It would quickly turn me mad.

I promise, oh, I swear, that I never will allow it to come to me again." Poor Dorothy's time of madness was not far distant nor was the evil that was to follow in its wake.
John in writing to Dorothy concerning his journey to Scotland had unhesitatingly intrusted to her keeping his honor, and, unwittingly, his life.

It did not once occur to him that she could, under any conditions, betray him.

I trusted her as John did until I saw her vivid flash of burning jealousy.

But by the light of that flash I saw that should the girl, with or without reason, become convinced that Mary Stuart was her rival, she would quickly make Derbyshire the warmest locality in Christendom, and John's life might pay the cost of her folly.