Salt the prepared Serum with Chloride of Sodium or Ammonium; in quantity about eight or ten grains to each fluid ounce, and render sensitive upon the same Bath as that recommended for the Citrate Process. A NEGATIVE PRINTING PROCESS UPON IODIDE OF SILVER. Iodide of Silver is more sensitive to the reception of the invisible image than the other compounds of that metal; and hence it is usefully employed in printing _enlarged_ Positives from small Negatives, by means of the Camera. The great stability of the proofs upon Iodide of Silver will also be a recommendation of this process when unusual permanency is required. Take of Iodide of Potassium 160 grains. Water 20 fluid ounces. The best paper to use will be either Turner's Calotype, or Whatman's or Hollingworth's Negative; the foreign papers do not succeed with the above formula (p. 258). Float the paper on the iodizing Bath until it ceases to curl up and lies flat upon the liquid: then pin up to dry in the usual way. Render sensitive upon a Bath of Aceto-Nitrate of Silver containing 30 grains of Nitrate of Silver with 30 minims of Glacial Acetic Acid to each ounce of water. When the sheet is quite dry, place it in contact with the Negative in a pressure frame, and expose _to a feeble light_. About 30 seconds will be an average time upon a dull winter's day, on which it would be impossible to print at all in the ordinary way. On removing the Negative nothing whatever is seen upon the paper, the image being strictly invisible in this process unless the exposure has been carried too far. Develope by immersion in a saturated solution of Gallic Acid, prepared in the manner described at page 261. The image appears slowly, and the process may last from 15 minutes to half an hour. If the exposure has been correctly timed, the Gallic Acid appears at length almost to cease acting; but when the proof has been over-exposed, the development goes on uninterruptedly, and the image becomes too dark, partaking more of the character of a Negative than a Positive. The usual rule, that _under_-exposed proofs develope slowly but show no half-tones, and that the _over_-exposed develope with unusual rapidity, is also observed in the process with Iodide of Silver.