When the developer is poured over the plate it attacks first the free silver nitrate, and causes it to deposit extremely fine particles of metallic silver. The question arises: How is it these particles arrange themselves to form an image? This is explained by the physical movement known as molecular attraction or affinity. These particles are attracted first to the portions of the plate where there is most sub-iodide and sub-bromide. In the shady parts less silver is deposited. When the image is once started it follows that particles of silver produced by the iron developer will cause more to fall down on the face of those already present, and the image is, of course, built up if the silver nitrate be all consumed on the plate. The developer then becomes useless or injurious. The presence of acetic acid checks the reduction of the silver, and the alcohol facilitates the flow when the bath becomes charged with ether and spirit. The molecular attraction just mentioned is made plainer by reference to the simple lead tree experiment. We have here in this bottle a piece of zinc rod introduced into a solution of acetate of lead. A chemical change has taken place. The zinc has abstracted the acetic acid and the lead is deposited on the zinc, and will continue to be so until the solution is exhausted. The irregularities of surface and arborescent appearance are well shown. If the change were rapidly conducted the lead particles would from their weight sink directly to the bottom instead of aggregating together like ordinary crystals. I have constructed a diagram of colored card, which will perhaps more clearly demonstrate the relation of the different constituents. The lower portion (Fig. a) represents a section of the glass plate or support, the collodion film (Fig. b) having upon its surface a thin layer of bromo-iodine silver (Fig. c), which, when exposed to a well-lighted image, as in a camera, changes into different gradations of sub-bromide and sub-iodide, as indicated by irregular, dark masses in the film. The dotted marks immediately above these are intended for the silver deposit (Fig. d)--clusters of granules, more abundant in the well lighted and less in the shaded parts of the picture, corresponding to the amount of sub-bromide and iodide beneath. [Illustration: SECTION OF SENSITIVE PLATE AFTER EXPOSURE AND DURING DEVELOPMENT. d Silver deposit--Image, c Sub-bromide and sub-chloride (gradations of), b Collodion film--Substratum, a Section of glass plate--Support.]