I shall touch very lightly on the manipulative part of photography, as that would be unnecessary; but a brief account of the chemicals in use is essential to a right appreciation of the theory of developing the image. In the first place, our object is to get a film of some suitable material coated with a thin layer of a sensitive salt of silver--say a bromo-iodide. By mixing certain proportions of ammonium iodide and cadmium bromide, or an iodide and bromide of cadmium with collodion--which is pyroxyline, a kind of gun-cotton dissolved in ether and alcohol--a plate of glass is coated, and before being perfectly dry is immersed in the nitrate of silver bath. The silver nitrate solution, adhering and entering to a slight extent the surface of the collodion, becomes converted by an ordinary chemical action of affinity into silver iodide and bromide. The ammonium and cadmium play a secondary part in the process, and are not absolutely necessary in forming the image. The plate is now extremely sensitive to light. When we have entered it into the dark slide and camera, and then exposed to light, the change I mentioned has taken place. The film is transformed into different quantities of sub-iodide and sub-bromide of silver, according to brilliancy of light. In addition, there is on the plate an amount of unchanged silver nitrate which becomes useful in the second stage, or development. The image is not seen as yet, being latent, and requiring the well-known developing solution of sulphate of iron, acetic acid, alcohol, and water. Practically we all recognize the effect of a nicely-balanced wave of developer worked round a plate. The high lights are first to appear as a darker color, till the details of shadow come out; when this is reached the developer is washed off. The chemical action is briefly thus, and it can be shown by solutions without a photographic plate, as in a test tube: Pour into this glass a solution of silver nitrate, AgNO, and add a solution of ferrous sulphate, FeSO_4. The ferrous sulphate combines with the nitric acid, forming two new salts--ferric nitrate and ferric sulphate. The silver is deposited. Any other substance which will remove oxygen from silver nitrate without combining with the silver would do the same, and metallic silver would be thrown down. The formula, as shown on the diagram, explains the interchange.