176 BACTERIOLOGY 



by rubbing the bottom of the slide with a towel wet with 

 cold water. Usually the preparation is firmly fixed after 

 this treatment; a little practice is necessary, however, in 

 order not to overheat and crack the slide. The method is 

 applicable only to cover-slip preparations, and cannot be 

 safely used with tissues. 



Impression Cover-slip Preparations. Impression prepara- 

 tions differ from ordinary cover-slip preparations in only 

 one respect: they present an impression of the organisms 

 as they were arranged in the colony from which the prep- 

 aration is made. They are made by gently covering the 

 colony with a thin, clean cover-slip, lightly pressing upon it, 

 and, without moving the slip laterally, lifting it by one of 

 its edges. The organisms adhere to the slip in the same 

 relation to one another that they had in the colony. The 

 subsequent steps of drying, fixing, staining, and mounting 

 are the same as those just given for ordinary cover-slip 

 preparations. 



By this method constancies in the arrangement and group- 

 ing of the individuals in a colony can often be made out. 

 Some will always appear irregularly massed, others show 

 growth in parallel bundles, while others, again, will be seen 

 as lorjg, twisted threads. 



NOTE. From a colony of bacillus subtilis make a cover- 

 slip preparation in the ordinary way; now make an impres- 

 sion cover-slip preparation of another colony of the same 

 organism. Compare the results. 



ORDINARY STAINING SOLUTIONS. 



The solutions commonly employed in staining cover-slip 

 preparations are, as has been stated, watery solutions of 



