THE DECLINE OF ROME.-The rise of a city, which swelled into an empire, may deserve, as a singular prodigy, the reflection of a philosophic mind. But the decline of Rome was but the natural and inevitable effect of immoderate greatness. Prosperity ripened the principle of decay; the causes of destruction multiplied with the extent of conquest; and as soon as time or accident had removed the artificial supports, the stupendous fabric yielded to the pressure of its own weight. The story of its ruin is simple and obvious; and instead of inquiring why the Roman Empire was destroyed, we should rather be surprised that it had subsisted so long. The victorious legions who, in distant wars, acquired the vices of strangers and mercenaries, first oppressed the freedom of the republic, and afterward violated the majesty of the purple. The emperors, anxious for their personal safety and the public peace, were reduced to the base expedient of corrupting the discipline, which rendered them alike formidable to their sovereign, and to the enemy; the vigor of the military government was relaxed, and finally dissolved by the partial institutions of Constantine; and the Roman world was overwhelmed by a deluge of barbarians.* * Gibbon's Decline and Fall of the Roman Empire. CHEMISTRY. II. Common coal always contains sulphur united with iron, as iron-pyrites. When this is burned, both the sulphur and the iron oxidize. The iron remains as oxide of iron, or iron rust, in the ash, and the sulphur goes into the air as sulphurous acid gas. The latter is also produced whenever sulphur or brimstone is burned, and is well known by its odor as that of the burning brimstone match. From drains and sewers, sulphuretted hydrogen is often set free. Mercifully, the odor is so offensive that it is at once detected. Its injurious effects are easily prevented, as it is quickly decomposed by chlorine. But, otherwise, so fatal is it, that its presence in the air, to the extent of one fifteen-hundreth of its bulk will destroy the life of small birds, while a horse will die where the air contains one two-hundred-fiftieths of its volume of this gas. But setting aside such minor differences as are occasioned by the presence of slight casual impurities, how does it happen that, as regards its chief constituents, the air maintains its compositions so uniformly in all open places? Samples brought from distances far apart from each other, exhibit a sameness of composition; and to such an extent is this the case, that the oxygen and nitrogen of the air were long believed to be chemically united. And yet we know that this is not the case; that the air is not a compound. In a chemical compound the elements lose their individual characters; in a mixture, these are retained. Oxygen is a supporter of combustion; so is the atmosphere. Oxygen supplies combustion far better than the air; the air contains four times as much of nitrogen as it does of oxygen, and nitrogen does not support combustion. Oxygen is more soluble in water than nitrogen; if air be shaken with water, the latter dissolves oxygen in proportions twice as great as it does of nitrogen. Air, therefore, must be a mixture, and this, too, in spite of the fact that all the gases contained in the air are of different densities, and that there is a far greater difference between the respective weights of nitrogen and carbonic acid, than between oil and water. The subject of this intermixture of gases of different densities has been thoroughly examined by Graham: it is called diffusion. Owing to the absence of cohesion between their particles, vapors and gases mingle very freely and completely—a peculiarity of such mixtures being that they never again separate into their component parts. To such an extent is this tendency to mingle carried into action, that if two gases, say hydrogen and oxygen (the latter being sixteen times heavier than hydrogen), be allowed access to each other through a tube several feet in length, they will be found to have mixed thoroughly in the course of a few hours. [The rapidity with which diffusion takes place is inversely proportional to the square root of the densities of the different gases. Oxygen being sixteen times heavier than hydrogen, it follows that only one volume of oxygen would diffuse itself in the same time that four volumes of hydrogen would be able to do so. This is the law deduced by Graham.] The diffusive power of gases is of the utmost importance. Were it not for this law, the carbonic acid evolved in such large quantities in our large towns would, from its weight, collect, and speedily destroy their inhabitants. The foul and noxious gases constantly arising from the numerous operations of a large city would spread disease and pestilence around it. But for this, the perfumes of flowers would fall from their own weights on the senseless earth. In common with every form of matter, the atmosphere is possessed of weight. This property was first demonstrated by Torricelli, whose attention was called to it by the following circumstances. In sinking a well at Florence in 1643, it was found that, after