YEAST 35 



takes place in the water ? Is this result at all hke 

 that obtained in the first experiment ? Explain. 



3. Take two test-tubes, prepare the first with baryta 



water as in the first experiment, fill the second 

 test-tube about half full of yeast which is ac- 

 tively giving off bubbles of gas, connect the 

 two tubes as before so that the gas will bubble 

 up through the baryta water. If the gas stops 

 forming, add a little sugar to the yeast. Does 

 the baryta water change as in the first experi- 

 ment ? As in the second ? Is, then, the gas 

 which is given off by a liquid in which yeast is 

 growing the same as the gas formed by the ac- 

 tion of an acid upon bicarbonate of soda, and 

 also the same as that exhaled from the human 

 lungs ? 



4. Put a half - teacupf ul of actively growing yeast 



into a loosely corked bottle, and set the bottle 

 in a warm place for an hour or so. Is the cork 

 blown out ? Why ? Prepare another bottle in 

 a similar manner, but tie the cork down with 

 a cord or wire. Do bubbles of gas still form 

 in the fluid? What do you conclude, then, 

 regarding the energy with which the gas is 

 formed in the fermenting liquid, in spite of the 

 pressure on its surface due to the accumulation 

 of gas in the upper part of the bottle? The 

 pressure of the accumulated gas may be meas- 

 ured directly by means of a manometer. This 

 consists of a U-shaped tube partially filled with 

 mercury. First mark the level of the mercury 

 in the two branches; then, by means of a rub- 

 ber tube, connect one branch with a glass tube 

 run through the stopper of the bottle of yeast. 



