Final answer:
Stevenson's mood at the end of the chapter can be characterized as anxious and absorbed.
Step-by-step explanation:
In the given excerpt from the chapter, Stevenson's mood can be characterized as anxious and absorbed. The narrator describes Stevenson as being pale as death and hung over the furnace. Additionally, the passage mentions that Stevenson's expression is different from his previously sanguine and joyous mien. These details suggest that Stevenson is in a state of intense concentration and concern.