g. _Decomposition of Pyroxyline a source of Injury to Collodion Photographs._--Collodion Positives and Negatives are usually esteemed permanent; but some have been exhibited which, having been put away in a damp place, gradually became pale and indistinct. The change commences at rough edges and isolated points, leaving the centre, as a rule, the last affected. On examination, numerous cracks are often visible, thus seeming to indicate that the Collodion film has undergone decomposition. The result of this would be the liberation of corrosive Oxides of Nitrogen, which destroy the image. Substitution compounds containing Peroxide of Nitrogen are known to be liable to spontaneous change. The bitter resin produced by acting upon white sugar with Nitro-Sulphuric Acid, if not kept perfectly dry, will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept; the solution of the resin has then a strong acid reaction, and rapidly fades an ordinary Positive Print. These facts are interesting, and indicate that Collodion Pictures, containing in themselves the elements of their destruction, should be protected from moisture by a coating of varnish. _Comparative Permanence of Photographic Prints._--There is every reason to think that the Photographic Image, however formed, is permanent, if certain injurious conditions are avoided;--in other words, that prints do not necessarily fade, in the same manner as fugitive colours, by a simple exposure to light and air. But supposing a case, which is the common one, of injurious influences which cannot altogether be removed, it may be useful to inquire what mode of printing gives the greatest amount of stability. Positives produced by a short exposure to light and subsequent development with Gallic Acid, may be expected to be more permanent than ordinary sun-prints; not that there is any reason to suppose the chemical composition of a developed image to be peculiar, but that the use of the Gallic Acid enables us to increase the intensity of the red picture first formed, and to add to its stability by precipitating fresh Silver upon it. This point has not always been attended to. It has been recommended to remove the print from the developing solution whilst in the _red_ and early stage of development, and to produce the dark tones subsequently by means of gold; but this plan, although giving very good results as regards colour and gradation of tone, appears to lessen the advantage which would otherwise accrue from the adoption of a Negative process, and to leave the picture, as regards permanency, much in the condition of an ordinary print obtained by direct action of light. The original Talbotype process, in which the latent image is formed upon Iodide of Silver, produces, next to Collodion, the most stable image; but the difficulty of obtaining bright and warm tints on Iodide of Silver, will stand in the way of its adoption.