STAINING AND EXAMINATION OF MICRO-ORGANISMS 51 



of a looped needle to another cover-glass, and spread it 

 as evenly and thinly as possible over the surface. Care 

 must be taken that practically none of the culture 

 material is inti-oduced along with the organisms, as this 

 spoils the preparation, rendering it indistinct and dirty. 

 When the cover-glass vp'ith its thin film of material 

 has become perfectly air-dry,^ but not before, it should 

 be taken up with a pair of forceps at one corner and 

 heated in a spirit flame or bunsen-burner. This heating 

 of the preparations is a matter of great importance, for 

 unless the material is treated in this manner it will not 

 adhere to the cover-glass, and when subsequently rinsed 

 with water it will be washed away. The degree of 

 heating is also a matter requiring great care, for should 

 the preparation be over-heated the bacteria will not 

 take up the stain satisfactorily, whilst the albuminous 

 matters in insufficiently heated specimens often give rise 

 to precipitations on adding the stain. Experience, how- 

 ever, has shown that, as a general rule, by passing the 

 cover-glass without stopping three times in succession 

 vertically through the flame of a bunsen-burner, the 

 requisite degree of heating is secured. Of course in 

 this, as in all other manipulations, only j)Tactice will 

 enable the student to attain the judgment necessary for 

 carrying out successfully such details. When the pre- 

 paration is thus fixed it is held with the forceps in the 

 left hand, whilst with the right a few drops of stain are 

 poured on to the cover-glass by means of a narrow 

 glass pipette ; the stain must be evenly distributed, and 

 entirely coat the surface of the cover-glass. After from 

 one to ten minutes, according to the strength of the 

 stain and the nature of the organism, the cover-glass 



"■ This air-drying may be accelerated by means of a desiccator, or by a 

 rapid current of air directed on to the cover-glass with a spray-producer, 

 or by very gentle heat, but the latter requires the greatest caution. 



