_The Preparation of the Sensitive Paper._--The conditions which are required for producing a sharp and well defined print are--that an even layer of Chloride of Silver should exist upon the very surface of the paper, and that the particles of this Chloride should be in contact with a sufficient excess of Nitrate of Silver. These points have been already referred to at an early part of the Work (p. 19). The material used for _sizing_ the paper is of importance. English papers are usually sized with Gelatine, which is a photographic agent, and acts chemically in forming the image. Foreign papers on the other hand being sized with starch only, require an addition of Gelatine, Caseine, or Albumen, to retain the Salt at the surface of the paper, and to assist in producing the picture: if otherwise, the print will be flat and "mealy," as it is termed. Albumen especially produces a beautifully smooth surface, and is advantageously employed in printing small portraits and stereoscopic subjects. The uniform surface distribution of the Chloride of Silver is sometimes interfered with by a faulty structure of the paper, causing it to absorb liquids unevenly, and in consequence the pictures, when removed from the printing frame, appear _spotted_. Another cause producing the same effect, is the employment of too weak a solution of Nitrate of Silver, or the removal of the sheet from the Nitrate bath before the Chloride of Ammonium has been perfectly decomposed; it is thus rendered unequally sensitive at different portions of the surface, and the prints have the characteristic marbled appearance above referred to. A sufficient excess of Nitrate of Silver being essential, it is important to bear in mind, that the quantity of this salt eventually remaining in the paper, is much influenced by the manner in which the solution is applied. If it be laid on by _floating_, then the proportion of Nitrate to that of Chloride of Sodium should be about as 3 to 1 (the atomic weights are nearly as 5 to 2); but if the plan of brushing or spreading with a glass rod be adopted, 7 to 1 or 8 to 1 will not be too much. _The Darkening of the Sensitive Paper by Light._--The operator should be familiar with the changes of colour which indicate the progress of the reduction of the sensitive layer. Much in this respect depends upon the kind of organic matter used, but there is always a regular sequence of tints; in the case of a paper prepared simply with Chloride of Ammonium and Nitrate of Silver, it is as follows: pale violet, violet-blue, slate-blue, _bronze_ or copper-colour. When the _bronzed_ stage is reached, there is no further change. On immersion in the fixing bath of Hyposulphite, the violet tones due to Subchloride of Silver are destroyed, and the print assumes a red or brown colour, which is deepest and most intense in the parts where the light has acted longest.