It was like Monte Cristo--it was like-- 'I say,' cried Anthea, suddenly, 'come out! There's always bad air in places that have been shut up.

It makes your torches go out, and then you die.

It's called fire-damp, I believe.

Come out, I tell you.' The urgency of her tone actually brought the boys out--and then every one took up its jacket and fanned the dark arch with it, so as to make the air fresh inside.

When Anthea thought the air inside 'must be freshened by now,' Cyril led the way into the arch.
The girls followed, and Robert came last, because Jane refused to tail the procession lest 'something' should come in after her, and catch at her from behind.