After the excited iodized paper is exposed in the camera--and the time of exposure cannot be taught, as that speciality is only acquired by experience, and may vary from five to thirty minutes, or even more--the invisible picture is developed and rendered visible, not by exposure to the vapour of mercury, as in Daguerre's process with silver plates, but by a mixture of one of No. 1 with four of No. 3. The development is carefully watched by looking through the negative placed before a lighted candle, and the time of development may vary from ten to thirty minutes, and all the time the picture must be kept wet with the solution, so that it is better perhaps to make a bath of the solution and lay the picture on its surface than to pour the liquid over the picture. After the development is matured, the picture is now washed in clean water, and fixed temporarily, if required, by immersion in a bath containing 200 grains of bromide of potassium in one pint of water, or permanently by the hyposulphite of soda, made by mixing one part of a saturated solution with five or ten of water, or one ounce of the salt to six or twelve of water; but, as before mentioned, it is better to keep a Winchester quart full of a saturated solution of hyposulphite of soda, and then it is always ready for use instead of employing the weights and scales, and continually weighing out portions of the salt. The picture after fixing is thoroughly washed with water, and being [Page 144] dried is now placed between the folds of a wax book--_i.e._, some leaves of blotting-paper are kept saturated with white wax, and when a picture is placed between them, and a hot iron passed over the outside sheet, the wax enters the pores of the paper, and after removing any excess of wax by passing the picture through a book of bibulous paper, over which the hot flat iron is passed, the negative picture at last is ready for use, and any number of positive copies may be taken from it, as already described in the first experiment, page 139. This mode of manipulation is called the Talbotype, and before dismissing the subject another process of iodizing the paper may be explained. To a solution of nitrate of silver of twenty, thirty, or fifty grains to the ounce of water, a sufficient number of the crystals of iodide of potassium is added, first to produce the yellow iodide of silver, and then to dissolve it, so that the yellow precipitate appears with a small quantity, and disappears with an excess of the iodide. If this solution is spread over sheets of paper, and these latter then placed in a bath of water, the iodide of silver is precipitated on the surface, and after plenty of washing to remove the excess of iodide of potassium, the paper may be dried, and will keep for any length of time without change. This paper may be excited, exposed, developed, fixed, and waxed, as already explained. _Fourth Experiment. The Wax-paper Process._