a certain depth, the common suction-pump ceased to bring water to the surface. It had been previously known that when a tube was dipped in water and the air above it withdrawn, the water would rise to a very considerable height; but the only explanation of this phenomenon that was offered, was the vague assertion "that nature abhorred a vacuum," and that the water rose in the tube to fill the vacuum caused by the exhaustion of the air. The attention of Torricelli having been called to the fact that the water in the suction-pipe of the pump would not rise beyond a height of thirty-four feet, it occurred to him that its rising at all was to be attributed to the pressure of the atmosphere outside the pipe, forcing the water to occupy the vacuum caused by the exhaustion within it. Torricelli presumed that, as the volume of water was sustained by the pressure of the air, the two must exactly balance each other; and, therefore, that a volume of water thirty-four feet high must be of equal weight with a similar column of the whole height of the atmosphere. Suppose this to be the case, then a denser fluid exchanged for the water ought to rise in the exhausted tube less than the water; in fact, in exact proportion to the greater density of the fluid employed. To prove this theory, Torricelli filled a glass tube, three feet long, and closed at the lower end, with mercury, and inverted it in a vessel containing the same liquid. The mercury immediately fell 6 inches in the tube, leaving a column 30 inches in height. Now, as the weight of the mercury is 13%1⁄2 times greater than that of the water, the truth of his opinion was fully proved, viz., that the weight of a column of the atmosphere was equal to a similar column of water 3334 feet, or of mercury 30 inches high. In 1648 Pascal repeated Torricelli's experiment, and varied it by the employment of other liquids of different densities. But, as many still affected to doubt whether the rise of the fluid was really owing to atmospheric air, he determined to bring the question to a definite issue. Having filled two tubes with mercury at the base of the Puy de Dome, a mountain in Auvergne, he inverted them as in the previous experiment, and saw that the mercury stood at the same height in both. One tube he left at the foot of the mountain, and carried the other to the summit. As he ascended the mountain, the mercury fell steadily in the tube, till, on reaching the summit, it stood at 3 3-10 inches lower than it did at the base. On descending, the Combustion is a simple chemical process, consisting in the union of the body to be burned with Oxygen. After the union, the body so burned is said to be oxidized. And when any substance has once entered as fully into combination as is possible, it can not be either set on fire or burned. Now, as combustion consists in the union of any substance with Oxygen, it is clear that the substance, after it has been burned, must be increased in weight by just so much as the weight of the Oxygen with which it has united. To speak of the destruction of a body by burning, can not therefore be correct, for all matter is indestructible. The body is not destroyed, but has only changed its form. This is clear enough in such a case as the burning of Phosphorus; there we see, and easily collect and weigh, the Phosphoric acid produced. But it is not so clear in the case of a candle, because the new substances into which they are transformed are gaseous and invisible, and are dispersed into thin air. We can, however, collect and weigh these gases, and prove that in their case also, the burned substances have actually increased in weight. Combustion is accompanied by a development of heat, and generally by light. Every combustible substance has a certain fixed temperature at which it burns in air, but this temperature differs greatly for different substances. If Some are so eager for oxygen that they take fire the moment they are exposed to the action of the air; others at a temperature but slightly above that of summer heat; while others again can only be induced to change their state by the urgent persuasion of intense heat. Neither of these classes of substances could answer the purpose of fuel. But there is a large class of bodies, principally of vegetable origin, which possesses the necessary characteristics. The only point in which these bodies agree, whatever may be their visible properties, is in containing both Carbon and Hydrogen, the leading feature being the very large proportion of Carbon. As no vegetable matter can be formed without the help of certain mineral constituents, which are unalterable by fire, it is clear that Charcoal from wood is not Carbon in a pure form. And so the purest Charcoal leaves an ash when burned with full access of air. It is, however, with Coal that we have to deal, a substance of well-known vegetable origin. In it we see the remains of a vegetation which covered the earth in long bygone ages. The former existence of surface-land has been proved by the occurrence of numerous, upright, fossil trees, with their roots terminating downwards