Rugs, Carpets and Carpeting - Old Carpet Glue

Just pulled up the carpet with hopes that the hardwood floor underneath was in good condition. This carpet had edge tack strips and a pad that wasn't glued down. Okay, right? Wrong. It appears that this was rug #2 in this seventy year-old home. It must be that rug #1 WAS glued down because this ugly looking grey/black shtuff had been spread around with a toothed trowel. It is a very thin layer of glue that rug #2 adequately covered up.

In years past, was this a typical thing to do...glue carpet to a hardwood floor? What kind of glue could it be?

If I'm refinishing the floors, can I just chew threw this glue with the floor sander or should I make an attempt to remove the glue first? Is there a way to remove this glue safely without damaging the floor and my lungs?

First I would make sure that the old glue does not contain asbestos. If it is black, there is a good chance. But usually a sander will take it off, although you will go thru paper big time.