I'm sure back in the days they put them there for a reason. You don't usually find them in the floor cavity, in the 20's they used them in wall cavities, most likely you have bricks inside your exterior walls also, this type of construction was called "Brick Noggin". I doubt they will fall out, they haven't moved in 100 years... If you worried provide additional bracing.

I gently touched them and they began crumbling and there is a slight buckle and slope, so I am not convinced, but I'm glad to hear it may not be an issue! It's going to be an exposed ceiling in an income property. I'm worried that if someone jumps or jostles them just right, they may fall?

If they lose and the ceiling will be exposed, you don't want to take any chances, have your contractor brace them or investigate further and see if they can be removed. Keep in mind this will be a messy job getting them removed.

Good luck

I have concrete trenchers out and they know what it is! They said it is like a flue so that smoke from the 1920 wood fed boiler would bypass the main chimney. He said it was sturdy and no danger and that he's seen them before. I think I'll talk to someone about some cool industrial supports for the "just in case" scenario, but it's a relief to know why it is there. Thanks for the advice!

Brace and support or remove. (Unless your trenchers happen to be structural engineers.)