venice turpentine. An 

 account has already been given of the singular change, at a par- 

 ticular temperature, of hydrated metaphosphate of soda into bi- 

 pyrophosphate of soda, occasioned by an atom of water becom- 

 ing basic to the acid, which before was constitutional to the salt 

 (page 353). 



The bipyrophosphate of soda undergoes several changes, 

 under the influence of heat before it becomes metaphosphate. 

 At a temperature of 500, the salt becomes nearly anhydrous, 

 and affords a solution which is neutral to test paper, but in 

 other respects resembles the bipyrophosphate. But at tempe- 



