38 BACTERIOLOGY 



evaporate to a small volume, and test for formic and acetic acids 

 as follows : 



To a small portion of the test add a few drops of ferric 

 chloride solution, and then, drop by drop, dilute ammonia 

 water, nearly to saturation. 



A red color shows the presence of acetic or formic acid. 

 Mercurous nitrate throws down formic and acetic acids as a 

 white precipitate. With silver nitrate acetic acid salts are 

 thrown down in the cold as a. white precipitate, while the corre- 

 sponding formic acid salts are precipitated only in concentrated 

 solution, and upon boiling. A solution of formic acid salt 

 heated with mercuric chloride gives a precipitate of mercurous 

 chloride and a metallic mirror after a long time, while acetic 

 acid salts give no precipitate with mercuric chloride. 



To a small portion of the distillate containing butyric acid, 

 add a small quantity of alcohol and several drops of strong 

 sulphuric acid. If butyric acid be present, the fragrant odor of 

 pineapple is evolved on heating. 



The residue in the retort after the distillation of the volatile 

 acids may contain lactic, oxalic, succinic, or glycocholic acid. 



Evaporate the residue to a syrupy consistency, and extract 

 with ether by agitation in a separatory funnel. The ether 

 dissolves the fixed organic acids. Distil off the ether until the 

 residue has a syrupy consistency ; add water, and boil with an 

 excess of oxide of zinc. Filter. The filtrate contains zinc lactate 

 in solution, the residue zinc oxalate and succinate. Evaporate 

 the filtrate to dryness. The residue is nearly pure zinc lactate. 



Nencki showed that different bacteria produce different 

 isomeric forms of lactic acid, whose zinc salts behave differently 

 in the polariscope. One, the inactive acid, has no effect upon the 

 polarized ray ; another, the solution of whose zinc salt rotates the 

 polarized ray to the left, is termed right-handed acid ; and a third 

 whose zinc salt rotates the ray to the right is termed left-handed acid. 



Thus, when lactic acid is present, it is important to determine 



