Sentence:  "It speedily became apparent that the entire strangeness of our circumstances and surroundings—great loss of weight, <WSD>attenuated</WSD> but highly oxygenated air, consequent exaggeration of the results of muscular effort, rapid development of weird plants from obscure spores, lurid sky—was exciting my companion unduly." 
Meaning of the word marked with <WSD> tags:  To rarefy.