The returned invitation

An original fiction passage set in the nineteenth century.

The envelope came back unopened. George recognised his own careful handwriting and the small blot beside the address. He had imagined many replies, but not this silent return. He placed it beside the breakfast plate and tried to speak about the newspaper. The envelope gave no answer, yet it seemed to occupy the whole room. His sister waited until he was ready to pick it up again.

Identify the phrase which suggests that George cannot stop thinking about the envelope.