CARBON AND ITS OXIDES.

CARBON.-SYMBOL, C; ATOMIC WEIGHT, 12.

No solid plays a more prominent part in the economy of nature than carbon, as it forms the wood of vegetation. It is a very remarkable substance, for it appears in three perfectly distinct states-the diamond, graphite, and charcoal.

1. The diamond is found in alluvial débris-that is, in water-worn deposits of gravel. It is presumed that the gem was formed by crystallisation when the rock was in a fluid state, and when in after ages it became broken in pieces by the action of water and other geological forces, the hard diamond was delivered from its matrix, and mixed with the débris. The chief diamond mines are those of Golconda and Bundelcund, in India, Borneo, and Brazil.

When found, the stone has the appearance of a piece of white glass; occasionally there is an approach to the crystal form of the octahedron.

It is the hardest of all known bodies, and is capable of dispersing light-that is, of breaking up white light into its coloured component rays-in a greater degree than any other body except chromate of lead. To exhibit this property to its full advantage, the gem must be cut. This was once an operation of the greatest difficulty, and could only be executed by the Dutch diamond-cutters, who fastened two stones in cement, and then rubbed them against each other until a facet was produced. Now, diamond-cutting is much less laborious. The stone is fixed, as before, in metallic cement, and pressed upon a disc of steel about eight inches in diameter, which revolves horizontally with a great velocity. As with all crystals, there are certain directions in which the diamond is more readily cut. It is the skill of the cutter to place the stone upon the disc in the right position. The steel, were this not done, would itself be cut, instead of making any impression on the diamond. The secret of the disc being enabled to wear down the hard mineral is, that the minute interstices of the metal become filled with dust from the diamond. This is in many instances applied to the plate mixed with olive oil; but when a disc has been some time in work, it is sufficiently impregnated with the dust not to need this addition.

In the Brazil mines is found a dark brown carbonaceous matter, in small pieces, which is as hard, if not harder, than the diamond itself; and it commands as high a price on account of its use in forwarding the cutting of the stones.

The most important use of the diamond is the cutting of glass. This is effected by a natural face of the crystal. If the edge be formed by the intersection of two artificial faces, the cut produced on the glass is not a true cut, but only a scratch with rugged edges. The natural faces of the diamond are frequently curved. The diamond may be heated intensely in an atmosphere of any gas except oxygen, but if it be suspended in a cage of platinum wire, and heated to a bright redness, and then plunged in a jar of that gas, it burns with a steady red light, producing carbonic acid gas (CO2).

It was reserved for Sir H. Davy to show that this gas was the sole product of the combustion of the diamond, though the fact that it was combustible was known in 1694 to the philosophers at Florence. The combustion, however, is not complete, as there always remains an ash, which is generally in the form of a cellular network-the skeleton, as it were, of the gem, and which consists of silica and the oxide of iron. With this excep. tion the diamond is pure carbon. When submitted to the most intense of heats, that of the voltaic arch, the diamond loses its transparency, begins to swell, and is converted into a black mass resembling coke, the amorphous form of carbon. In this state it is a good conductor of electricity, a property it does not possess in its transparent condition.

veins, always in rocks of the earliest formations. The most celebrated mine is that of Borrowdale, in Cumberland. Here it is found in "nests" in trap traversing clay slates. It is a good conductor of electricity, and is as difficult to burn as the diamond. It is chiefly used for manufacturing lead pencils. Being very friable, it leaves its particles on paper when passed across it. The particles themselves, however, are extremely hard, and soon wear out the saws with which the graphite is cut.

Formerly good pencils could only be made from lumps sufficiently large to permit of long pieces being cut. The small masses and dust were cemented together with sulphur, but by this the "marking" quality of the graphite was so injured that only the coarsest pencils could be made of it. But it has been discovered that by submitting this dust to enormous pressure it will cohere, forming plates fit for the manufacture of the best pencils.

Graphite is used for lubricating machinery, and also for making crucibles. For this purpose it is mixed with fire-clay. These crucibles are not so liable to crack as those made of clay only.

3. The third form of carbon has no appearance of crystallisation. It is therefore said to be "amorphous," or without form. This state is shown in coal, charcoal, soot, etc. Of the composition of coal we shall treat in the next chapter. Charcoal is got from the "destructive distillation" of wood-that is, the wood is heated in vessels of iron, closed so that no air can cause the carbon in the wood to burn. Of course there is a pipe to carry off the gases liberated by the heat. If blood or bones be submitted to this process, the result is animal charcoal.

