COVER-SLIP PREPARATIONS 161 



hottest, and becoming gradually cooler toward the other end 

 of the plate, which may be of a very low temperature. By 

 dropping water upon the plate, beginning at the hottest 

 point and proceeding toward the cooler end, it is easy to 

 determine the point at which the water just boils; it is at a 

 little below this point that the cover-slips are to be placed, 

 bacteria side up, and allowed to remain about ten minutes, 

 when the fixing wUl be complete. In very particular com- 

 parative studies this plan is to be preferred to the process 

 of passing the cover-slips through a flame, as the organisms 

 are always subjected to the same degree of heat, and the 

 distortions which sometimes occur from too great and 

 irregular application of high temperatures may be elimi- 

 nated. The fixing consists in drying or coagulating the 

 gelatinous envelope surrounding the organisms, by which 

 means they are caused to adhere to the surface of the cover- 

 slip. It is sometimes desirable to fix the preparations with- 

 out the use of heat, as in the case of pus or other exudates. 

 In this event, after drying the thinly spread material care- 

 fully in the air, the cover-slip on which it is placed is im- 

 mersed in a mixture of equal parts of absolute alcohol and 

 ether for about 15 minutes. At the end of this time it may 

 be removed and stained. The advantage of this method is 

 that there is less distortion and, as a rule, less precipitation 

 (or, perhaps better, no charring) of extraneous matter. 



The majority of bacteria with which the beginner will 

 have to deal stain readily with watery solutions of any of 

 the basic aniline dyes, such, for instance, as f uchsin, methyl- 

 ene-blue, or gentian-violet. 



To stain the fixed cover-slip preparation, it is taken by 

 one of its edges between forceps, and a few drops of a watery 

 solution of either of the dyes named are placed upon the 

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