Volume disappears behind transmission

I am trying to make a glass cube with a foggy volume inside it. I have two cubes, one hollow transparent one (The cube) and one inner one (The fog).

The fog on its own looks fine, but it disappears when the transparent cube is placed over it.

This is what it looks like with the transparent cube.

This is what it looks like without (Sorry for multiple posts, New users can only embed one image)