Wood charcoal is also made by cutting the wood into logs, and arranging them on end, then covering the whole with sods, and setting fire to the heap at some central point. A portion of the wood is consumed, but the heat thus produced converts the remainder into charcoal. Charcoal is a bad conductor of heat and electricity. It is very porous, in virtue of which, like spongy platinum, it absorbs gases, the quantity varying with the nature of the gas. Thus, boxwood charcoal absorbs of

This power is also shown in what are termed the antiseptic properties of charcoal-that is, the power it has of removing offensive smells. If putrefying meat or fish be packed in charcoal, all smell is removed; for the gases which cause the unpleasant effluvia are absorbed by the charcoal; and while in this state the oxygen, previously in its pores, attacks and changes the various gases, or oxidises the volatile organic matter. The process of putrefaction, however, is not arrested, but rather increased. This is often resorted to by unprincipled butchers and fishmongers who present tainted meat or fish for sale, which escapes detection for the moment by its inodorousness. Yet it possesses all the deleterious properties of unwholesome food. Charcoal, especially animal charcoal, clears coloured liquids which are passed through it. This may be well illustrated by shaking a little of it with a few ounces of port wine in a bottle, and then filtering the mixture; the liquid which passes the filter will be colourless.

Sugar is clarified by means of charred bullock's blood. After being in use for some time the charcoal is found to lose its

The numerals refer to the volumes--that is, the carbonic acid formed is of the same volume as the carbonic oxide burnt. This may be proved by mixing in the eudiometer carbonic oxide, and half its volume of oxygen; after the spark has passed, nothing but carbonic acid will remain, whose volume is that of the carbonic oxide. This gas has a great affinity for oxygen, and therefore is a powerful reducing agent; in iron-smelting furnaces the reduction of the ore is chiefly due to its action. A solution of cuprous chloride (Cu, Cl,), in hydrochloric acid absorbs carbonic oxide, as also does melted potassium. Carbonic Acid (symbol, CO,; atomic weight, 44; density, 22).When limestone or chalk, which are both carbonates of lime (CaOCO,), are heated in a kiln, the heat drives off the gas (CO,), leaving the lime (CaO). The gas, being half as heavy again as air, collects in the fire-pit, and in any hollows close by the kiln; and here many a wanderer has slept his last sleep, poisoned by the gas. It is also the chief product of fermentation, and collects at the bottom of vats; its presence is ascertained by lowering a lighted candle, which will be extinguished if the gas be present. The escape of this gas causes the effervescence of wines, the froths of porter, ale, etc., and makes bread "rise." It is best prepared by pouring dilute hydrochloric acid on chalk, or, what is better, on pieces of marble (Fig. 36). On

is better not to use sulphuric acid, as the fragments of marble are then coated with calcium sulphate, which is insoluble, and thus the action is retarded. The gas is unable to support life, and when breathed, spasm of the glottis prevents its entrance into the lungs. If its presence in air exceed four per cent., it acts as a narcotic poison.

It is exhaled from the lungs, which are organs composed of a membrane some 160 square yards in surface, which has the property of absorbing the oxygen from the air inhaled, and thus bringing it in contact with the venous blood beneath. This blue blood

owes its colour to the presence of carbon. The oxygen combines with this carbon, forming carbonic acid, which is exhaled.

This may easily be proved muman by blowing through a glass tube into some lime water,

Fig. 37.

or barytic water. The calcium, or barium carbonate, is formed, which renders the water milky. This chemical action is not only carried on in the lungs, but over the whole body through the pores of the skin. If the hand be introduced into a jar of oxygen standing over water, in a short time that gas will be found to be converted into carbonic acid. Hence from this simple experiment will be seen the imperative necessity of cleanliness.

This chemical action is the source of animal heat, and is exactly that which goes on more vigorously in a coal fire. In this case the oxygen entering in at the bottom of the grate combines with the coal, forming carbonic acid. As this passes upward through the fire, it takes another atom of carbon, forming, as we have seen, carbonic oxide, which burns again into carbonic acid when it reaches the top of the fire. The blue, flickering flame seen over a cinder fire is carbonic oxide burning into the higher oxide.

