io ELEMENTARY BIOLOGY. [CHAP. i. 



5. [Take two test tubes; in one place some yeast, with 

 Pasteur's solution containing sugar; in the other place 

 baryta water, and then connect the two test tubes by 

 tightly fitting perforated corks and a bent tube passing 

 from above the surface of the fluid in the first tube to 

 the bottom of the baryta water in the second ; pass a 

 narrow bent tube, open at both ends, through the cork 

 of the baryta water 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 carbonic 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 thermometer ; 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 

 carbonic anhydride that may be in the atmosphere. 



