Final answer:
Mrs. Drover reflects on a profound emotional experience that has left an indelible mark on her and the house, symbolically suggested to be remembered by the physical space as a 'spiritual environment’.
Step-by-step explanation:
The memory that Mrs. Drover thinks about after she finds the letter appears to be related to a deeply emotional experience, anchored in the intense and high moments that have left a lasting impact on her life.
Despite the particulars of the memory not being explicitly mentioned in the provided text, it is clear that what Mrs. Drover recalls is connected to a profound and dramatic event—possibly a traumatic one—that has imprinted itself not just on her, but on the physical space of the house as well.
The narrative suggests that the house, symbolically, has not forgotten this intense emotion or action, and Mrs. Drover is drawn into these reflections upon encountering a reminder in the form of a letter.
The impressions and memories surrounding this event are described as being potent enough to create a 'spiritual environment' that's more real than the material reality itself, underscoring the depth and permanence of whatever incident is being remembered.