172 BACTERIOLOGY. 



is to be removed from the surface of the slip with 

 blotting-paper, and the slip loosened, or rather floated, 

 from the slide by allowing water to flow around its 

 edges. It is then taken up with forceps, carefully de- 

 prived of the water adhering to it by means of blotting- 

 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 



