Sentence:  ... he threw down his book, stretched his legs towards the embers in the grate, and clasped his hands at the back of his head, in that agreeable <WSD>afterglow</WSD> of excitement when thought lapses from examination of a specific object into a suffusive sense of its connections with all the rest of our existence ... 
Meaning of the word marked with <WSD> tags:  The mildly euphoric feeling experienced after a pleasurable experience, especially after an orgasm or drug-induced high.