GENERAL BACTERIOLOGY 77 



4. Spread over the cover slip by two or three sweeps 

 of the needle. The water should spread easily and not 

 run together. If the water does not spread well the 

 cover slip has not been sufficiently cleaned. 



5. Dry by moving high over the flame. 



6. Pass rapidly three times back and forth through 

 the flame. This process precipitates albuminous matter 

 and causes the bacteria to adhere firmly to the glass. 



Note. — ^The same object may be accomplished by allowing 

 absolute alcohol to evaporate from the cover slip. This method 

 has some advantages, since the bacteria do not shrink from the 

 heat. 



7. Cover with stain for lo-ij seconds. 



8. Wash in water. 



9. Blot with filter paper, dry in the air or high over 

 the flame, and mount in Canada balsam. 



. 10. Label and preserve this preparation. 

 Try to avoid the mistake, made by most beginners, 

 of taking too much growth on the needle. For hang- 

 ing-drop preparations less material should be used than 

 for stained preparations. 



EXERCISE 7 



Method of making preparations according to Gram — 



1. Prepare a film of the organism to be examined, 

 as for the ordinary stained preparation. 



2. Cover with gentian violet for i minute. 



3. Wash in water, and remove the water by means 

 of filter paper, leaving the surface moist. 



4. Cover with Gram's iodin solution for 2 minutes. 



5. Pour Gram's iodin solution off and, without 

 washing, place in a staining dish, film side up, and 

 cover with 96 per cent alcohol. 



