VMI (Victor Mercury Intensifier) uses Mecuric chloride.
This is one of the most poisonous substances around so you
are not likely to find anyone putting the stuff commercially
any more.
  Mercury intensifier produces a great increase in density.
The image is not permanent. There are safer intensifiers
that produce great intensification. Kodak states that In-6,
Quinone-Thiosulfate Intensifier, produces the greatest
increase in density of all formulas. This also has some
hazardous ingredients in it but less so than the Mercuric
cloride of VMI.
  I will post the formula for a couple of intensifiers if
you desire. Write directly to me as well as posting. You
will get a spam blocker message from Earthlink/Netcom but I
will get the message nonetheless.