156 BACTERIOLOGY. 



paper, and allowed to dry. When dry it is mounted in 

 xylol-Canada-balsam by placing a small drop of the 

 balsam upon the surface of the film, and then inverting 

 the slip upon a clean glass slide. It is sometimes de- 

 sirable to have the balsam harden quickly, and a method 

 that is commonly employed to induce this is as follows : 

 the slide, held by one of its ends between the fingers, is 

 warmed over a gas-flame until quite hot ; a drop of 

 balsam is then placed on the centre of it, and it is again 

 warmed ; the cover-slip is then placed in position, and 

 when the balsam is evenly distributed the temperature 

 is rapidly reduced by rubbing the bottom of the slide 

 with a towel wet with cold water. Usually the prepara- 

 tion 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. Impres- 

 sion preparations differ from ordinary cover-slip prep- 

 arations in only one respect : they present an impression 

 of the organisms as they were arranged in the colony 

 from which the preparation 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 organ- 

 isms 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 prep- 

 arations. 



By this method constancies in the arrangement and 

 grouping of the individuals in a colony can often be 



