i. _Relative proportions of Ether and Alcohol in Collodion._--It was shown at p. 84 that the addition of Alcohol to Collodion lessens the contractility of the film, and renders it soft and gelatinous. This condition is favourable to the formation of the invisible image in the Camera, the play of affinities being promoted by the loose manner in which the particles of Iodide are held together. It is therefore usual to add to Collodion as much Alcohol as it will bear without becoming glutinous, or leaving the glass; the exact quantity required varying with the strength of the spirit or its freedom from dilution with water. k. _Decomposition in the Collodion._--Collodion iodized with the metallic Iodides generally, excepting the Iodide of Cadmium, becomes brown and loses sensitiveness in the course of a few days or weeks. If the free Iodine, the cause of the brown colour, be removed, the greater part, but not the whole, of the sensitiveness is regained. The experiments of the Author, and of others, have proved that a solution of Pyroxyline in contact with an unstable iodide, slowly undergoes decomposition, the result of which is that Iodine is set free, and an equivalent quantity of the base remains in union with certain organic elements of the Collodion. Decomposition also gradually ensues when iodized Collodion is placed in contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid, Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain a constant sensibility for any length of time. Even plain Collodion uniodized cannot be preserved many months without a small but perceptible amount of change. l. _Decomposition in the Nitrate Bath._--A Collodion Nitrate Bath which has been much used, often gives a less sensitive film than when newly made. It is known also that many organic substances which reduce Nitrate of Silver, if added to the Bath, produce a state which is favourable to sensitiveness whilst the decomposition is taking place, but is eventually unfavourable; hence the solution will be injured by adding either Gallic or Pyrogallic Acid, and by organic matters generally if exposed to light. _Recapitulation._--The conditions most favourable to extreme sensitiveness of the Iodide of Silver on Collodion may be condensed as follows:--perfect neutrality of the solutions employed; a soft, gelatinous state of the film; absence of Chlorides and other salts which precipitate Nitrate of Silver; an undecomposed Collodion, containing no organic matter of that kind which is precipitated by basic Acetate of Lead, and combines with oxides of Silver. THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE. The general theory of the development of a latent image by means of a reducing agent, having been simply explained in the third Chapter, may now be more fully examined in its application to the Iodide of Silver on Collodion.