Surreysinger: OK, stick to "ion" or "iorn" if you like, but don't be surprised if it doesn't form two distinct syllables as it seems to have done at the time the verse was written. Older pronuncations are often closer to the spelling, and far from being "foreign", they go back for centuries. It still comes very naturally to many hundreds of thousands of native speakers of English to say "i-ron", as written, and in this case it makes the song more singable.

And as for the Global Warming issue - I think Rosetti would have been perfectly aware that Bethlehem did not suffer freezing cold winters. The poem was written for an audience for whom Christmas was closely associated with the depths of winter, and she's simply linking the Christan imagery to the climate and weather that we generally experience during the festive season. Poetic licence - why not? It's a song, not a historical account.

Marje