HAL.—I think probably on vegetable matter, and, perhaps, partially, on two elementary principles, iodine and brome, which it certainly contains, though these are possibly the results of decayed marine vegetables. These give a yellow tint, when dissolved in minute portions in water, and this, mixed with the blue of pure water, would occasion sea green. I made, many years ago, being on the _Mer de Glace_, an experiment on this subject. I threw a small quantity of iodine, a substance then recently discovered, into one of those deep blue basins of water, which are so frequent on that glacier, and, diffusing it as it dissolved with a stick, I saw the water change first to sea green in colour, then to grass green, and lastly to yellowish green: I do not, however, give this as a proof, but only as a fact favourable to my conjecture. POIET.—It appears to me to confirm your view of the subject, that snow and ice, which are merely pure crystallized water, are always blue, when seen by transmitted light. I have often admired the deep azure in crevices in masses of snow in severe winters, and the same colour in the glaciers of Switzerland, particularly at the arch where the Arve issues, in the Valley of Chamouni. We thank you for your illustration. HAL.—In return, I ask you for some further remarks on this grand waterfall. You said just now, you preferred the fall of the Velino for picturesque effect to any other waterfall you have seen; yet it is a small river compared even with the Traun, and nothing compared with the Gotha, the Rhine, or, above all, the Glommen. POIET.—Size is merely comparative: I prefer the fall of the Velino, because its parts are in harmony. It displays all the force and power of the element, in its rapid and precipitous descent, and you feel, that even man would be nothing in its waves, and would be dashed to pieces by its force. The whole scene is embraced at once by the eye, and the effect is almost as sublime as that of the Glommen, where the river is at least one hundred times as large; for the Glommen falls, as it were, from a whole valley upon a mountain of granite, and unless where you see the giant pines of Norway, fifty or sixty feet in height, carried down by it and swimming in its whirlpools like straws, you have no idea of its magnitude and power: yet still, I think, considering it in all its relations, this is the most awful fall of water I have seen, as that of Velino is the most perfect and beautiful. I am not sure, that I ought not to place the fall of the Gotha above that of the Rhine, both for variety of effect and beauty; and the river, in my opinion, is quite as large, and the colour of the water quite as beautiful.