in seams of coal. Our coal mines furnish almost a complete fossil flora; a history of many of the now lost species which once decorated the face of the earth. There is a great resemblance between the plants of the coal-formation, and the flora of New Zealand. A large proportion of the purest kind of coal has been formed from plants which grew on the spot, by a process similar to that which is now seen at the bottoms of marshes, lakes, and rivers, in the formation of peat. The vegetable matter, submerged in water, undergoes decomposition, losing its Water and some of its Hydrogen, and leaving its Carbon behind: this is subsequently covered by accumulations of clay and sand, which, in process of time, harden into slate and gritstone. The pressure of the deep overlying strata has not only prevented the evolution of gas, thus giving the coal the property of burning with flame, but has destroyed most traces of vegetable structure, and given to the pit-coal the close and compact quality of stone. The weight of a column of mercury 30 inches in height, and covering one square inch, amounts to fifteen pounds. As the air presses upon the earth's surface with that force, it follows that objects of all kinds, at the sea-level, have to support an average pressure of fifteen pounds on each square inch of surface. Taking the surface of a man's body at 2,000 square inches, the pressure upon it must amount to about thirteen and one-half tons; yet, owing to the perfect equality of its pressure, and to the fact that every cavity within him is expanded by aid of the same elastic force, he is enabled to sustain this enormous load not only without inconvenience, but generally without even a consciousness of its existence. From the perfect mobility of the particles of air, the force of its pressure is evenly communicated throughout, and exerted equally in all directions. THE NATURE OF BURNING.-It has already been stated that the great supporter of combustion is Oxygen: that under ordinary circumstances, burning bodies obtain their Oxygen from the air; that the intensity of the combustion is in proportion to the amount of Oxygen present in a free state; that therefore it is most energetic in pure Oxygen,— so energetic, in fact, that, for the ordinary operations of nature, it requires to be restrained by the dilution of the Oxygen of the atmosphere with four times its volume of Nitrogen. The several varieties of Coal differ greatly in appearance and composition. All coal consists chiefly of Carbon with a somewhat varying percentage of Hydrogen, Oxygen, and Nitrogen, and with very variable quantities of Sulphur and of Ash. The principal varieties of coal in common use are block racite contains more Carbon than any other kind. For the same reason it burns without flame, and, of course, never smokes; it gives out much heat, and leaves but little ash. Cannel or gas-coal burns with a brilliant flame, because it contains so much Hydrogen which unites with the Carbon: it derives its name from the practice among the Scotch farmers of burning it for its light, instead of candles, or, as they pronounce it, "cannels." Lignite, or brown coal, which is used as fuel in some parts of the world, rarely occurs in extensive fields as does pit-coal. It has usually a brown color, burns with a dark smoky flame, and a very disagreeable odor; it consists, in fact, of vegetable matter very little altered. Peat consists also chiefly of the remains of plants which have undergone comparatively little change. Wood may be said to contain nearly equal weights of Carbon united with Hydrogen and Oxygen in the same proportions in which they are found in water. From this statement it may be inferred that wood fires will not only burn with much flame and little smoke, but will also give out intense heat and consume very fast. Combustible as are all kinds of coal, they must nevertheless be heated to some extent before they will inflame. When the coal is heated below redness by lighted wood, its Hydrogen unites partly with its own Oxygen and partly with the Oxygen of the air, and escapes up the chimney in the form of steam. The heat being at first below that at which Carbon ignites, a quantity of it passes off unconsumed as smoke. Soon, however, the heat increases by continued chemical action; gases are formed which kindle on contact with the air, and burn as long as the coals continue to supply volatile matter. After the flame has ceased, the coals retain their glow as long as any Carbon is left, and at length nothing remains in the grate but the incombustible ash. The chief results of the full combustion of Coal are Water and Carbonic acid, with smaller quantities of Sulphurous acid. The waste of fuel which takes place when much of it escapes in the form of smoke is mainly due to the improper supply of air, which may be to a great extent prevented by supplying plenty of air. The effect of heat on any substance depends then upon the presence or absence of air, that is, of Oxygen. The results of heating coal in air have been sufficiently explained. But, if it be heated in the absence, or with very