82 THE BENZOIC ACID GROUP. 



have been obtained from benzine, we may mention nitrobenzide, 

 C 12 H 5 NO 45 a yellow fluid with a sweetish taste and a cinnamon- 

 like odour, which is not decomposed by the alkalies. If an 

 alcoholic solution of this nitrobenzide be treated with hydrated 

 potash and then distilled, there is produced a non-oxygenous, 

 nitrogenous body, azobenzide, C 12 H 4 N, forming large, red, fusible, 

 and volatile crystals, which neither corresponds with the nitriles 

 nor possesses basic properties like the organic, non-oxygenous 

 bases. 



Preparation. Benzoic acid is found in many of the resins or 

 balsams, but occurs in the largest quantity in the resin known as 

 gum-benzoin, from which it is ordinarily prepared either by subli- 

 mation, or, in the moist way, by dissolving the resin in spirit of 

 wine, adding an aqueous solution of carbonate of soda, and then 

 precipitating the benzoic acid by the addition of hydrochloric acid 

 to the filtered and concentrated fluid. 



Tests. Benzoic acid is less to be distinguished from other 

 substances by its volatility, than by its property of separating in 

 crystalline scales from very concentrated aqueous solutions on 

 the addition of an acid. But in carrying on investigations 

 in relation to benzoic acid we must be especially careful 

 respecting the evaporation of the fluid, since it volatilises very 

 readily with the steam ; we may easily perceive delicate crystals 

 on the paper covering of the evaporating basin, when acid 

 fluids of this nature have been evaporated without due 

 care ; it is therefore better not to add an acid to the fluid till 

 after evaporation, or if it be already acid, to render it alka- 

 line previously to evaporating it. I have found the following 

 method applicable to the discovery of small quantities of benzoic 

 acid in the animal fluids : the alcoholic extract of the fluid in 

 question (for the alkaline benzoates and benzoate of lime are 

 soluble in alcohol) must be mixed with a little acetic, hydrochloric, 

 or lactic acid ; if distinct crystals of benzoic acid do not now 

 separate, the mass must be extracted with ether, and the ethereal 

 solution submitted to spontaneous evaporation ; from this ethereal 

 extract, which is usually of an oily fluid character, the benzoic acid 

 separates in a crystalline form on the addition of water. When 

 too much fat is present, we must treat the separated mass with 

 dilute spirit, which dissolves the benzoic acid without acting on 

 the fat ; on the evaporation of this spirituous solution, we obtain 

 the benzoic acid in a tolerably pure crystalline state, mixed with 

 other free but fluid acids. Under the microscope it appears in 



