METABOLIC PRODUCTS 465 



{<!) Butyl and higher alcohols may ho detected )>y 

 their odour ; the solution is redistilled and the distillate 

 saturated with sodium chloride when the alcohols separate 

 as oily drops. If present in sufficient quantity they can 

 be submitted to fractional distillation and determination 

 of the boiling points. 



(e) Esters may be detected by their odour, and by 

 fractional distillation and determination of the boiling 

 points. 



2. Volatile Acids. — The residue from the distillation 

 of the volatile neutral products is made acid with sul- 

 phuric acid or, preferably, j)hosphoric acid (to prevent 

 charring) and distillation continued. The distillate is 

 tested for : 



(a) Formic acid, by reduction of ammoniacal silver 

 nitrate and the usual tests. 



(6) Acetic acid, by the odour ; by the production of 

 the red-brown colour with ferric chloride. 



(c) Propionic acid. 



(d) Butyric acid. The latter two acids may be separ- 

 ated and determined by use of Duclaux's " distillation 

 ratios " (see Beitrand and Thomas' " Manipulations de 

 Chimie biologique "). Information as to the approximate 

 composition of the mixture may be obtained by deter- 

 mining the acid equivalent by titration with sodium 

 hydroxide using phenolphthalein as indicator, or by 

 formation of the silver salts. 



3. Non-volatile Acids. — If the fermentation has been 

 carried out in the presence of calcium carbonate the non- 

 volatile acids may be precipitated from the metabolism 

 solution as the calcium salts by the addition of four 

 volumes of alcohol. 



(a) Bacterial products. The precipitate will probably 

 be calcium succinate or lactate. The salt is acidified to 

 Congo red with sulphuric acid and the solution extracted 



