Iodide of Silver is dissolved by Hyposulphite of Soda more slowly than Chloride of Silver, and the amount eventually taken up is less. This is explained as follows:-- During the solution of Iodide of Silver, _Iodide of Sodium_ is formed, and this alkaline Iodide has a prejudicial effect upon the continuance of the process. _Chloride_ of Sodium has not the same action, neither has Bromide of Sodium, consequently the corresponding Silver Salts dissolve to a greater extent than the Iodide. ALKALINE CYANIDES. The chemistry of Cyanogen is sketched in Part III. The Cyanide of _Potassium_ is the salt most frequently employed in fixing. It occurs in commerce in the form of fused lumps of considerable size. In this state it is usually contaminated with a large percentage of Carbonate of Potash, amounting in some cases to more than half its weight. By boiling in proof Spirit the Cyanide may be extracted and crystallized, but this operation is scarcely required as far as its use in Photography is concerned. Cyanide of Potassium absorbs moisture on exposure to the air. It is very soluble in water, but the solution decomposes on keeping; changing in colour and evolving the odour of _Prussic Acid_, which is a Cyanide of Hydrogen. Cyanide of Potassium is highly poisonous, and must be used with caution. Solution of Cyanide of Potassium is a most energetic agent in dissolving the insoluble Silver Salts: far more so, in proportion to the quantity used, than the Hyposulphite of Soda. The Salts are in all cases converted into Cyanides, and exist in the solution in the form of soluble double Salts, which, unlike the double Iodides, are not affected by dilution with water. Cyanide of Potassium is unadapted for fixing positive proofs upon Chloride of Silver; and even when a developer has been used, unless the solution is tolerably dilute, it is apt to attack the image and dissolve it. CHAPTER V. ON THE NATURE AND PROPERTIES OF LIGHT. The present Chapter is devoted to a discussion of the more remarkable properties of Light; the object being to select certain prominent points, and to state them as clearly as possible, referring, for information of a more complete kind, to acknowledged works on the subject of Optics. The Chapter will be divided into five Sections:--first, the compound nature of Light; second, the laws of refraction of Light; third, the construction of Lenses and of the Camera; fourth, the Photographic action of coloured Light; fifth, on Binocular Vision and the Stereoscope. SECTION I. _The Compound Nature of Light._ The ideas entertained on the subject of Light, before the time of Sir Isaac Newton, were vague and unsatisfactory. It was shown by that eminent philosopher, that a ray of sunlight was not _homogeneous_, as had been supposed, but consisted of several rays of vivid colours, united and intermingled.