10 ELEMENTARY BIOLOGY. [l. 



tube, so that its outer end dips just below the surface of 

 some solution of potash 1 . All gas formed in the first tube 

 will now bubble through the baryta water in the second, 

 and, from thence, any that is not absorbed will pass out 

 through the potash into the air. An abundant precipitate 

 of barytic carbonate will be formed which can be collected 

 and tested. The fermenting fluid, therefore, evolves car- 

 bonic anhydride.] 



6. [Grow some yeast in Pasteur's solution (with sugar), in a 

 nearly closed vessel (say a bottle with a cork through which 

 a long narrow open tube passes) : as soon as the evolution of 

 gas seems to have ceased, distil the fluid in a water bath 

 and condense and collect the first fifth that comes over : 

 redistil this after saturation with potassic carbonate, and 

 test the distillate for alcohol by its odour and inflammability, 

 and by the sulphuric acid and potassic dichromate test.] 



7. [Determine that heat is evolved by a fluid in which active 

 alcoholic fermentation is going on. Place 200 cc. of fresh 

 yeast in a flask, and add 1 litre of Pasteur's fluid with 

 sugar : put another litre of the fluid alone in a similar flask, 

 cover each flask with a cloth and place the two side by side 

 in a place protected from draughts. When gas begins to be 

 actively evolved from the yeast-containing solution, take the 

 temperature of the fluid in each flask with a good thermo- 

 meter; the temperature of the one in which fermentation is 

 going on will be found the higher.] 



1 The object of the potash is to shield the baryta water from any car- 

 bonic acid that may be in the atmosphere. 



