It's not what you think it is, not a double entendre.
"But Ikey was timid, and his hopes remained insoluble in the menstruum of his backwardness and fears. "

MENSTRUUM
Webster's 1913 Dictionary

Definition: \Men"stru*um\, n.; pl. E. {Menstruums}, L. {Menstrua}.
[L. menstruus. See {Menstruous}.]
Any substance which dissolves a solid body; a solvent.

The proper menstruum to dissolve metal. --Bacon.

All liquors are called menstruums which are used as
dissolvents, or to extract the virtues of ingredients
by infusion or decoction. --Quincy.

Note: The use is supposed to have originated in some notion
of the old chemists about the influence of the moon in
the preparation of dissolvents. --Johnson.