RESPIRATION AND FERMENTATION 



137 



inquiry into the other products which may be formed during 

 the process. Their determination offers great experimental 

 difficulties, and hence the student must ascertain this point from 

 his accessible literature. The results of his study will no doubt 

 recall to the student another very conspicuous case in which, 

 without free oxygen, carbon dioxide and alcohol are formed by 

 actively working tissues, namely, in yeast fermentation. This 

 process, partly because of its connection with our present sub- 

 ject and partly because of its immense economic importance, 

 should be thoroughly understood by the student. As a basis 

 for such understanding he should personally observe for himself: 

 What -are the exact products of yeast fermentation? 



This may be determined by inducing fermentation under conditions 

 such that the products may be collected and submitted to analysis. 



EXPERIMENT. In an ERLENMEYER, or conical, flask of about 200 

 cc. capacity, place about 100 cc. of a fermentable solution, and add 

 thereto a half cake of com- 

 pressed yeast (such as FLEISCH- 

 MANN'S, weighing about 12 

 grams) ; close the flask with a 

 tight rubber stopper through 

 which passes a thermometer 

 and a glass tube bent as in 

 the accompanying figure (Fig. 

 36) ; carry the free end of the 

 latter into a dish of water 

 over which is suspended an 

 inverted and water-filled test- 

 tube. Allow the gas from the 

 flask to escape until all the 

 air originally present must 

 have been expelled, then allow 

 the gas to rise into the test- 

 tube until full, after which 

 apply the test for carbon dioxide *- 

 (insertion of a lump of caustic FlG - 36. -ARRANGEMENT FOR THE STUDY OF 

 potash). Some hours later, FERMENTATION; X*. 



after all gas evolution has ceased, examine the liquid in the flask, 

 noting its smell, etc. Then proceed to distill it at a temperature 

 below boiling. Examine the distillate and apply to it a test for alcohol. 



PRECAUTIONS. It is best to have the flask of a capacity at least double 

 that of the liquid used, as otherwise the frothing during fermentation will 



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