STUDY OF MICROORGANISMS 43 



cavity ground in a thicker piece of glass, the slide. The 

 evaporation of the drop in the unsealed space causes cur- 

 rents, the effect of which makes it difficult to determine 

 whether the movement of the contained bacteria is a really 

 independent one, or is due to the currents. By sealing the 

 edges of the cover-glass to the slide with vaseline, evapora- 

 tion is prevented. 



All finely divided matter suspended in a liquid shows a 

 purely physical movement known as the Brownian move- 

 ment. Each particle has a trembling or vibratory motion, 

 but no progressive movement of the particles from place to 

 place is noted. The bacteria are sufficiently small to show 

 this molecular motion. Those forms of bacteria that are 

 provided with flagella have the power of independent pro- 

 -ive motion, which can be readily demonstrated under 

 the microscope. 



Staining. The live bacteria are difficult to demonstrate 

 in any material, except in a liquid in which they are the 

 only bodies in suspension. In such materials as milk, blood, 

 and other body fluids, the recognition of bacteria by the 

 microscope is impossible, unless the material can be so 

 treated as to differentiate the organisms from the other 

 materials present. This is done by staining the organisms 

 with solutions of certain dyes. The material to be exam- 

 ined is spread in a thin film on a slide or cover-glass, and 

 allowed to dry. The film is then treated in a way that will 

 cause it to adhere firmly to the glass when it is subsequently 

 moistened. This process is termed ''fixing" the prepara- 

 tion, and is accomplished by gently heating the dried film 

 in a gas flame to such a temperature as to cause the protein 

 to coagulate, or by treatment with some chemical that will 

 produce the same effect. The film can now be flooded with 

 a dye and the material will not be washed from the slide. 



The stains most commonly used are the aniline dyes, 