slight access, of air, by a kind of smothered combustion, as in the manufacture of charcoal, the coal is much altered, but not burned. Gas is driven off, and the greater part of the Carbon remains unburned as Coke-a variety of Carbon, containing in addition all the ash of the coal. Coke is a good conductor of heat, and therefore more difficult to ignite than coal, but it is well adapted for fuel, as it burns without smoke and with an intense, steady heat. The great difference, then, between coal and coke, is the presence of volatile matter in the former, and its absence in the latter. The power of coal to burn with flame depends upon this volatile matter, which on the application of heat is converted into gas. The manufacture of Coal-gas is at present a trade of great importance. The Coal is placed in closed, oblong cylinders of cast-iron, called retorts, which are ranged in furnaces and kept at a red-heat, the volatile products being conveyed by pipes, in connection with the retorts, to a condensing vessel kept cold by immersion in water. Here all the condensible vapors, such as Water, Tar, and Ammonia, together with other impurities are kept back; and the Coalgas, still very impure from the presence of Carbonic acid and Sulphuretted hydrogen, is passed over Lime, in vessels called Purifiers. In this process the lime unites with, and retains, the Sulphur of the Sulphuretted hydrogen, and the Carbonic acid, but does not act on the Gas, which, further purified, is passed into the Gas-holders, where it is stored for subsequent distribution, by means of iron pipes, to the places where it is required for use. For gas-illumination, the value of the coal is proportionate to the quantity of volatile matters which it disengages at a red-heat. Coal-gas is a mixture of several gases, the most important of which are the two Hydro-carbons, Hydrogen carbide and Hydrogen di carbide. Hydrogen carbide, or Methane, is the principal component of coal-gas. In many coal-mines it is also found abundantly collected between the seams of coal, escaping from which, it mingles with the air of the mine, and is the common cause of fatal accidents by its explosion. It is known to miners as Fire-damp, and Wild-fire. It is likewise evolved in the decay of moist vegetable matter; and, as decay, like life, is in perpetual activity upon earth, the gas is generally contained in the air. It may be readily obtained by stirring the mud at the bottom of stagnant pools (whence it has the common name of Marsh-gas), and collecting it as it escapes in an inverted bottle full of water. This marsh gas burns with a yellowishwhite flame. If mixed with 10 measures of air (or 2 measures of Oxygen) it explodes with violence on the application of flame. As it is a compound of Carbon and Hydrogen, the sole products of its combustion in air are Carbonic acid and Water. It is the lightest gas next to Hydrogen. Hydrogen di-carbide is likewise contained in Coal-gas, and is, indeed, its most important constituent for the purpose of illumination. It is also known by the name Olefiant gas and Ethylene. It burns quietly, with a white light, when set on fire; but when previously mixed with 15 measures of air (or 3 measures of Oxygen), it detonates with great violence when ignited. Carbonic oxide is also, to a lesser degree, a constituent of Coal-gas. This gas is produced abundantly in the open fire, and is often seen as a blue flame flickering over burning coke and coal. It contains less Oxygen than Carbonic acid, and, therefore, as it is not completely burned, it may be kindled. When fully burned, it becomes Carbonic acid. Carbonic oxide is also abundantly produced in the combustion of charcoal in stoves. It is a very poisonous gas, and is often the cause of fatal accidents when respired. Of course a small quantity of air is always present in Coalgas; also some Hydrogen. The odor of coal-gas is partly due to the presence of minute quantities of Benzol and of Naphthalin. The removal of the odorous principles in coal-gas offers special difficulties, and it may be doubted whether it is desirable that they should be completely gotten rid of, considering that the strong smell is an excellent safeguard against accidents, and that well-made gas-fittings, in good order, are amply sufficient to prevent its escape. Apart from this, however, it may be safely asserted that the purification of coal-gas could with propriety be carried considerably beyond the present practice. THE NATURE OF FLAME.