EXAMINATION OF SOIL 169 



4. Boil the solution. If a precipitate forms, it is 

 due to undigested albumins. 



5. Filter and add ammonium sulphate to the nitrate 

 to saturation. Albumoses will be precipitated. 



6. Allow the mixture to stand over night, then 

 filter. 



7. Dissolve the precipitate in distilled water. 



8. Add to the dissolved precipitate an excess of 

 NaOH and a few drops of copper sulphate solution. 

 This will produce the characteristic color of the biuret 

 reaction for the albumoses present. 



9. Add to the nitrate left from No. 6 an excess of 

 NaOH and a few drops of copper sulphate solution. 

 This will give the biuret reaction for peptons. 



EXERCISE 3. THE DIGESTION OF GELATIN 



1. Prepare gelatin plates with a dilution of garden 

 soil suspension. 



2. Isolate one of the liquefying colonies by trans- 

 ferring it to slant agar. Make subcultures and 

 determine the species. 



3. Sterilize 200 grams of gelatin in a 500 c.c. flask 

 and inoculate this heavily with a culture of the lique- 

 fying organism obtained in the previous experiment. 



4. Incubate at room temperature until the gelatin 

 is completely liquefied. 



5. Determine the composition in the same manner 

 as in Exercise i. 



EXERCISE 4. THE DIGESTION OF CASEIN 



1. Precipitate the casein in a quantity of milk by 

 means of dilute acetic acid. 



2. Filter. 



