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.
Soda water is water charged with carbon dioxide under pressure. This pressure is maintained from the time it leaves the manufacturer to the time it reaches the consumer by the strong walls of the syphon or bottle. Immediately this pressure is released, the greater part of the excess gas escapes, producing effervescence. It is, however, curious to note that all the gas which ought to escape when the pressure is reduced does not do so at once. If soda water is allowed to stand in an open glass until it becomes “flat,” a brisk effervescence can be started again by dropping a lump of sugar into the quiescent liquid. Soda water remains supersaturated with gas for some time after the pressure has been released.
Calcium Carbonate. The salts of carbonic acid are called carbonates. Calcium carbonate is one of the most abundant substances in Nature. The white cliffs of the east and south coasts of England, and those of France across the intervening sea, are the exposed parts of enormous beds of chalk or calcium carbonate. Whole mountain ranges in various parts of the world are composed of limestone, which in some cases is mainly calcium carbonate, and in others a mixture of this substance with magnesium carbonate. Marble, whether white, black, or variegated, is almost pure calcium carbonate, the differences of colour being due to insignificant traces of iron and other foreign matter. In Iceland spar and calc spar, sometimes called dog-tooth spar, we have two transparent crystalline forms of this same substance.
Connected with the animal kingdom there are forms of calcium carbonate no less varied in appearance. Egg shells are composed of this substance, and so are oyster shells and the hard external coverings of some of the lower animals. The mother-of-pearl lining of the oyster shell, and also the pearl itself, are secretions of calcium carbonate. The beauty of the last-named variety is due to the external form and to minute inequalities of the surface, which cause the resolution of white light into colours seen in the spectrum or in the rainbow. The coral reefs or atolls of the Southern oceans, which may be miles in breadth and hundreds of miles in length, are all composed of calcium carbonate, which a tiny marine animal has formed for its own support and protection.