THE BENZOIC ACID GROUP. 79 



Hypotheses of the most varied kinds, chiefly grounded on the 

 products of decomposition, have heen set up regarding the rational 

 constitution of these acids. These hypotheses are, however, for 

 the most part limited to the constitution of benzoic acid, and as 

 but few of them are applicable to the other members of this group, 

 we may regard this as an evidence of their untenability. This is 

 partially the case with the hypothesis of Fehling, who, previously 

 to Kolbe, regarded benzoic acid as a conjugated oxalic acid, whose 

 adjunct was phenyl, C 12 H 5 . Hitherto, however, the evidence in 

 favour of any one of these hypotheses has not been sufficiently pre- 

 ponderating to warrant its unconditional acceptance. 



All these bodies present an analogy in their relations of com- 

 bination and decomposition. Thus each of these acids presents a 

 series of lower stages of oxidation not dissimilar to the aldehydes 

 of the first group, and containing 1 atom of hydrogen more and 1 

 atom of oxygen less than the corresponding acid in the anhydrous 

 state. These lower oxides are sometimes acid, sometimes basic, 

 sometimes indifferent volatile oils, some of which occur pre-formed 

 in the vegetable kingdom. 



Volatile oil of bitter almonds C 14 H 6 2 corresponds with benzoic acid C 14 H 5 O 3 



Salicylous acid C 14 H 6 O 4 salicylic acid C 14 H 5 O 5 



Hydride of cinnamyl .... C 18 H 8 O 2 cinnamic acid C 18 H 7 O 3 



Cumarin .... .... .... C 18 H 8 O 4 cumaric acid C 18 H ? O 6 



Cumin C 20 H 12 O 2 cumic acid ^so^ii^s 



In all these combinations 1 equivalent of hydrogen may be 

 replaced by 1 equivalent of chlorine, bromine, iodine, or sulphur. 



From the chlorine-combinations of this class, we can obtain the 

 corresponding amides by the action of ammonia ; thus, for instance, 

 in the case of benzamide, the action is shown by the equation, 

 C 14 H 5 2 C1 + HgN^ HC1 + H 2 N.C 14 H 5 2 . 



On submitting to dry distillation, the ammonia-salts of the 

 acids containing 3 atoms of oxygen we obtain the corresponding 

 nitriles, which, like the nitriles of the first group of acids, are 

 volatile, inflammable fluids. They are likewise decomposed both 

 by strong acids and alkalies into ammonia and the corresponding 

 acid, and when heated with potassium they yield cyanide of 

 potassium and carbo-hydrogens. 



The hydrates of the acids containing 3 atoms r of oxygen, 

 when heated with caustic alkalies, lime, or baryta, yield to them 

 2 atoms of carbonic acid, and become converted into non- 

 oxygenous oils : 



