"Like the water, the Walden ice, seen near at hand, has a green tint, but at a distance is beautifully blue, and you can easily tell it from the white ice of the river, or the merely greenish ice of some ponds, a quarter of a mile off. Sometimes one of those great cakes slips from the ice-man’s sled into the village street, and lies there for a week like a great emerald, an object of interest to all passers. I have noticed that a portion of Walden which in the state of water was green will often, when frozen, appear from the same point of view blue. So the hollows about this pond will, sometimes, in the winter, be filled with a greenish water somewhat like its own, but the next day will have frozen blue. Perhaps the blue color of water and ice is due to the light and air they contain, and the most transparent is the bluest. Ice is an interesting subject for contemplation. They told me that they had some in the ice-houses at Fresh Pond five years old which was as good as ever. Why is it that a bucket of water soon becomes putrid, but frozen remains sweet forever? It is commonly said that this is the difference between the affections and the intellect."
Henry Thoreau