NEUTRAL RED IN HISTOLOGY AND BACTERIOLOGY. 
627 
to act only just long - enough to dehydrate the tissue, then clarify 
with xylol (which does not remove the dye) and mount in balsam. 
The dye seems to have an especial affinity for the chromatin 
of the cells, staining this substance more or less intensely and 
leaving the remaining tissue colorless. Under proper conditions 
it would seem as if it might exert some special action on the 
neutrophilous tissue elements. 
In addition to its histological uses neutral red has been found 
ser\ iceable for some bacteriological purposes. As the name in¬ 
dicates the salt is of a neutral character, and a trace of it in a 
small beaker of water causes the mixture to assume a pinkish 
color. This solution has been found useful as a test and a more 
delicate one than litmus paper for determining the alkalinity of 
culture media. If the media be acid or neutral there is no marked 
change in the red when a portion of the media is added unless 
there be a slight deepening of the pink color; but if the media 
be alkaline and a few drops be added to the neutral red solution 
and slightly agitated the mixture immediately changes to a 
brown color. 
In aqueous solutions the neutral red, if treated with either 
sulphuric, hydrochloric or nitric acid, at first shows no change, 
but if a sufficient amount of either acid be added the color 
changes from red, first to purple and then to a deep blue. If 
acetic acid be added to the aqueous solution of the neutral red 
there is no change except perhaps a slight deepening of the color. 
In an alcoholic solution, the neutral red presents a light 
brown or amber color ; upon the addition of an alkali (sodium 
hydrate) this is not changed except for a general lightening of 
the color due to the dilution. If the alcoholic solution be treated 
with hydrochloric acid added in increasing quantities it changes 
first to a red and then to a light blue ; if treated with nitric acid 
the solution changes to a red and then to a dark blue ; if sul¬ 
phuric acid be employed the change is first to a red and then to 
a bottle-green color. Acetic acid changes the alcholic solution 
to a pinkish-red color. 
As a result of these crude experiments, the idea quite 
