12 MICRO-ORGANISMS AND DISEASE [CHAr. 



sufficiently long to remove this undesirable staining, then 

 washed in water, dried and mounted. In some instances, 

 washing with alcohol removes also the dye from the 

 bacteria, but as a rule it is better to first over-stain the 

 cover-glass specimen, then wash well in alcohol so as to 

 remove the dye from all except the bacteria, but do not 

 wash with alcohol too long, then rinse in distilled water, dry 

 and mount. 



A method extensively used and yielding the best speci- 

 mens is the one known as the method of making impression 

 specimens — Klatschpmparate of the Germans. This method 

 aims at representing in stained films the impression of bac- 

 teria in the actual position on a solid culture medium. Be 

 the bacteria growing in a streak or in isolated colonies on 

 the surface of gelatine or agar in a plate cultivation (see 

 below), by pressing a clean cover-glass on to the surface of 

 the growth an impression is obtained of the growth, the 

 cover-glass is heated, ' and stained and treated as before. 

 When it is successful the bacteria are seen in the exact 

 position which they occupied in the culture, be that in a 

 streak or in separate colonies ; the manner in which they 

 arrange themselves and the manner in which the growth 

 proceeds at the margin is well shown. Care must be taken 

 to make impression specimens of young growths, for if late 

 the impression is too thick ; but even in such cases the 

 second or third impression of the same colony gives the 

 desired result. 



In the case of liquefying bacteria impression preparations 

 must be made from gelatine growths at an early stage 

 before liquefaction commences {vide Fig. 44) of a young 

 colony of anthrax bacilli on gelatine. 



The most useful dyes in the examination of animal tissues 

 for bacteria are those aniline dyes that are soluble in water ; 



