52 THE BUTYRIC ACID GROUP. 



Preparation. The methods of producing and obtaining acetic 

 acid are so well known that we need not here advert to them. 



Tests. Some light will be thrown on the importance of the 

 modes of testing for acetic acid when we have to treat of the 

 assumed or actual occurrence of acetic acid in the animal fluids. 



As in the case of most organic substances, we must first sepa- 

 rate it from most of the substances with which it is mixed, before 

 we can apply the appropriate tests. This separation is compara- 

 tively easy because the acid admits of being distilled ; hence it can 

 only be confounded with volatile acids exhibiting reactions homo- 

 logous or similar to it. It may be readily distinguished from 

 formic acid, in consequence of the property which this latter acid 

 possesses of being decomposed by oxide of mercury; hence these 

 two acids can hardly be mistaken for one another. How it is to 

 be separated and distinguished from the homologous acids, as, for 

 instance, metacetonic acid, &c. 5 will be explained when we treat of 

 these acids. 



If we have isolated acetic acid as completely as possible by 

 distillation, and then by crystallisation of one of its salts, the fol- 

 lowing reactions may be established, independently of the examination 

 of the form of the crystals ; nitrate of suboxide of mercury added to 

 a not too dilute solution of an acetate at first yields no precipitate, 

 but, after a short time, minute crystalline specks are formed which 

 slowly gravitate in the fluid like fatty glistening scales. Since the 

 acetates, in common with the meconates and sulphocyanides, yield a 

 somewhat intense red colour on the addition of a solution of a per- 

 salt of iron, acetic acid, in a mixed fluid, might be mistaken for one of 

 these acids; but acetic acid maybe readily distinguished from meconic 

 aci6$ by the solubility of the acetate of lime (the meconate of lime 

 being insoluble in water), and from sulphocyanic acid by the circum- 

 stance that the red solution of sulphocyanide of iron, on the addition 

 of ferricyanide of potassium, and on being warmed, very soon preci- 

 pitates Prussian blue, which is not the case with . any other persalt 

 of iron. 



Physiological Relations. 



Occurrence. We learn from pure chemistry that acetic acid is 

 formed in various processes of decomposition of vegetable sub- 

 stancesin their fermentation as well as in their dry distillation : 

 we shall, however, presently see that it often occurs as a product 

 of distillation of several nitrogenous animal substances. It was 

 formerly believed that it much more frequently existed pre-formed 