To exhibit the presence of carbon in carbonic acid, it is only necessary to pass the gas over a piece of heated potassium. The arrangement is shown in Fig. 37. The metal deprives the gas of its oxygen, and the liberated carbon is deposited on the interior of the bulb.

When submitted to a pressure of 35-4 atmospheres, the gas becomes a liquid. This is the means by which it is accomplished. A and c (Fig. 39) are two wrought-iron vessels. A

mixture of water and sodium bicarbonate is introduced into A. and the tube b, filled with sulphuric acid, is placed upright in the "generator," A. The top is now fixed, and the tap, s, turned. The vessel is inverted, being placed on stands for that purpose, and the acid is emptied into the solution of sodium bicarbonate. Sodium sulphate is formed, and a vast quantity of carbonic acid gas is liberated. By means of the pipe, d, the generator is connected with the "condenser," c, which is packed in ice. So great is the pressure of the gas, that when both the taps are turned the liquid carbonic acid distils over. When the operation is complete, the two vessels are disconnected. A little of the liquid can be received on a piece of wool by turning M, the pressure of the gas in c forcing the liquid up the tube. So rapid is the evaporation, that the liquid on the wool frozen. When mixed with ether, a paste is formed, which possesses an extremely low temperature, by which mercury is

CaOCO, + 2HCl = CaCl, + H,O + CO,,

Fig. 38.

Carbonic acid, though a very weak acid, forms a numerons class of "carbonates." All these effervesce with dilute nitric

the delicacy and yet the power of the wings-the splendour of the colours, and elaborateness of pattern, whether expressed in these gorgeous hues, or marking and chasings-are all so exquisite that the class is a general favourite with all. With man, excellency in the execution of any one plan seems to be inconsistent with diversity; but how different is it with the Creator. As if to exhibit how unlimited might be the variety while the ground-plan is the same, we find a greater number of species in this class than in all the rest put together. One order of insects, the Coleoptera, has not less than 80,000 different kinds known to, and already described by naturalists; and yet so imperfectly has the search for these hiding and burrowing insects been carried out, that it would not be a matter of surprise if a few years should double the number of known species. We shall describe the insects first, because they occupy a central position, and follow so closely on to the class Myriapoda, that some naturalists have included these latter in the class. The Myriapods, however, show so marked a difference to the insects in the greater number of rings, in the similarity of these to one another, in the whole of them being furnished with limbs, and in never presenting any traces of wings, that they may be well classed alone; while the name Insecta, or notched animals, is confined to the true hexapod (six-legged) order.

The reader must have seen insects so often that it seems superfluous to describe their general form and constant peculiarities; yet we are so often accustomed to see without examining, and to examine without noting, that perhaps the fact that a fly or a gnat has six legs may be new to some persons who have been plagued by these creatures all their summers. The body, then, of a typical insect in its final and perfect state consists of three well-defined divisions, called (beginning from the front) the head, the thorax (chest), and abdomen. So deep is the notch which divides them from one another, and so small is the stalk or connection which unites them in bees and flies, that the divisions of the bodies of these insects cannot have escaped notice. In beetles and butterflies, the divisions, though not quite so marked, are evident enough, but in such insects as crickets and plant-bugs they are traced with some difficulty. To the head is deputed the faculty of sensation and prehension; to the thorax the office of locomotion; while almost all the functions of organic life, such as digestion and reproduction, are delegated to the abdomen. The head is variously shaped, commonly resembling a disc, and presenting a flattened but still convex surface forwards, on the expanse of which are situated two antennæ or feelers. These are almost constantly present, but their form is so modified in different insects, that no general description can be given of them. Usually they are jointed, but the numbers of the joints, their relation, size, and shape, and all connected with them, are so different in different families, that they form an important means of distinguishing one family from the other. The mouth opens on the bottom part of the edge of the disc, while the large complex eyes cover the lateral edges, and extend often both in front and to the middle line at the top of the head. When this is not the case, it is no uncommon thing for three simple eyes to be placed on the very apex of the head, in the form of a triangle. These, however, are by no means constant in all insects. The organs which, standing round the mouth, minister to all the accessory functions of gaining and swallowing food, have, though very diverse in shape, been harmonised by the labours of entomologists so as to represent one plan. There is in front the labrum, or upper lip, then two pairs of jaws, one pair behind the other, but each single jaw playing from the side to meet its fellow in the mid-line. Behind these is the under lip, which is sometimes very complex, being split into three or five divisions. Additional feelers like the antennæ, but usually smaller and of fewer joints, are often placed both on the hind pair of jaws and the lower lip. When the mouth organs are spoken of as lips and lateral jaws, it must be remembered that these organs are so much modified that in some insects the terms seem hardly applicable. Thus, a reference to the illustration of the head of t! hich is found too commonly in our metropolis, four jaws, though springing from separate form a single style-like puncturing appaclosed in the lower lip, which is a tube are sucked.

