214 GENERAL MICROBIOLOGY 



3. What is the lowest per cent of formaldehyde that has 

 inhibitive action? That has preservative action? What 

 terms are applied to these different percentages in each 

 case? 



4. Make a "ring" test for formaldehyde as follows: 

 Add several drops of concentrated commercial H2SO<i to 

 each tube of plain milk, allowing it to run down the side of 

 the tube as in making an ordinary " ring " test. A violet 

 coloration at the junction of the H 2 S04 with the milk 

 demonstrates the presence of formaldehyde in the milk. 

 The presence of ferric chloride, an impurity in commercial 

 sulphuric acid, is essential to this test. 



5. Did all percentages of formaldehyde used give this 

 test? Did all percentages which preserved give the test? 



Is formaldehyde a desirable preservative for milk? 

 Why? Are any chemicals more desirable for this purpose 

 than formaldehyde? 



What are the main uses of formaldehyde? What is 

 paraformaldehyde? Its use? 



6. State your results in full and draw any conclusions 

 that follow. What practical applications may be made? 



REFERENCES 



MARSHALL: Microbiology, p. 179. 



SAVAGE: Milk and Public Health, pp. 383-391. 



HAWK: Practical Physiological Chemistry, 4th Ed., p. 239. 



EXERCISE 31. TO ILLUSTRATE SYMBIOSIS 



Apparatus. Sterile 5 c.c. pipettes; three sterile 200 c.c. 

 Erlenmeyer flasks; 450 c.c. skim milk; three tubes of litmus 

 milk (sterile). 



Cultures. Bad. lactis acidi; Oospora lactis. 



Method. 1. Place 150 c.c. milk in each flask and sterilize 

 (Tyndall method). 



2. Mark the flasks, A, B and C. Inoculate flask A 



