58 MICRO-ORGANISMS IN WATER 



which is heated for about ten seconds over a small 

 flame until it begins to steam ; the mordant is then 

 shaken off, and the cover-glass inclined and washed in a 

 stream of distilled water, care being taken that the pre- 

 paration is not detached. The mordant is then run on 

 again, heated, and the cover-glass washed as before, 

 which process should be repeated altogether three or four 

 times. The under-surface of the cover-glass as well as the 

 points of the forceps must be carefully dried after each 

 washing, otherwise on subsequently applying the mor- 

 dant it will extend on to the under- side of the glass 

 as well as on to the forceps. The staining is very 

 simple, and may be effected by applying Ziehl's fuchsine 

 solution to the surface of the preparation, and heating 

 it once or twice for a quarter of a minute, or by using 

 even the ordinary aqueous solutions of violet. After 

 the stain has been washed off in water the preparation 

 is ready, and may be examined in the usual way under 

 the microscope. Thus the addition of an acid or alkali 

 is omitted, these authors stating that it is not only 

 tedious, but does not serve any useful purpose, equally 

 good results being obtained by applying the mordant 

 three or four times, instead of only once as recom- 

 mended by Lijffler. 



Drop-cultures of Micro-organisms 



The most convenient mode of studying micro-organ- 

 isms in the living state under the microscope, e.g. in order 

 to ascertain whether they are possessed of motility or 

 not, is in what is known as drop-culture. An excavated 

 glass slide is, after careful cleaning, sterilised by heating 

 in a bunsen-flame ; a little vaseline is then applied round 

 the cup, and it is placed under a glass bell jar. It is best 

 to hold the slide with the excavated side downwards when 

 sterilising it in the flame, and to keep it in this position 



