PRESENCE OF MICROORGANISMS IN AIR, ETC. 147 



2. Sterilize it in the flame. 



3. Moisten the needle with sterile water. 



4. Rub it along the floor, and then, 



5. Draw it lightly across the surface of the agar in the 

 fourth Petri dish. 



III. Desk. 1. Sterilize the needle and repeat operation 

 (II, 5) obtaining the inoculum from the surface of a desk 

 which has not just previously been washed with 1 : 1,000 

 mercuric chloride. 



2. Then wash the surface of the desk well with this 

 solution and when the desk top is dry, repeat the operation, 

 using the sixth plate. 



3. Mark all plates with the date on which they were 

 exposed or inoculated and place them at a constant tem- 

 perature. 



4. Watch any developments from day to day. What 

 organisms predominate on the plates? Why? 



5. Examine different colonies in a hanging drop. What 

 types of bacteria are found? 



6. Upon what does the species and number of micro- 

 organisms depend? What becomes of them when air cur- 

 rents are present? When the floor is swept in the ordinary 

 way? Mopped? When the desk is washed with water? 

 With mercuric chloride? 



7. Are these types deleterious to health? Why should 

 and how may they be avoided in the laboratory? Out- 

 side of the laboratory? 



8. State your results for I, II and III in detail, draw any 

 conclusion possible and point out any practical operations. 



REFERENCES 



MARSHALL: Microbiology, pp. 185-191. 



BESSON: Practical Bacteriology, Microbiology and Serum Therapy, 



pp. 862-863. 



CONN: Bacteria, Yeasts and Molds, pp. 114-123. 

 CONN: Practical Dairy Bacteriology, pp. 65-67. 

 TYNDALL: Floating Matter of the Air. 



