A weather picture

A piece of white blotting paper is immersed in a solution having two parts cobalt chloride to one part common salt. While wet the paper will stay pink, but when dried in the sun or near a Bunsen burner it turns blue. This is the basis of the weather pictures sold in the shops. A home made one works just as well. A picture having sky or water can be cut from a book and an inset of this prepared blotting paper made to change say, the sky. The picture should then be mounted on a card and hung near a window where it will rapidly respond to changes in the hygrometric state of the atmosphere