This might seem like a chemistry question, but the principle behind it is mainly physics. How does keeping the thistle tube below the acid level prevent gas from seeping through it in a acid and metal reaction? In case you are wondering the thistle tube is the tube with a bulb-like opening at the top in the attachment. In the diagram, the thistle tube is above the acid level, but that's not the point, since it's just to show you what it looks like. Please explain this. Thanks.