To do the experimenet, soak small pieces of filter paper in 5% solution of cobalt chloride or cobalt nitrate and dry them over a flame. The cobalt paper is deep blue when dried, but in contact with moisture they turn pink. Place two dried cobalt papers, one on the upper and the other on the lower surface of a thick, healthy leaf. Cover them completely with glass slides or with a Jeaf-clasp and clamp them properly to the leaf. Then quickly seal the sides with vaseline to prevent atmospheric moisture from coming in contact with the papers.