MATERIALS. 319 



Dunham's Solution. 



Peptone, i gm. 



Sodium chlorid, o . 5 gm. 



Distilled water, 100 c.c. 



Dissolve, place in test-tubes and sterilize as usual. After sterilization 

 inoculate tubes with several different pure cultures of bacteria and allow 

 to- grow for 10 days. Add i c.c. of a o.oi per cent, solution (fresh) of 

 potassium nitrite and a few drops of concentrated sulphuric acid. Heat 

 gently. If a pink color appears it indicates the formation of indol, a charac- 

 ter used to distinguish certain species of bacteria (e.g., B. coli). 



No. 21. Putrefaction. Place in a series of test-tubes with a little cold 

 water the following: a. A bit of raw meat; b. some white of egg; c. 

 some flour; d. some crushed beans; e. sugar;/, starch; g. a bit of melted 

 butter. Set in a warm place for two or three days and determine which 

 will putrefy and which will not. 



From the tube containing the meat and the egg, remove a bit of the liq- 

 uid as soon as putrefaction begins and examine under a miscrocope (both 

 stained and unstained). Examine the liquid in the other tubes in the same 

 way. Remove a little of the putrefying mass from one tube and dilute by 

 placing it in a water blank. Transfer a platinum loopful of this dilution 

 to a melted gelatin tube and another to a melted agar tube. Mix by gentle 

 agitation and pour into Petri dishes. Allow to grow for two days and ex- 

 amine the colonies. Are both liquefiers and non-liquefiers present? 



No. 22. Ammoniacal Fermentation of Urea. Fill a test-tube or a flask 

 half-full of urine and allow to stand for a day or two in a warm place. 

 Note the odor of ammonia. Suspend a bit of red litmus-paper* in the 

 mouth of the tube and note that it turns blue from the ammonia fumes. 

 Remove a bit of the liquid with a platinum loop and examine (stained) 

 under microscope. Note the immense number of bacteria. 



No. 23. Manure and Sewage. Place a small drop of sewage and a small 

 bit of manure in separate water blanks. After thorough mixing remove a 

 loopful in each case and transfer to a second water blank for further dilu- 

 tion. After again mixing transfer a loopful of this second dilution to melted 

 agar or melted gelatin, gently agitate and then pour into Petri dishes. 

 Allow to grow for two or three days and note the number of colonies, in- 

 dicating the great numbers of bacteria in the original materials. A 

 quantitative determination can be made if desired by using i c.c. of the 

 sewage or i gram of manure, and diluting 100,000 times. 



No. 24. Soil Bacteria. For general study use standard media as already 

 described, adjusting the reaction to 0.5 per cent, acid instead'of 1.5 per cent, 

 and using 1.2 per cent, agar and 12 per cent, gelatin. Plates made with 

 these media inoculated with soil will not fail to show numerous soil organisms, 

 bacteria and molds being very abundant. To obtain proper samples of 



* Filter-paper moistened with Nessler's solution (used by chemists) is better, which should 

 turn yellow to reddish-brown if ammonia fumes arise. 



