DUCKWALL'S METHOD. 189 



a saturated solution of tannin, and a solution of liquor 

 ferri sesquichlor. of the strength of 1 : 20 of distilled 

 water. To 3 parts of the tannin solution add 1 part of 

 the dilute iron solution. To 10 c.c. of such a mixture 

 add 1 c.c. of concentrated watery solution of fuchsin. 

 This mordant is not to be used fresh, but only after 

 standing exposed to the air for several days (better for 

 several weeks). After preparing the cover-slip with all 

 precautions necessary to cleanliness the filtered mordant 

 is allowed to act cold for about five minutes, after which 

 it is slightly warmed ; the slip is then washed in 

 water, dried, and faintly stained with carbol-fuchsin. 

 No addition of acid or alkali to the mordant is neces- 

 sary. 



DUCKWALL'S METHOD l is a modification of the Lof- 

 fler method, and the results obtained thereby are very 

 satisfactory. 



Preparation of the Staining Agents. The fixing agent 

 is mordant, and the stain is carbol-gentian-violet or, 

 preferably, carbol-fuchsin. 



The Mordant. 



Desiccated tannic acid 2 grammes. 



Cold saturated solution ferrous sulphate (aqueous) 5 " 



Distilled water 15 c.c. 



Saturated alcoholic solution of fuchsin 1 " 



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 a certain 

 amount of sodium hydroxide, a 1 per cent, solution, 

 varying from 0.5 to 1 c.c. The best grade of filter- 



1 The Canner, vol. xx. p. 23. 



