What kind of reaction the dry ice causes depends on what else is in the cauldron. I think it's just to lend more spookiness to Barn's treatments. As for the chair, the treatments seem to consist of Barnabas sitting in the chair that is not visibly connected to the noisy chemical apparatus that Julia runs. I'm surprised they couldn't spring for a piece of rubber tubing that, say, might be wrapped around Barn's arm, with the other end leading back to some test tube or something.
Julia had to ditch her medallion because too many people knew about it. It's very convenient for her that Vicki was interested in remodeling the West Wing. It made a good excuse for Julia to substitute a piece of chandelier.
The battle between Julia and Barnabas continues to escalate. I am reminded of Julia's comment about the fine line between love and hate--it's certainly true in her case right now.
According to my notes, Barn's hand is Jonathan's own, spectacularly made up.