6oO LABOKATORY EXERCISES 



Directions for Making Plant Infusion. 



1. Add looo c.c. of boiling distilled water to 50 grams dry weight of the sub- 

 stance of the equivalent, chopped or ground fine. 



2. Macerate in a closed vessel for thirty minutes. 



3. Strain through cheese cloth or filter as for other media and pass distilled water 

 through the filter to make 1000 c.c. If a clear medium is desired the white of an 

 egg may be added: 



Directions for Making a Plant Decoction. 



1. Add 1000 c.c. of cold distilled water to 50 grams dry weight of the substance, 

 or the equivalent, chopped or ground fine. 



2. Heat in a cooker over a gas burner and boil for fifteen minutes, stirring suffi- 

 ciently to prevent burning. 



3. Filter as for infusion and clear, if desirable. Decoctions are preferable to 

 infusions since there will be a somewhat more complete extraction of the nutrients. 



Laboratory Study.— In the use of the culture fluids observe the rapidity, density 

 and persistency of the growth. Record the formation of acids, alkalis, odors, gas 

 bubbles, stains, etc. 



LESSON 8 



Milk. — Nearly all bacteria grow in milk. Ordinary cow's milk is used. The 

 cream is separated off and the skim milk used. Ordinary milk as sold is contami- 

 nated with fecal bacteria, those found in cow's dung and around stables. Conse- 

 quently the milk before it is used must be thoroughly sterilized. It may be used 

 in this form, or a tincture of blue litmus is added until a pale blue color is obtained. 

 Different organisms react differently with this milk; some render the litmus more 

 deeply blue, others are indifferent, some give an acid reaction. 



The milk should not be acid to taste and should not contain formaldehyd, or 

 other antiseptic substance which milk dealers sometimes add to milk to improve its 

 keeping qualities. It should be steamed in wire-crates fifteen minutes at ioo°C. 

 on each of four consecutive days (loo-c.c. portions in test-tubes) and should not be 

 used until at least a week after the last steaming. Such milk should be kept under 

 observation at least si.x or eight weeks. 



Litmus milk is prepared from fresh milk which has been passed through a separa- 

 tor (centrifuge) or from milk which has stood eighteen or twenty hours at 2o°C. and 

 has had the cream removed by skimming. To each 100 c.c. of this milk is added 

 2 c.c. of a saturated solution of high-grade lime-free blue litmus (litmus i gram, water 

 15 c.c). This gives a lavender color of just the right degree, which reddens distinctly 

 under the action of acids and blues with the development of alkalis. After adding 

 the litmus water, the milk should be pipetted in lo-c.c. portions into cotton-plugged 

 test-tubes and heated as directed above. This is a very useful medium. 



Litmus Whey (After Eyre). 



1. Curdle fresh milk by adding rennet (or by acidifying with hydrochloric acid). 



2. Filter off the whey into a sterile flask. 



3. Heat in the Arnold sterilizer for one hour. 



4. Filter into a sterile flask. 



