De la Rue and Miiller on Terephthalic Acid and its Derivatives. 543 



crystalline state. When collected on a filter, these crystals dry in 

 paper-like masses of a silky lustre. Terephthalic acid is not per- 

 ceptibly soluble in ether, chloroform, acetic acid, water, or the other 

 usual solvents. Concentrated sulphuric acid dissolves it to a con- 

 siderable extent, especially when warm, without the formation of 

 sulpho-terephthalic acid, and the acid separates unchanged on the 

 addition of water. On heating, terephthalic acid sublimes without 

 previously fusing. The sublimate, which is indistinctly crystalline, 

 has the same composition and properties as the original acid, and 

 therefore, unlike other bibasic acids, terephthalic acid cannot be 

 converted into an anhydride by merely heating it. Terephthalic 

 acid exhibits a remarkable deportment with regard to its salts ; for 

 although bibasic, there appear to exist no double salts ; and even 

 acid salts are only prepared with the greatest difficulty. 



The alkaline terephthalates are all very soluble in water, but are 

 insoluble in alcohol. The potassium, sodium, and ammonium com- 

 pounds can be obtained in well-crystallized forms. The calcium and 

 barium salts are less soluble than the before-named, and may be ob- 

 tained in small scaly crystals. The copper salt is a pale blue 

 crystalline powder. The silver and the lead salt occur as curdy 

 precipitates when obtained by double decomposition. The com- 

 pounds of terephthalic acid with the alcohol radicals possess a par- 

 ticular interest, as they furnish the most direct proof of the bibasic 

 nature of the acid. There exist neutral and acid compounds. The 

 neutral ethers are obtained either by the action of chloride of 

 terephthalyle on the alcohols, or by means of the iodide of the alco- 

 hol radicals and terephthalate of silver or of potassium. 



The methyl-terephthalic ether, C 8 H 4 (CH 3 )' 2 0\ is the most cha- 

 racteristic compound, and consequently may be used to detect the 

 existence of terephthalic acid in the presence of other acids. It 

 forms beautiful flat prismatic crystals several inches long, which fuse 

 at a temperature above 100° (Cent.), and sublime without decom- 

 position. It is readily soluble in warm alcohol, and slightly soluble 

 in cold alcohol. 



The ethyl-terephthalic-ether forms long prismatic crystals resem- 

 bling urea, and is readily soluble in cold alcohol. 



The amyl-terephthalic-ether forms scaly crystals of pearly lustre, 

 is readily soluble in alcohol, and fuses in the temperature of the hand. 



Phenyl-terephthalic- ether, a white crystalline substance, fuses at 

 above 1 00° C. 



The acid compounds are generally formed 'in small quantities, 

 along with the neutral ethers, by the action of the iodide of the 

 alcohol radicals on terephthalate of silver. They are well-defined 

 monobasic acids, and form crystallizable substances soluble in alcohol. 



Nitro-terephthalic acid, C 8 H 5 (NO") O 1 . This acid is formed by 

 acting with a mixture of nitric and fuming sulphuric acid on tere- 

 phthalic acid. When crystallized from certain solvents, it forms well- 

 developed prismatic crystals of a faint yellow colour. From water, 

 it deposits in cauliflower-like aggregations. 



Nitro-terephthalic acid is readily soluble in warm alcohol and in 



