152 PATHOGENIC BACTERIA. 



stained with alum carmin or any stain not requiring 

 acid for differentiation, after which the sections are 

 dehydrated in absolute alcohol, cleared in xylol, and 

 mounted in Canada balsam. 



As will be mentioned hereafter, certain of the bacteria 

 which occur in tissue do not allow of the ready penetra- 

 tion of the color. For such forms a more intense stain 

 must be employed. One of the best of these stains, 

 which can be employed by the given method both for 

 cover-glasses and tissues, is Loftier' s alkaline methylene 

 blue : 



Saturated alcoholic solution of methylene blue, 30 ; 



1 : 10,000 aqueous solution of caustic potash, 100. 



Some bacteria, as the typhoid-fever bacillus, decolorize 

 so rapidly as to contraindicate the use of acid for the dif- 

 ferentiation, washing in water or alcohol being sufficient. 



Gram's Method of Staining Bacteria in Tissue. — 

 Gram was the fortunate discoverer of a method of stain- 

 ing bacteria in such a maimer as to saturate them with 

 an insoluble color. It will be seen at a glance what a 

 marked improvement this is on the method given above, 

 for now the stained tissue can be washed thoroughly in 

 either water or alcohol until its cells are colorless, with- 

 out fear that the bacteria will be decolorized. Its prose- 

 cution is as follows : The section is stained from five to 

 ten minutes in a solution of a basic anilin dye, pure 

 anilin (anilin oil) and water. This solution, first devised 

 by Ehrlich, is known as Ehrlich's solution. The ordinary 

 method of preparing it is to mix the following : 



Pure anilin, 4 ; 



Saturated alcoholic solution of gentian violet, 11 ; 

 Water, 100. 



Instead of gentian violet, methyl violet, Victoria blue, or 

 any pararosanilin dye will answer. The rosanilin dyes, 

 such as fuchsin, methylen blue, vesuvin, etc., will not 

 react with iodin. The mixture does not keep well — in 