-We have learned that the difference between coal and coke, the one burning with flame, the other without, depends on the presence or absence of gas. The removal of the gas prevents the formation of flame. What, then, is flame? Flame is only burning gas. This explanation was at one time thought to be without application to the flames from solid and liquid bodies, as candles or oil. But oil, when heated, is readily decomposed and changed into gas, and that the same is true of a candle, is proved by a very simple experiment with which every one is familiar. If a candle with a glowing wick is blown out, there arises from it for a few seconds a small cloud of smoke, which may be lighted, even at a distance from the candle. What is this but gas? Flame, then, is burning gas. And inasmuch as combustion is the result of the union of the gas with the Oxygen of the air, it is clear that the gas can only burn when it comes into contact with this Oxygen effects of cooling. A mixture of Gas with a considerable excess of air beyond what is necessary to burn it, brings about the same result. If Methane, the Fire-damp of coal mines, is mixed with more than sixteen times its bulk of -that is, at the surface. Hence flame is hollow: it is a luminous envelope to a quantity of gas. The hollow structure of flame may be readily shown by experiment. By inserting into the flame of a candle one end of a glass tube, the unburned gases of the interior may be drawn off and in-air, it can no longer be inflamed, and much less exploded, flamed at the other end of the tube. The simple burning of a gas would not, however, account for the light it gives. The feeble light of burning Hydrogen has already been noticed. It is scarcely visible in sunlight. | And even the flame of Oxy-hydrogen, though possessing the greatest heat of all artificial flames, is in no degree more luminous. Oxy-hydrogen is a mixture of Oxygen and Hydrogen in the proportions to form Water. If, then, these two gases are mixed together in the proportion of two volumes of Hydrogen to one volume of Oxygen, we obtain Oxy-hydrogen. It is only on combustion that water, or hydrogen oxide, is formed. The flame is solid. The introduction of any solid object, as platinum, into such a flame, is immediately rendered luminous. One of the most brilliant lights known, the Lime-light, is obtained by introducing a piece of Lime into the feeble Oxy-hydrogen flame; when its rays of light are concentrated by a proper mirror the light has been visible at a distance of 112 miles. The solid matter which gives light to the flame of a candle or of gas is finely divided Carbon. The gases, under the influence of heat are at first decomposed into Carbon and Hydrogen. In the portion of the flame where this takes place there is only enough Oxygen to burn the Hydrogen, which inflames on account of its greater attraction for Oxygen. The Hydrogen, in burning, forms water and gives out so much heat that the liberated Carbon, instead of being deposited in the form of soot, is heated to whiteness, but does not burn until it reaches the very outermost part of the flame, where, finding a free supply of Oxygen, it is converted into carbonic acid. The presence of free Carbon, in every light-giving flame, is easily shown by depressing a plate, or any other cold object, upon it; the Carbon is immediately deposited as lamp-black. This, then, is the mechanism of flame. It may be seen to consist of three parts. A quantity of undecomposed gas, ready to burn, forms the center of the flame as a dark cone; in the bright, or light-giving part, this gas is kindled and decomposed, and the burning Hydrogen ignites the particles of Carbon; whilst in the outermost portion of the flame this Carbon itself burns, forming Carbonic acid, which, with the Water from the burned Hydrogen, forms a thin atmosphere around and above the flame. The hottest part of the flame is near the top, where the combustion is complete, while the temperature in the center is so low, that in a large flame gunpowder may be placed without being ignited. At the bottom of the flame, as well as at the outside, the gas is in perfect contact with the air, and here also combustion takes place, so that in this portion of gas-flame a colorous semi-circle may always be seen. If a plate be depressed into this part of the flame, no soot is deposited on it. Advantage is taken of this fact in the various gasburners employed for heating purposes. A Bunsen burner is so constructed that the coal-gas mixes, in the tube, with the air which is admitted through small holes near the base. The mixture burns with a pale flame, and with very slight luminosity, owing to the complete oxidation of the carbon at the same time as the hydrogen. If the combustion is quickened by a jet of air, the heat of a flame may be considerably increased; upon this principle depends the action of the blow-pipe. Flame requires a very high temperature for its existence, so that if it be cooled down below a certain point, it is extinguished. The deposit of soot or carbon from a flame, on a cold plate, or on a lamp chimney, are illustrations of the by reason of this cooling power possessed by so large a volume of air. Sir Humphrey Davy found that a mixture of explosive gases could not be inflamed through a narrow tube, owing to the cooling influence exerted by the tube on the flame. If wire-gauze with about 400 meshes to the square inch be depressed into the candle or gas flame, the upper part will be cut off, because the unburned gas is reduced in temperature below the point of combustion. It may be kindled by the application of a light above the gauze, and the gas will then continue to burn both above and below. This principle finds its most perfect