138 PRACTICAL BACTERIOLOGY. 



necessary in fixing these preparations. If they are 

 passed three times through a Same, the rate being 

 determined by the pendulum of a regulator, the pre- 

 parations as a rule are a success. It must however be 

 borne in mind that the flagella are of much more 

 delicate construction than the cells, and if the above 

 method does not succeed, the cover-glass may be 

 taken between the forefinger and thumb, and passed 

 quickly through the flame, until it just begins to be 

 unpleasantly warm. If they are too much heated, 

 flagella invariably lose all their power of absorbing 

 stain. 



After having been fixed, the preparations must be 

 treated with the mordant, which is prepared in the 

 following manner : 5 ccm. of a cold saturated solution 

 of ferrous sulphate (green vitriol, sulphate of iron, 

 . FeSO^) are poured during constant stirring into 10 

 ccm.' of tannic acid solution (20 grm. tannic acid, 

 80 grm. water). To this mixture 1 ccm. of a con- 

 centrated aqueous solution of fuchsine is added; au 

 alcoholic solution may be used, but it is not so good. 

 The mordant, however, can only be used in this form 

 for a very few species. In some cases it must be aci- 

 dified, in others rendered alkaline, and often this must 

 be done very exactly. Each bacterium requires for 

 the staining of its flagella a specially prepared mor- 

 dant, the acidity or alkalinity of which can only vary 

 to a very limited degree. We prepare by titration 



