In floating paper one corner is first laid down, and care taken not to enclose any air bubbles, which would prevent the fluid wetting the paper, whilst the remainder of the paper is slowly laid upon the surface of the fluid. The albumen paper is excited by laying it for five minutes on a solution of nitrate of silver, seventy-two grains to the ounce of water, [Page 142] and when dry it will keep for three days. This copying paper is used in the same manner as the last, and fresh eggs only must be used in its preparation, because stale ones soon cause the copy to change and blacken all over from the liberation of sulphur, which unites with the silver. The colour of the copy is sometimes improved by a solution of hot potash, and by dipping the well-washed picture, after the use of the hyposulphite of soda, in a very dilute solution of hydrosulphuret of ammonia. _Third Experiment._ In the Daguerreotype process, a silver plate, after being thoroughly cleaned and polished, is exposed to the vapour of iodine, and is thus rendered so sensitive that it may be at once exposed in the camera. In the Talbotype process, the same principle is apparent, and paper is prepared by first covering its surface with iodide of silver, which is afterwards rendered sensitive to the action of light by means of an excess of nitrate of silver, as follows:-- One side of a sheet of selected Cansan's paper is first covered (by means of a spreader) with a solution of nitrate of silver (thirty grains to the ounce of water), hung up in a dark room and dried; it is then immersed in a solution of iodide of potassium of five hundred grains to a pint of distilled water, for five or ten minutes, and immediately changes to a yellow colour in consequence of the precipitation of the yellow iodide of silver; it is then well washed with plenty of water, and being dried, may be kept for any length of time, and is called "iodized paper." Light has no action whatever upon it. To render the paper sensitive, three solutions are prepared in separate bottles, and marked 1, 2, 3. No. 1, contains a solution of nitrate of silver, fifty grains to the ounce of water. No. 2, glacial acetic acid. No. 3, a saturated solution of gallic acid.