and useful application in the Davy Safety-lamp, which consists of an oil-lamp enclosed within a cylinder of wire-gauze. The chief use of the Davy Lamp ought to be, to enable the viewer of the coal mine to examine it carefully before the miners are allowed to enter the mine. An explosive atmosphere is of necessity unfit for respiration; so that good ventilation is the miner's complete protection from danger to health and life. The above principles find their application in the varieties of flame used for illumination. As the light depends on Carbon at a white heat, it follows that the richer a gas is in Carbon, the higher is its illuminating power. The degree of light given out by a flame depends on the amount of air supplied to it. If it receives too little air, the combustion is incomplete, the flame smokes, and gives out only an imperfect light. If it receives too much, it is cooled down and again its light is diminished. Hence the best burner will be that which admits the air to the flame in just sufficient quantity to obtain the greatest light. By the Bat'swing and Swallow-tail burners (in the former of which the Gas issues through a narrow slit, and in the latter through two slits), the flame is made as thin and wide as possible, so as to present a large surface to the air. These, accordingly, give considerable light for a small quantity of gas. In the Argand burner, the air is made to pass through the interior of the flame, as well as round it. This flame, therefore, condensing a large surface into a small space, is the brightest and most economical. Many persons object to the burning of gas in inhabited rooms, and it does render the air impure more than any other artificial source of light. One volume of good Gas produces three volumes of Carbonic acid and a large supply of watery vapor. And all contains some Sulphur-compounds, which, when burned, still further contaminate the air by the Sulphurous acid which they contribute to it. These several impurities produce very injurious consequences to the health and comfort of many who breathe them. The deleterious effects of Gas-burning have been sometimes strikingly shown by the injury it has inflicted on articles in rooms, such as metal-ornaments, curtains, books, and so forth. These effects can only be remedied by perfect ventilation. Petroleum is a product of various parts of the world, and at present furnishes light to many houses in our own and other lands. It is generally obtained by drilling into those strata which contain it in abundance. Pennsylvania produces large quantities of this oil. It is often accompanied by Methane, or marsh gas, which forces it out; otherwise it must be pumped to the surface. It is a mixture of Hydro-carbons, and gives the best quality of light when properly rectified. The steady light of a good oil lamp is much preferable to the flickering flame of a gas jet, the effect upon the eyes being much less noticeable in the former than in the latter. The chemistry of burning petroleum does not differ from that of burning gas, save that the oil is brought to the air by the wick, through which it is drawn by capillary attraction, and here turned into vapor by the flame. The products of the burning gases of oil are nearly the same as those of burning coal-gas. Candles made from tallow, paraffine or wax burn in the same manner. The solid is made liquid by the heat; the liquid is made gaseous and unites with the oxygen of the air. Many other oils, both vegetable and animal, are used, and all depend on the same elements for their lightgiving qualities: namely, Carbon, Hydrogen, and Oxygen. Oxygen was given from two Greek words which signify "acid making," as many of the elements burn in oxygen and form acids. Oxygen occurs both in the free state and in combination; indeed it is the most abundant of all the elements. It is a component of nearly all minerals. Lastly, oxygen is contained in all vegetable and animal matters. It is difficult to withdraw oxygen from the air, since this can only be accomplished by oxidizing some body, as a metal. Any of the oxides of metals may be employed to make oxygen. When mercuric oxide is strongly heated in a tube of hard German glass, it yields mercury and oxygen. Potassium chlorate yields oxygen most readily. It is a salt consisting of Potassium, Oxygen, and Chlorine. When heated, it melts, and at a higher temperature loses the whole of its oxygen; Potassium chloride remains. Mix Potassium chlorate with manganese di-oxide in the proportion of 4 parts to 1, put into a copper retort or test tube and THE CHEMISTRY OF HYDROGEN, OXYGEN, CARBON AND NITROGEN.