158 BACTERIOLOGY. 



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 allow^ed to act cold for about five minutes, after which 

 it is slightly warmed; the slip is then washed off in 

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

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

 sary. 



The Method of Van Ermengem. — Another 

 method of demonstrating the presence of flagella is that 

 suggested by Van Ermengem. It is somewhat more 

 complicated than either of the preceding methods. The 

 steps in the process are as follows: 



In the centre of a perfectly cleaned cover-slip place 

 a drop of a very dilute suspension, in physiological salt- 

 solution, of a 10- to 1 8-hour old agar-agar culture of 

 the organism to be studied. The suspension of the 

 organisms in the salt-solution should be very dilute in 

 order to favor the isolation of single cells on the slip 

 and also to obviate the occurrence of excessive j)recip- 

 itation. The slips are then to be dried in the air and 

 in the gas-flame in the usual manner. 



The mordant used consists of: 



Osmic acid (2 per cent, solution) 1 part. 



Tannin (10-25 per cent, solution) 2 parts. 



To this 4 or 5 drops of glacial acetic acid may be 

 added, but experience has shown this to be hardly 

 necessary. 



Place a drop or two of this mordant on the cover-slip 

 to be stained, and allow it to act for one-half hour at 

 room temperature, or for five minutes at 50° to 60° C. 



