Read the excerpt from The Strange Case of Dr. Jekyll and Mr. Hyde. Six o’clock struck on the bells of the church that was so conveniently near to Mr. Utterson’s dwelling, and still he was digging at the problem. Hitherto it had touched him on the intellectual side alone; but now his imagination also was engaged, or rather enslaved; and as he lay and tossed in the gross darkness of the night and the curtained room, Mr. Enfield’s tale went by before his mind in a scroll of lighted pictures. He would be aware of the great field of lamps of a nocturnal city; then of the figure of a man walking swiftly; then of a child running from the doctor’s . . .

What is the mood of the excerpt?