STAINING OF FLA CELL A. 139 



with, standard oxalic acid solution an exact 1 per 

 cent, solution of caustic soda, and also a solution of 

 sulphuric acid in water of such a strength, that one ccm. 

 of the latter exactly neutralizes one com. of the former. 

 In order to preserve the caustic soda solution for a 

 considerable tiii;ie, it is best to fill several small bottles 

 up to the brim with it, to close them with well-fitting 

 india-rubber stoppers, and to fasten them down with 

 paraffin. In this manner the solution may be kept in 

 perfectly good condition for a considerable time. 



In order to stain the flagella of the bacillus subtilis, 

 twenty-eight to thirty drops of the 1 per cent, sodium 

 solution must be added to 16 ccm. of the mordant* 

 After these have been thoroughly mixed, a few drops 

 are placed upon the cover-glass, until it is covered 

 right to the edge with a single arched drop. If the 

 cover-glass is not completely covered, ridges of dried 

 mordant are formed, which — as very frequently they 

 cannot be removed — are liable to spoil the prepara- 

 tion. The cover- glass is then held high up over a 

 lowered Bunsen-burner flame, and is moved backwards 

 and forwards until the liquid just begins to steam. 

 On no account should it be allowed to boil, for if 

 bubbles once form, the preparation as a rule is spoiled, 

 a fine precipitate of mordant being formed, which 

 becdmes stained later on, and forms a rose-coloured 

 background, from which the bacilli can hardly be dis- 

 tinguished. The cover-glass should only be warmed; 



