STAINING 191 



First, be sure the slide is clean and free from grease. This 

 is accomplished most readily by scouring a few minutes with 

 finely ground pumice stone and a little water, then washing 

 and drying with a grease-free cloth, handkerchief, or piece 

 of cheese-cloth. With the "loop" needle place in the middle 

 of the slide a small loop of water. This is best done by 

 filling the loop by dipping in water, then tapping it gently 

 so that all that remains is the water that just fills the loop 

 level full, and this amount is placed on the slide by touching 

 the flat side of the loop to the glass. Then the straight 

 needle is sterilized, dipped into the culture and just touched 

 once into the small drop of water on the slide. The remain- 

 der of the culture on the straight needle is then burned off 

 and the needle is used to spread the drop of water contain- 

 ing the bacteria into a thin even film, which will result, pro- 

 vided the slide is free from grease. This is dried and then 

 "fixed"' by passing three times through the Bunsen flame at 

 intervals of about one second, passing through slowly for 

 thick slides and a little more rapidly for thin ones. If the 

 culture is in a liquid medium, the use of the loop of water is 

 unnecessary; a loop of the fluid from the surface, middle or 

 bottom as the cultm-e indicates is spread out to a thin film, 

 dried and fixed. 



After the film is fixed the stain desired is dropped on, 

 allowed to act for the proper time, which will depend on the 

 stain and the preparation, washed in water, dried thor- 

 oughly and examined with the oil-immersion lens, without 

 a cover. If it is desired to preserve the preparation it may 

 then be mounted in balsam. This is not necessary, as they 

 keep just as well, provided the immersion oil is removed. 

 To do this, fold a piece of filter paper so that at least three 

 thicknesses result. Lay this on the slide and press firmly 

 several times, when the surplus oil will be taken up by the 

 paper. Slides not mounted in balsam are more apt to 

 become dusty than those that are. This is the only disad- 

 vantage. 



Gram's Method of Staining. — It has been ascertained that 

 some bacteria contain a substance, possibly a protein, which 

 forms a compound with gentian violet and iodine, which 



