The syrupy fluid was _iodized collodion_. This is made by dissolving gun-cotton in ether with alcohol, and adding some iodide of ammonium. When a thin layer of this fluid is poured on the glass plate, the ether and alcohol evaporate very speedily, and leave a closely adherent film of organic matter derived from the cotton, and containing the iodide of ammonium. We have plunged this into the bath, which contains chiefly nitrate of silver, but also some iodide of silver,--knowing that a decomposition will take place, in consequence of which the iodide of ammonium will become changed to the iodide of silver, which will now fill the pores of the collodion film. The iodide of silver is eminently sensitive to light. The use of the collodion is to furnish a delicate, homogeneous, adhesive, colorless layer in which the iodide may be deposited. Its organic nature may favor the action of light upon the iodide of silver. While we have been talking and waiting, the process just described has been going on, and we are now ready to take the glass plate out of the nitrate-of-silver bath. It is wholly changed in aspect. The film has become in appearance like a boiled white of egg, so that the glass produces rather the effect of porcelain, as we look at it. Open no door now! Let in no glimpse of day, or the charm is broken in an instant! No Sultana was ever veiled from the light of heaven as this milky tablet we hold must be. But we must carry it to the camera which stands waiting for it in the blaze of high noon. To do this we first carefully place it in this narrow case, called a _shield_, where it lies safe in utter darkness. We now carry it to the camera, and, having removed the ground glass on which the camera-picture had been brought to an exact focus, we drop the shield containing the sensitive plate into the groove the glass occupied. Then we pull out a slide, as the blanket is taken from a horse before he starts. There is nothing now but to remove the brass cap from the lens. That is giving the word Go! It is a tremulous moment for the beginner. As we lift the brass cap, we begin to count seconds,--by a watch, if we are naturally unrhythmical,--by the pulsations in our souls, if we have an intellectual pendulum and escapement. Most persons can keep tolerably even time with a second-hand while it is traversing its circle. The light is pretty good at this time, and we count only as far as thirty, when we cover the lens again with the cap. Then we replace the slide in the shield, draw this out of the camera, and carry it back into the shadowy realm where Cocytus flows in black nitrate of silver and Acheron stagnates in the pool of hyposulphite, and invisible ghosts, trooping down from the world of day, cross a Styx of dissolved sulphate of iron, and appear before the Rhadamanthus of that lurid Hades.