In cyanotypes, a class of photographs discovered by Sir John Herschel, in which cyanogen in its combinations with iron forms the ground, the pictures are Prussian blue and white. In the chrysotype of the same eminent philosopher, the image is first received on paper prepared with the ammonia-citrate of iron, and afterwards washed with a neutral solution of gold. It is fixed by water acidulated with sulphuric acid, and lastly by hydriodate of potash, from which a white and purple photograph results. It is vain to attempt to describe the various beautiful effects which Sir John Herschel obtained from chemical compounds, and from the juices of plants; the juice of the red poppy gives a positive bluish purple image, that of the ten-week stock a fine rose colour on a pale straw-coloured ground. Pictures may be made by exposure to sunshine, on all compound substances having a weak chemical affinity; but the image is often invisible, as in the Daguerreotype, till brought out by washing in some chemical preparation. Water is frequently sufficient; indeed Sir John Herschel brought out dormant photographs by breathing on them, and some substances are insensible to the action of light till moistened, as for example, gum guaiacum. Argentine papers, however, are little subject to the influence of moisture. The power of the solar rays is augmented in certain cases by placing a plate of glass in close contact over the sensitive surface. All these various experiments, though highly interesting, have now been superseded. It was found that paper did not always answer for photography, on account of imperfections in its structure; silver plates were too expensive; and glass was found to be unimpressable. Nevertheless, M. Niepcé de Victor obtained beautiful results upon glass coated with albumen mixed with sensitive substances, which suggested the medium by means of which the art has been brought to its present perfection, and that final step is due to Mr. Scott Archer. He coated a plate of glass thinly with collodion, that is, gun-cotton dissolved in ether and alcohol, which dries into a delicate transparent film of extreme adhesiveness, and of such intense sensibility that the action of light upon it is so instantaneous that it arrests a stormy sea or a fleeting cloud before they have time to change. Now landscapes in chiaroscuro are produced of great beauty, which by the slower methods were mere masses of deep shade and broad light. Architecture is even more perfectly obtained, but it fails to give a pleasing representation of the human countenance.