All is clear to me.

The exclamation of Elizabeth Device, which, like my ruthless husband, I looked upon as an artifice to save the infant's life, I now find to be the truth.

Her child perished instead of mine.
How or why she exchanged the infants on that night remains to be explained, but that she did so is certain; while that she should afterwards conceal the circumstance is easily comprehended, from a natural dread of her own husband as well as of mine.

It is possible that from some cause she may still deny the truth, but I can make it her interest to speak plainly.

The main difficulty will lie in my public acknowledgment of you.