Hydrobromic and Hydriodic Acids are not much used, but their salts, the bromides and iodides respectively, are of great technical importance. Silver chloride, bromide, and iodide, are sensitive to light, and mixed with gelatine they form the emulsion which is spread over photographic plates and papers. Potassium bromide and iodide are also well known to photographers. When the halogen salts of silver are exposed to light, an extremely subtle chemical change takes place, which is only made apparent when the plate or paper is developed. Then the silver salts on which the light has fallen are reduced to metallic silver, and this reduction is greatest where the light was most intense, and in other places is proportional to the light intensity. A very faint image may appear on the plate while it is in the developer, but generally the image is only brought out clearly when the plate, film, or paper is placed in “hypo” solution, which dissolves out the silver salts which have not been changed, leaving the metallic silver unaffected. CHAPTER V CARBONIC ACID AND CARBONATES Carbon. When any product of animal or vegetable life is strongly heated in a vessel from which all air currents are excluded, a mixture of gases and liquids is driven off, and a charred mass remains. This residue, from whatever source obtained, is composed mainly of the element carbon. It sometimes happens that a loaf of bread or a cake is left in the oven and forgotten. In popular language it is then said to be “burnt to a cinder”; in reality, the surface layers have been converted into carbon. Carbonic Acid. If carbon is heated in an open vessel provided with a good draught, it glows and in time disappears, because it combines with oxygen to form an invisible gas, carbon dioxide or carbonic acid gas, which, when dissolved in water, forms carbonic acid. Compared with the acids which have been described in the foregoing chapters, this is a very feeble acid; it changes the colour of litmus to a wine red, not a bright pink; its taste is just pleasantly acid, and its solvent action on metals and limestone is very small indeed. The solution of the acid, obtained by passing carbon dioxide into water, is, of course, very dilute, and it cannot be concentrated by evaporation, since this only results in expelling the carbon dioxide from solution, leaving pure water. Soda Water. In the case of most gases, the weight which dissolves in a given quantity of water is proportional to the pressure. This is true for carbonic acid gas. Under a pressure of 4 atmospheres, the weight of gas which dissolves is four times as great as under a pressure of one atmosphere.