•i6 MICRO-ORGANISMS AND DISEASE [chap. 



Behrens, and by Kanthack and Drysdale, will find all they 

 require not only with reference how to prepare the dyes and 

 how to apply them, but particularly in what cases and for 

 what tissues they were first employed and found most 

 useful. Without wishing in the slightest degree to convey 

 that those engaged in pathological work should not avail 

 themselves to the full of every method that is recommended 

 and that has been found useful, I venture to say here that 

 the methods of staining, mentioned in this book, which 

 after many years' experience have been successfully em- 

 ployed in my laboratory, have been found quite sufificient in 

 bacteriological work. 



2. One of the most useful methods for staining bacteria 

 in sections of hardened tissues or in films is Gram's method. 

 Film specimens or sections are kept for five to ten minutes 

 in absolute alcohol, are then placed in any of the above 

 mixtures of aniline water and dye (fuchsin, magenta, Hum- 

 boldt's red or gentian-violet, methyl-blue or methyl-violet), 

 and kept there for from two to five minutes or more ; they 

 are then washed in alcohol for from one to three minutes, 

 and are then transferred into the following solution : one 

 part of iodine, two parts of iodide of potash, 300 parts of 

 distilled water ; they are kept here till their colour com- 

 pletely changes (as a rule into dark purple), they are then 

 transferred into alcohol till all colour has apparently gone. 

 If successful, such sections when examined under the micro- 

 scope, show only the bacteria stained, while the tissue- 

 elements are quite colourless. To bring out these latter 

 more strikingly the sections are stained in a contrast dye, 

 vesuvin or Bismarck-brown, if red, violet, or blue has been 

 used as the first dye. 



This method is of great diagnostic value, inasmuch as it 

 represents an important distinction between species which 

 otherwise may be difficult to distinguish ; one kind becom- 



