21 



LABORATORY WORK. Practice staining cover-glass 

 preparations made from the different potato cultures, em- 

 ploying the five aniline dyes mentioned above. Also 

 stain preparations made from the white matter on the 

 teeth, near the edge of the gums, and look for comma 

 bacilli, spirilla, and leptothrix threads. 



To make permanent stained preparations, the speci- 

 men which has proven satisfactory on preliminary exam- 

 ination in water as above, can be floated off the slide by 

 first bringing a drop or two of water near the edge of the 

 cover-glass. If any oil is on the upper side it should be 

 carefully removed by rotating the cover-glass on a piece 

 of filter-paper. The specimen is then allowed to dry in 

 the air, or by gently waving over aflame. A clean glass 

 slide is then selected and a suitable drop of Canada bal- 

 sam placed in the center. The dry cover-glass is then 

 inverted, specimen side down, and carefully lowered until 

 it touches the balsam. If necessary, gentle pressure is 

 applied so as to cause the balsam to spread out under the 

 cover-glass. 



The following synopsis will be of service : 



Simple Stain. Cover-glass preparation. 

 Air- dried. 

 3 x through flame. 

 Dilute stain (^ 1 min.). 

 Water (and examine). 

 Air-dried. 

 Canada balsam. 



GELATIN PLATE CULTURE. 



Tha- object of this method, as with the dilution potato 

 culture already made, is to isolate the several kinds of 

 bacteria that may be present. The isolated organisms 

 developing in a solid, transparent medium, form colonies 

 which are easily perceived and from which transplan- 

 tations can be readily made. Pure cultures of the differ- 

 ent kinds of bacteria 'are thus obtained. 



