PREHNITIC ACID. 369 



Crystallizes from water with two molecules of water 

 of crystallization, in colorless prisms; but slightly 

 soluble in cold water, easily in hot water and in alco- 

 hol ; fuses at 264, and when distilled is converted into 

 the anhydride, C 10 H 2 6 , which forms large crystals and 

 fuses at 286. 



Barium pyromellitate, C 10 H 2 8 Ba 2 , and Calcium 

 pyromellitate, C 10 H 2 8 Ca 2 , are white precipitates, 

 insoluble in water. 



Ethyl pyromellitate, C 6 H 2 (CO.O.C 2 H 5 ) 4 . Short, 

 flat needles, insoluble in water; fusing point, 53. 



Hydropyromellitic acid, C 10 H 10 8 . Is slowly 

 formed by the action of sodium-amalgam on an aqueous 

 solution of ammonium pyromellitate. Colorless syrup, 

 gradually congealing in crystalline form ; very easily 

 soluble in water ; when heated alone, it is converted 

 into the anhydride of tetrahydrophtalic acid (p. 364) ; 

 when heated with concentrated sulphuric acid, it 

 yields carbonic anhydride, pyromellitic anhydride, tri- 

 mellitic and isophtalic acids. 



2. Prehnitic acid. Is formed, together with car- 

 bonic anhydride, trimesic and mellophanic acids, by 

 heating hydro- and isohydromellitic acids (p. 371) with 

 concentrated sulphuric acid. Large prisms united in 

 groups ; contain two molecules of water of crystalliza- 

 tion ; easily soluble in water ; fuses at 237-250, the 

 anhydride being formed at the same time. 



Hydroprehnitic acid, C 10 H 10 8 . Is obtained like 

 hydropyromellitic acid. Syrupy. When heated with 

 sulphuric acid, it yields prehnitic and isophtalic acids 

 and carbonic anhydride. 



3. Mellophanic acid. Is formed together with 

 the preceding acid. Small, indistinct crystals, united 

 in crusts, without water of crystallization; fuses at 

 215-238, giving rise to the formation of the anhy- 



