The experiment with Iron sulphate is repeated during the night and again it can be observed, that the solution rises higher than during the day time. The pattern formed is much softer, compared with the day experiment, although different from the waves formed by Silver nitrate. The colours are just the same as those produced during the day and have the same tendency to darken slightly in time. But they never darken beyond a rich ochre yellow. Fig. 3 and Fig. 4 on Plate 2 are typical examples for such experiments with Iron sulphate.