196 CIVIC BIOLOGY 



fermentation was supposed to be a purely chemical process, 

 and accordingly brewers and vintners had 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 by 

 wild yeasts and other microorganisms, and the problem is a 

 logically simple one of weeding them out of the cultures. 

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

 isms involved, in pure cultures, and this is equally true of any 

 germ disease of plants, animals, or men. 



EXPERIMENT 1. First necessary step: Get the yeast plants single; 

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



^ 3 ^ well-r.ubbed-up suspension 



in a slender test tube or 

 straight vial, and force 

 down through this a tight, 

 hard plug of sterilized ab- 

 FIG. 96. A lifter, cut from tin, or, better, sorbent cotton. The liquid 

 from thin sheet aluminium a b ove the cotton will be 



It is sterilized by holding the end in a flame for pretty sure to contain noth- 

 an instant, giving it only time to cool before j n g- b u t single yeast plants, 

 using, a, sheet of metal indicating how the lift- FVPFRIMKKT O Str-mid 



ers are cut. (One half natural size) 



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 w T ater, 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 keep them from flow- 

 ing together and getting mixed up, and clear enough so that we can see 

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

 Starch jelly made with sweetened water (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 

 warm to the hand), and pour in a thin layer into a Petri dish (or on 

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

 Keep in a warm place away from the light, and in a day or two whitish 

 specks begin to appear, if the work has been carefully done, scattered 



