Almost anything wil at least darken the brick a little bit.
UGL drylock masonry sealer is one possible fix. It is resin in solvent (the paint thinner clean up stuff is better than the latex/water claen up).
It will have about the smallest change you can get.

The crumbly nature of your brick may be attributable to 180 years of water penetration through the porous bricks.

To stop this water penetration, you should seal the brick on the exterior of the building.

Full disclosure, I work for Applied Technologies, which makes a brick sealer for just this application. You can learn more at; http://appliedtechnologies.com/pages/content/masonry_brick_sealer.html

P. S. The color won't change either.

Here is a link that might be useful: Brick Sealer

Sorry, the link should have been:

Here is a link that might be useful: Brick Sealer

Skootch, thanks for the info, but we have a brick wall in our kitchen over the cooktop. Should we use this same sealer? It mentions sealing against water penetration, but I'm also concerned about grease and dirt. I have no idea how to keep this clean. Thanks so much
Pattie

We did that I have a brick arch with an Aga stove in it.I put a couple layers of sealer I too didnt want splashes etc ruining the brick.

I sealed the old chimney we uncovered in our kitchen with 2 coats of satin water based poly. It darkened a little but that made the brick prettier I think anyway as it enhanced the color. They are completely undusty now.