Respiration, Digestion, and Fermentation. 101 



chemical combination with either the original substance or 

 any of its derivatives. Fermentation may also serve an im- 

 portant purpose in digestion, in which its chief purpose is to 

 reduce the foods to soluble and diffusible form. 



104. Exhalation of carbon dioxide by germinating 

 peas. Fill a glass cylinder, of a capacity of about a liter, 

 one-third full of peas which 



have lain in water at a proper 

 temperature for a day. Cover 

 tightly with a glass plate or 

 wooden top sealed with vase- 

 line. Twelve or fourteen hours 

 later provide a section of can- 

 dle 2 or 3 centimeters long 

 with a holder of bent wire. 

 Carefully slide the cover to one 

 side without creating currents 

 of air, and lower the burning 

 candle into the jar. Note result. 

 Repeat two or three times. 

 Make the test also with a jar 

 containing the same quantity of 

 dry peas. 



Make a fresh solution of lime 

 or baryta water. Pour some 

 into clean dishes. Set one inside 

 the cylinder containing the grow- 

 ing seeds, and another in one with dry seeds. Cover 

 carefully as before. Examine a few hours later. The lib- 

 eration of carbon dioxide will be denoted by the formation 

 of a film of carbonate of calcium or barium in the liquid. 

 The series of tests given above may be repeated with a quan- 

 tity of flowers of clover or sunflower. 



105. Estimation of the amount of carbon dioxide 



Fig- 75. Method of testing gas 

 liberated by germinating peas. 

 After Sachs. 



