Sentence:  ... there were times when she would lie looking at her, with such a still, watchful, almost dangerous expression, that Helen would sigh, and change her place, as persons do whose breath some cunning orator had been sucking out of them with his <WSD>spongy</WSD> eloquence, so that, when he stops, they must get some air and stir about, or they feel as if they should be half-smothered and palsied. 
Meaning of the word marked with <WSD> tags:  Having the characteristics of a sponge, namely being absorbent, squishy or porous.