>I asked the druggist if his product is the dihydrate or not, and the druggist
>did not know.  He said that the label mentions two molecules of water in the
>crystals, but did not say 'dihydrate', just oxalic acid.

If the the crystal have two molecules of water then they are dihydrate.

>To me, a dihydrate should be an acid molecule with two molecules of water.
But,
>I should also have imagined that the dihydrate would be liquid.  Maybe not.

The water is locked up in the crystal structure.  So for examples the old
"bluestone" crystals, or copper sulphate pentahydrate is CuSO4.5H2O
Heating them drives off the water, leaving a powder called anhydrous copper
sulphate.

>To those who use have used oxalic acid dihydrate:  what exactly is the
dihydrate

I haven't used oxalic acid (still don't have varroa here in PEI) but I am
sure the chemistry is the same.  The crystal structure contains the water so
it is not liquid.  It does affect the molecular weight for mixing, so
proportions in a recipe should indicate whether the dihydrate or anhydrous
forms are to be used.

Regards, Stan