ORGANIZED FERMENTA. 65 



sugar formed by Fehling's quantitative test (Experiment 

 33), or, approximately, by the subnitrate of bismuth test 

 (Experiment 30). There has been little or no fermenta- 

 tion in the cold liquid. 



158. In the same manner cool another portion of 

 the saliva and, after ten minutes, warm to body-tempera- 

 ture and show by its decomposing starch that the ferment 

 is not destroyed by the cold. 



159. PREPARATION OF LACTIC ACID BY FERMENTATION. 

 In 150 cubic centimeters of boiling water dissolve 30 grammes of 

 cane-sugar and add about 30 milligrammes of tartaric acid. Let 

 the solution stand two days, then add 40 cubic centimeters of sour 

 milk and about half a gramme of old cheese. After the addition 

 of 15 grammes of zinc oxid allow the mixture to stand ten days at 

 a temperature of 40 to 50, with repeated stirrings. At the end 

 of that time heat to boiling, filter while hot, and allow to cool. 

 Zinc lactate will crystallize out on cooling if the solution is suffi- 

 ciently concentrated. If it is not it should be allowed to evapo- 

 rate to a smaller volume. It may be purified by recrystallizing. 

 The acid can be obtained by dissolving the lactate in water and 

 decomposing by hydrogen sulphid gas. Filter off the zinc sulphid, 

 evaporate the filtrate to a syrup, and after it is cold extract the 

 acid by dissolving it in ether. 1 When the ether has evaporated 

 the acid will remain. It may be preserved for testing in the gastric 

 juice tests. 



160. PREPARATION OF BUTYRIC ACID. Make a mixture as 

 for the preparation of lactic acid, or use a part of that mixture. 

 Allow the fermentation to go on as before, but for three or four 

 weeks. In that time bubbles of hydrogen and carbon dioxid 

 appear and, after removing the zinc, butyric acid is found in the 

 ether. It can be identified by its acid reaction and characteristic 

 odor. 



1 Remember that the vapors of ether are extremely inflam- 

 mable. 



