THE ORDINARY STAINING SOLUTIONS. ]29 



monly employed to induce this is as follows : 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 preparation 

 is firmly fixed after this treatment ; but a little practice 

 is necessary 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. 



The Ordinary Staining Solutions. — The solu- 

 tions commonly employed in staining cover- slip prepa- 

 rations are, as has been stated, watery solutions of the 



