138 BACTERIOLOGY. 



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 bot- 

 tom of the slide with a towel soaked in cold water. 

 Usually the preparation is firmly fixed after this treat- 

 ment ; 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 used with tissues. 



IMPRESSION COVER-SLIP PREPARATIONS. The im- 

 pression preparations differ in value from the ordinary 

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

 sent an impression of the organisms as they were ar- 

 ranged 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 

 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, 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 

 an impression cover-slip of another colony of the same 

 organism. Compare the results. 



