That hollow might indeed be a pocket into which a gas flowed; just as in the mines the deadly coal damp collects in pits, flows like a stream along the passages.

It might be that--some odorless, colorless gas of unknown qualities; and yet-- "Did you try respirators ?" asked Dick.
"Surely," said Ventnor.

"First off the go.

But they weren't of any use.
The gas, if it is gas, seems to operate as well through the skin as through the nose and mouth.

We just couldn't make it--and that's all there is to it.