Steve Machin answers:

I don't know why aniline is in the staining solution.
As for differentiation, aniline does this very slowly, unlike acetone
or alcohol.  If the section is dry and put into aniline the dye comes
out very slowly. But if the section is damp it comes out much
quicker.
We shouldn't do this now because aniline is quite harmful, but the
trick is to do a bit of heavy breathing on the slide.  The moisture
in your breath turns the aniline milky and speeds up the
differentiation. The heavy breathing trick was taught to me by a
drunk Scotsman who will remain nameless.

--- Aidan Schurr  wrote: > Hi all,
> 
> Have an obscure request for a MacCallum - Goodpasture method for
> Gram positive and negative bacteria.  My question: what is the
> significance of the aniline called for in this method? (both in the
> staining solution and in the differentiator).
> 


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