-We have seen that the air contains four elements. Oxygen and Nitrogen in a free state; Carbon in the form of oxidized carbon, or Carbonic acid; Hydrogen, in the condition of oxidized hydrogen, or water, and of Ammonia, a compound of Nitrogen with Hydrogen. And we have learned that every compound containing carbon and hydro-heat over a gas flame or spirit lamp. Oxygen will be given gen, when fully burned, yields carbonic acid, and water. It is therefore necessary that we should know somewhat more accurately, what are the properties of these elements which are of such universal occurrence. When chemists wish to refer to any of the elements, they use the first letter of the name to designate that element; except when two have the same initial letter; then a second letter is added. This letter (or letters) is called the symbol of the element. Some of the elements have the first letter of their Latin name to designate them, thus: Ag. stands for silver, its Latin name being Argentum. Hydrogen* is a colorless gas, tasteless and odorless when pure, and scarcely ever found in the free state. Cavendish discovered it in 1766, and he prepared it from water. It is an element, the lightest of all the elements. On account of Hydrogen being the lightest of elements, it is used as the standard of weight for the other elements. It derives its name from its leading character; it comes from the Greek word signifying the formation of water. When heated in air Hydrogen inflames, burns at the expense of the oxygen, and forms water. Hydrogen is most readily prepared from its oxide, water. If water be boiled in a retort, and its steam passed over iron filings contained in a gun barrel, and made red-hot, Hydrogen passes over, and may be collected in a test tube inverted over water. Iron is an element: at a red-heat its affinity or attraction for oxygen is greater than that of the hydrogen for oxygen, and so an oxide of iron is formed, and Hydrogen is set free. Hydrogen is much more commonly prepared from water, by the aid of zine and sulphuric acid. Granulated zinc is placed in a bottle-which is either one having two necks (Wolff's) or one fitted with a cork having two perforations —and water is poured over it until it is covered. A short bent tube is put through one of the openings in the cork and a thistle tube through the other, the latter reaching below the water. A tube, filled with water and inverted in a vessel of water, is connected with the short tube, in the cork, by means of rubber tubing; sulphuric acid is then poured through the thistle tube and Hydrogen is liberated, which collects in the inverted tube. Care must be taken to reject the first portion of the gas that passes over, as it is mixed with the air which was in the generator. Oxygent is a colorless, tasteless, and odorless gas. It has great power of supporting combustion, but does not burn. In water Oxygen is somewhat more soluble than Hydrogen: but it is only soluble to the extent of three measures in one hundred measures of water. It is sixteen times heavier than the same measure of Hydrogen. The name * Symbol H. + Symbol O. off which is collected over water in the same manner as Hydrogen. Great care is necessary in preventing any organized material from being present in the substances used, as it will unite with the oxygen, when produced, and cause an explosion. Carbon* in its purest form is known as the Diamond. This highly-prized gem, as found, presents the appearance of a rounded pebble, enclosed in a thin, opaque crust. Freed from this coating it is generally colorless, is of a regular crystalline form, and is the hardest known body. Carbon also occurs as graphite or plumbago. In this form it has a lustrous appearance, and readily leaves a mark upon paper. It is quite unaffected by exposure to air. A third variety of Carbon is lamp-black. It was formerly obtained by collecting the smoke or soot of ill-trimmed lamps: hence the name. Its chief use is in the manufacture of printers' ink. Charcoal belongs to this third form of Carbon. It is a black, brittle, infusible substance, of wellknown appearance. As it undergoes no change from exposure to air or moisture, it is a frequent custom to char the ends of posts and piles, with a view of their preservation. Every variety of Carbon, when heated in air or Oxygen, changes its solid condition, and becomes converted into invisible Carbonic acid. Its weight compared with Hydrogen is twelve. Nitrogent is a colorless, tasteless, and odorless gas. It is not inflammable, neither does it support combustion. It is very little soluble in water. Nitrogen occurs in an uncombined state in the atmosphere, where it amounts to 79 per cent. It is readily obtained, as it remains after the removal of the oxygen from air by means of burning Phosphorus. It is fourteen times heavier than Hydrogen. The elements combine with one another in fixed proportions by weight. The smallest quantity of an element that can enter into union with another is called an atom. By the word we mean something that can not be further divided, and the idea it also conveys is of something very, very small. Although the atoms of Hydrogen, Nitrogen, Oxygen, and Carbon can not be further subdivided, yet are they possessed of definite weight. Water is a compound of Hydrogen and Oxygen in fixed proportions by weight. In eighteen parts by weight of water, sixteen parts of Oxygen are united to two parts of Hydrogen. Water is a true oxide. When Hydrogen is inflamed in air, it burns to hydrogen oxide, or water. The compound nature of water may be determined by electrolysis. |