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Why, in the photochemical substitution of a ligand in a carbonyl complex, is a water-cooled lamp jacket required to separate the solution and the mercury vapour lamp? It's shown in Figure 22.16 of Atkins' Inorganic Chemistry.
The main point is to stop the heat flow from the lamp, while allowing UV through. You want to do photochemistry, not thermochemistry.
Required, but never shown