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.

It is perhaps somewhat surprising at first to be told that all these forms are composed of the same chemical substance, yet on this point the evidence is definite and unmistakable. All the varieties dissolve readily in dilute hydrochloric acid with effervescence caused by the escape of carbon dioxide gas; moreover, if any of the purer forms, such as pearl, marble, or Iceland spar, are heated to redness for some time, they all lose about 44 per cent. by weight, leaving a residue which is pure lime.

Quicklime. The making of lime from limestone or chalk is called lime burning. The operation is carried out in a structure called a lime kiln, which is usually a barrel-shaped vertical shaft surrounded by substantial brickwork. There are two main methods of procedure, the one continuous and the other intermittent. In the continuous process, the kiln is filled up with limestone and fuel (generally coke) in alternate layers. Combustion is started at the bottom and maintained by a regulated draught. As the charge works down, the addition of limestone and fuel is continued from the top, while the lime is removed from the bottom of the kiln. The lime produced by this method has the ashes of the fuel mixed with it. To avoid this, the more modern type of kiln has four lateral fire grates outside the actual kiln.

For the intermittent method, a kiln is required which has a fireplace at the bottom. Over this a rough arch is built of large pieces of limestone, laid dry, and then the kiln is filled up with pieces of limestone which decrease in size from below upwards. The fire is kindled beneath the arch and urged by a regulated draught. The heating is maintained for three days and nights, after which time the charge is allowed to cool down.

Carbonic Acid Gas in Nature. Although the solvent action of carbonic acid is very small compared with that of strong acids, it is nevertheless great in comparison with that of water. This is shown especially in its action on limestone, an action from which several important consequences arise. Rain, as it falls through the air, dissolves a little carbon dioxide and, although this is only an exceedingly dilute solution of a very weak acid, its cumulative effect, especially in limestone districts, is very great; it hollows out enormous caves and causes the formation of those fantastic creations in stone known as stalactites and stalagmites.

When a drop of water charged with carbonic acid gas falls on limestone, it dissolves a little of that substance, forming calcium bicarbonate, which may be regarded as a compound of calcium carbonate, carbon dioxide, and water. Little by little, the solid rock is hollowed out and a cave, or perhaps an underground watercourse, is formed.

Again, the drop of water charged with calcium bicarbonate may find its way to the roof of a cave. As it hangs from the roof while it gathers strength to fall, a little of the carbon dioxide escapes, and a minute quantity of calcium carbonate is deposited. In this way, a stalactite looking like an icicle in stone gradually grows downwards.

When the drop reaches the floor of the cave, a little time elapses before it sinks into the ground; again a little carbon dioxide escapes, and a small quantity of calcium carbonate is formed. Little is added to little, and in the course of ages the stalagmite grows upward from the floor and ultimately meets the stalactite to form a continuous column of glistening crystallized calcium carbonate.

Hard and Soft Water. Water that is used for domestic or manufacturing purposes is described as either hard or soft. Soft water produces a soap lather almost at once; hard water forms at first a scum or curd which has no detergent properties, and only after a time gives the soap lather which is required. The difference is due to the relative amount of dissolved solid contained in the water.

Only distilled water or rain water collected in the open country is perfectly soft, for this is the only kind of water which on being evaporated to dryness leaves no solid residue. In districts where the underlying strata are composed of hard insoluble rock, such as granite or millstone grit, the water contains very little dissolved matter and is relatively soft. In a limestone or chalk country, water is very hard and in many cases has to be softened either before delivery or before use.

The chief impurities which cause hardness are the chlorides, sulphates, and bicarbonates of magnesium and calcium. The chlorides and sulphates are not affected in any way by boiling, and the hardness which is due to them is said to be “permanent.” The bicarbonates, on the other hand, are decomposed when the water is boiled, and then they cease to cause the water to be hard. This part of the hardness is spoken of as “temporary” hardness.

Let us now consider what calcium bicarbonate is and how it is formed. It is a compound of calcium carbonate and carbonic acid, and is formed by the solvent action of carbonic acid on limestone or chalk. The compound is soluble in water; but when the solution is boiled, the carbonic acid is broken up, carbonic acid gas is expelled from the solution, and calcium carbonate is formed.

Temporary hardness is the more troublesome. In the first place, the bicarbonates, especially that of calcium, often form the greater part of the dissolved impurity. Moreover, when the water is boiled, although the hardness is removed, the insoluble calcium carbonate is a source of trouble, for it gradually settles down into the hard mass known as “fur” in kettles and “scale” in boilers.

It is perhaps necessary at this point to emphasize the fact that matter suspended in water does not make it hard, and it is only matter which is dissolved which makes any difference in this respect.

Since the softening of temporary hard water by boiling has the undesirable feature of introducing solid matter into the boiler, it is customary now to treat this water chemically. The following is the process most generally used. Quicklime or slaked lime is stirred into the water until the mixture gives a faint brown coloration when a drop of silver nitrate is added to a small test portion. Unsoftened water is then added until a sample just ceases to give this test. The temporary hardness has then been removed, and it is only necessary to allow the suspended matter to settle.

The explanation of the method is as follows. The lime which is added neutralizes the carbonic acid combined with the calcium bicarbonate, and the result is the same as in the former case where this carbonic acid was decomposed by heating, for calcium carbonate is thrown out of solution.

For domestic purposes, water is softened by the addition of washing soda. Since this reacts with all the calcium and magnesium compounds forming the insoluble carbonates, all hardness, both temporary and permanent, is removed.