A careful study of the phenomena involved in this part of the process cannot fail to show that the ray of Light determines a molecular change of some kind in the particles of Iodide of Silver forming the sensitive surface. This change is not of a nature to alter the composition or the chemical properties of the salt. The Iodine does not leave the surface, or there would be a difference in the appearance of the film, or in its solubility in Hyposulphite of Soda.

The following diagrams may perhaps be useful in mechanically illustrating what is meant by a molecular change.

Fig. 1 represents a compound molecule of Iodide of Silver, the component atoms of which are closely associated.

Fig. 2. The same after the action of a disturbing force. The simple molecules have not altogether separated, but they are prepared to do so, touching only at a single point.

Fig. 1.Fig. 2.

Now the effect produced on this combination by a developer is understood, if we suppose that in the first case the affinity of the Iodine for Silver is too great to allow of its separation; but in the second, this affinity having been loosened, the structure gives way, and metallic Silver is the result.

This hypothesis has the merit of simplicity, and is not opposed to known facts; it may therefore for the present be received. The point however on which a doubt must rest is—whether the molecular disturbance produced by light upon Iodide of Silver leads to a reduction of that Salt by the developer. No image can be produced on the application of Pyrogallic Acid unless the particles of Iodide are in contact with Nitrate of Silver; and hence it may be the Nitrate and not the Iodide which is reduced—that is, the impressed molecule of Iodide may determine the decomposition of a contiguous particle of Nitrate, itself remaining unchanged. This view is supported to some extent by Moser's experiments, shortly to be quoted; and also by the fact that the delicate image first formed can be intensified by treating it with a mixture of the developing solution and Nitrate of Silver, even after the Iodide has been removed by a fixing agent. The following experiment will serve to illustrate this.—

Take a sensitive Collodion plate, and having impressed an invisible image upon it by a proper exposure in the Camera, remove it to the dark room, and pour over it the solution of Pyrogallic Acid. When the picture has fully appeared, stop the action by washing the plate with water, and remove the unaltered Iodide of Silver by Cyanide of Potassium. An examination of the image at this stage will show that it is perfect in the details, but pale and translucent. The plate is then to be taken back again to the dark room and treated with fresh Pyrogallic Acid, to which Nitrate of Silver has been added; immediately the picture becomes much blacker, and continues to darken, even to complete opacity, if the supply of Nitrate be kept up.

Now in this experiment it is evident that the additional deposit upon the image is produced from the Nitrate of Silver, the whole of the Iodide having been previously removed. Observe also, that it forms only upon the image, and not upon the transparent parts of the plate. Even if the Iodide, untouched by light, be allowed to remain, the same rule holds good;—the Pyrogallic Acid and Nitrate of Silver react upon each other and produce a metallic deposit; this deposit however has no affinity for the unaltered Iodide upon the part of the plate corresponding to the shadows of the picture, but attaches itself in preference to the Iodide already blackened by light.