Does the inter-floor insulation in my 1911 home contain asbestos?
Insulation between floors is often for sound isolation, but hard to know for sure with the original owners/builders presumably long since dead.

Asbestos is both easy and annoyingly expensive to know for sure - collect a sample and send it to a lab, if you can't just leave it alone, undisturbed. You can't tell just by looking at it, unless you have the right sort of fancy microscope and training in its use. It could also be in your plaster, sadly. Might not be in either place, but without testing, you can't know.