DUCKWALL'S METHOD. 171 



The tannic acid is dissolved in the water first by the 

 application of gentle heat, then the ferrous sulphate, 

 and then the alcoholic solution of fuchsin are added. 

 To these ingredients it is advisable to add from 0.5 to 

 1.0 c.c. of a 1 per cent, sodium hydroxide solution. 

 The best grade of filter-paper is used for filtering the 

 mordant, and there should be left a heavy precipitate. 

 After filtering, the color of this mordant should be of 

 a reddish-brown hue, not clear, but somewhat cloudy, 

 and this mordant must be used within five hours after 

 it is made. After that time it loses its fixing power. 

 This is indicated by its gradual clarification and 

 darkened color. It gives the best results when strictly 

 fresh, and accomplishes its work in a much shorter time, 

 so that very little if any heating is required when it is 

 placed on the cover-glass preparation. 



The Stain. To prepare the dye for this method take 

 about 1 gramme of granulated fuchsin (not the acid 

 fuchsin), put it in a bottle, and pour over it about 

 25 c.c. of warm, absolute alcohol. Shake vigorously and 

 let it stand for several hours before using. The carbol- 

 fuchsin is made by diluting the saturated alcoholic 

 solution four or five times with a 5 per cent, solution of 

 carobolic acid. Carbol-fuchsin should be freshly made, 

 heated, and filtered before using. 



The application of this method of demonstrating the 

 flagella varies with different organisms with regard to 

 the length of time the mordant and stain are allowed to 

 act, and the amount of sodium hydroxide solution used. 

 Usually, it is well to heat the mordant on the cover-slip 

 to steaming, and allow it to act from one-half to one 

 minute. It is then washed off with water and a small 



