True mercury glass has double walls. The liquid silvering solution is then inserted between the layer and sealed.
We often see vintage glass Christmas ornaments and garlands described as "mercury glass" but this is a misnomer, in most cases.
True mercury glass ornaments are much heavier than the usual glass ornaments and hard to find. It is difficult to tell real mercury glass from the usual glass just by looking at a photo.
If an item is described as mercury glass, contact the seller to make sure that it is, indeed, true mercury glass.
The ornament below is NOT mercury glass. Beautiful, but not mercury glass.