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consolidated that it would at first sight

suggest the idea that it consists of but one ring, corresponding with one annulus of a worm, and some have thought this was really the case. Others, however, believe, and apparently with reason, that short as it is in its fore and aft or axial diameter, it nevertheless is compounded of at least six rings, each of which has its pair of appendages. Thus, the first bears the eyes; the second, the feelers; the third, the undivided labrum; the fourth, the maxillae; the fifth, the mandibles; and the sixth, the more or less split and complex labrum. This view may appear fanciful, especially when the large rounded areas of pavement-like eyes are spoken of as appendages or limbs; but if we follow the comparison to the almost precisely similar organs in crustaceans, where they are sometimes set on long and jointed stalks, it ceases to be so.

The thorax, although it forms a more or less globular or cubical box, which lodges the muscles which ply the legs and wings, plainly consists of three rings or segments. This is apparent, not only on account of the number of appendages, but also, on examination, the plates of which it is composed show the lines of junction by sutures on the outside; while on the inside the edges of these are doubled in so as to form ridges, to which the muscles are attached. To the first segment, or prothorax, are attached a pair of legs. They spring from below, and are extended outwards. The second segment, or mesothorax, has a pair of legs below, and generally a pair of wings, springing from the back. The hind segment, or metathorax, has the same limbs as the preceding one. The legs are all jointed, the joints being of beautiful structure. The limb starts from a movable plate wedged in between the fixed plates of the body; this is called the coxa. Then comes a small joint which assists in allowing the limb to be rotated, and is called the trochanter. Beyond this is the femur, and to its end is attached, by a joint which only permits of an up and down movement, the usually long serrated or spined tibis. A string of five beaded joints forms the foot, the last of which is furnished with two curved hooks to lay hold on the minute roughnesses in the surface over which the insect crawls. Besides the claws, there are often two or three cushions of stiff hairs, which, aided by a sticky secretion, are very good sustainers of the light and strong creatures when they walk on the ceiling of a room. This description applies to the limb when most developed, as there is a vast variety in the composition of the limbs of insects. The legs are used not only for walking, but also for cleaning the body, the antennæ, and the wings. They are sometimes furnished with curious brushes and combs for effecting this purpose. The use which the working bee makes of its hind legs-namely, to store lumps of wax upon them, and so to carry a supply of this substance to its hive-will also occur to all bee-keepers.

The reader will probably wonder why the wings have not been spoken of as appendages to the body-rings. He will ask, if the number of so-called appendages is made to determine the number of rings of which the body is composed, why the wings do not count for limbs whereby to determine the number of the annuli of the thorax? A careful comparison of these organs throughout the class, with their mode of development, has led naturalists to suppose that the wings are modified gills corresponding to the gill covers of crustaceans, and not with the limbs of these. If this correspondence be genuine, it is a curious instance of how the same organ may have very different uses in different animals. The skin or integument of insects consists essentially of three layers. The outermost is a thin, transparent membrane; beneath this is the hard, horny-coloured layer, to the inside of which the live vascular skin is applied. The wing consists of an extension of the outer layer into a long bag, the two sides of which are smoothed down and applied to one another so as to form one sheet, while this is strengthened and kept in shape by a framework of stiff fibres derived from the second layer. Derivatives from the blood system and the respiratory system in some instances enter the fibres, but are not conveyed into the membranous part of the wing, so that the torn wing of an insect is never repaired. The pattern of the framework of fibres, or nervures, as they are called, is well worthy of study, not only because it is beautiful and made much use of in describing and distinguishing insects, but on account of its wonderful adaptation to the requirements of the wing, furnishing strength and resistance where strength and resistance are required. The wings are very variously deve