The bodies which produce Tetrathionate when added to a solution of Hyposulphite of Soda, and hence are inadmissible in the toning process, are as follows:--Free Iodine, Perchloride of Iron, Chloride of Copper, Acids of all kinds (in the latter case the acid first produces Sulphurous Acid, and the Sulphurous Acid, if present in any quantity, by reacting upon Hyposulphite of Soda, forms Tetrathionate and Trithionate of Soda). Chloride of Gold also produces a mixed Tetrathionate of Gold and Soda when added to the fixing Bath (p. 133); but as the quantity of Chloride used is small, the prints are far less sulphuretted than in the case of toning Baths prepared by Tetrathionate without Gold. SECTION III. _The Author's Researches in Photographic Printing._ Having been long engaged in conducting experiments upon the composition and properties of the reduced material forming the Photographic image, and especially with a view of determining the exact conditions under which the picture may be considered permanent, the Author has thought it advisable to give the results of these researches in the form of an abstract of the original papers read at the meetings of the Photographic Society. A previous perusal of these papers will put the reader in possession of the principal facts upon which are founded the precautions advised in the next Section for the preservation of Photographic prints. In order to keep the Work as nearly as possible within its original limits, and also for the purpose of distinguishing the present Section from the others, as one referring principally to scientific details, the type has been reduced to the size of that used in the Appendix. ON THE CHEMICAL COMPOSITION OF THE PHOTOGRAPHIC IMAGE. The determination of the chemical nature of the Photographic image in its various forms is a point of much importance, both as indicating the conditions required for the preservation of works of art of that class, and also as a guide to the experimenter in selecting bodies likely to have an effect as chemical agents in Photography. It has been stated by some who have given attention to the subject, that the image is formed in all cases of pure metallic Silver, and that any observable variations in its colour and properties, are due to a difference in the molecular arrangement of the particles. But this hypothesis, although involving much that is correct, yet does not contain the whole truth, for it is evident that the chemical properties of the Photographic image often bear no resemblance to those of a metal. One Photograph may also differ essentially from another, so that we are led to infer the existence of two varieties, the first of which is less of a metallic nature than the second.