144 BACTERIOLOGY. 



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

 warmed over the 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 soaked in cold water. Usually the prepara- 

 tion is firmly fixed after this treatment; a little practice 

 is necessary, however, in order not to overheat and not 

 to crack the slide. The method is applicable only to 

 cover-slip preparations, and cannot be safely used with 

 tissues. 



IjrPRESsiON Cover-slip Prjeparations. — The im- 

 pression preparations dii¥er in value from the ordinary 

 cover-slip preparations only in one respect: they pre- 

 sent 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 up by 

 oue 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, stain- 

 ing, and mounting are the same as those just given for 

 the ordinary cover-slip preparations. 



By this method constancies in the arrangement and 

 grouping of the individuals in a colony can often be 

 made out. Some will always appear irregularly massed 

 together, others will grow in parallel bundles, while 

 others, again, will be seen as long, twisted threads. 



Note. — From a colony of bacillus subtilis make a 

 cover-slip preparation in the ordinary way; now make 



