196 



CIVIC BIOLOGY 



fermentation was supposed to be a purely chemical process, 

 and accordingly brewers and vintners bad employed chemists 

 to try to relieve them from the great losses caused by diseases 

 of wine and beer. We now know that these were caused l)y 

 wild veasts and other microorganisms, and the problem is a 

 looficallv simide one of weedin;^ them out of the cultures. 

 The first requisite is to isolate and study the different organ- 

 isms involved, m jjure cultures, and this is equally true of any 

 germ disease of plants, animals, or men. 



Experiment 1. First necessary stej) : Get the yeast plants single; 

 that is, make a uniform suspension in water. To do this make a dilute, 



wel 1-rubbed-up suspension 

 in a slender test tube or 

 straight vial, and force 

 down through this a tight, 

 hard pUig of sterilized ab- 

 sorbent cotton. The liquid 

 above the cotton W'ill be 

 jM'etty sure to contain noth- 

 ing but single yeast plants. 

 Experiment 2. Second 

 step : Get the single plants 

 far enough apart so that we can work with them ; that is, dilute the 

 suspension. Add a drop to, say, one quart of boiled, filtered water, and 

 shake thoroughly. (If too many plants are still present, we may have to 

 repeat the dilution.) 



Experiment .3. Third step : Plant a drop or a few drops (according 

 to the dilution) in some medium solid enough to ke'ep them from flow- 

 ing together and getting mixed up, and clear enough so that w^e can see 

 them after each one has grown sufficiently to form a visible colony. 

 Starch jelly made with sweetened w^ater (or potato water filtered) 

 makes a good medium for yeasts and molds. Stir the drop of sus- 

 pension thoroughly into a tablespoonful of the jelly as soon as it is 

 cool enough not to injure the yeast (when it feels neither cool nor 

 w^arm to the hand), and pour in a thin layer into a Petri dish (or ou 

 a clean piece of glass which can be covered securely from the dust). 

 Keep in a warm place aw^ay from the light, and in a day or two whitish 

 specks l;)egin to api)ear, if the w^ork has been carefully done, scattered 



Fig. 96. A lifter, cut from tin, or, better, 

 from thin sheet aluminium 



It is sterilized by hol<ling the end in a flame for 

 an instant, giving it onlj' time to cool before 

 using, a, sheet of metal indicating how the lift- 

 ers are cut. (One half natural size) 



