Mercury does indeed vaporize at room temperature. Check out this health and safety vid: http://www.mwsi.com/new/videos/Mercury.mpg
The ultraviolet light is needed to visually see it, since the vapor is to...ummm...'thin' to see with the naked eye (don't know a better term).
I surely wouldn't do this since exposure to mercury is so incredibly damaging to the human nervous system. That said, film speed increases can be done with ammonia or peroxide since all you're trying to do is super-saturate the film emulsion with oxygen to help speed up the halide reactions. There's an old astrophotography book somewhere on my shelf that speaks of doing this when the old BW emulsions were still very slow and they needed the increase.