368 PYROMELLITIC ACID. 



above 300, and sublimes without decomposition ; 

 heated with an excess of lime, it is resolved into car- 

 bonic acid and benzene. 



Barium trimesate. The neutral salt, (C 9 H 3 6 ) 2 Ba 3 + 

 8H 2 0, is a crystalline precipitate, almost insoluble in 

 water. The acid salt, (C 9 H 5 6 ) 2 Ba + 4H 2 0, is thrown 

 down when barium chloride is added to a solution of 

 the free acid ; fine, colorless needles, but slightly soluble 

 in hot water. 



Ethyl trimesate, C 6 H 3 (CO.O.C 2 H 5 ) 3 . Long prisms, 

 of a silky lustre, fusing at 129. 



2. Hemimellitic acid (1:2: 3). Is produced, 

 together with phtalic anhydride, by heating hydro- 

 rmellophanic acid (p. 370) with concentrated sulphuric 

 acid. Colorless needles ; rather difficultly soluble in 

 water; from its concentrated aqueous solution it is 

 precipitated by hydrochloric acid ; fuses at 185, and, 

 when heated to a higher temperature, yields phtalic 

 anhydride and benzoic acid. 



3. Trimellitic acid (1:2: 4). Is formed, together 

 with isophtalic acid and pyromellitic anhydride, by 

 heating hydropyromellitic acid with concentrated sul- 

 phuric acid. Indistinct, verrucous crystals; fusing 

 point, 216 ; moderately easily soluble in water and 

 ether. 



Barium trimellitate, (C 9 H 3 6 ) 2 Ba 3 + 3H 2 0, forms 

 difficultly soluble, verrucous crystals. 



g. Tetrdbasic Acids. 



Benzenetetracarbonic Acids. 

 C 10 H 6 8 = C 6 H 2 (CO.OH) 4 . 



1. Pyromellitic acid. Is formed by careful dis- 

 tillation of mellitic acid ; and is obtained most readily 

 by heating sodium mellitate with sulphuric acid. 



