thoughts, had acquired the honorable title of Father of his Country. Cicero, born in a provincial town, and of citizen parents, had so distinguished himself by his talents, his industry, and his irreproachable conduct, that, although ignoble, he obtained the consulate. He had devoted himself in Athens and Rhodes with such zeal and success to the sciences of the Greeks, and especially to eloquence and philosophy, that he might be compared, both as a statesman and orator, to Demosthenes, and had composed profound works on rhetoric and philosophy. Though vain, boastful and weak, he possessed civic virtue, patriotism, and a strong sense of justice.* CONSPIRACY OF CATILINE.-During the consulate of Cicero, Catiline, a man of noble family, but disgraced by an infamous life, and loaded with debts, formed a conspiracy with certain other Romans of desperate fortunes, the objects of which were, to murder the consuls, to set fire to the city, to overthrow the constitution, and in the confusion to seize upon the government, by the aid of the soldiers of Sylla and the populace. But the vigilant consul, Cicero, had baffled this atrocious project. By his orations against Catiline, he unmasked the dissembling villain in the Senate, and compelled him to fly into Etruria, where he met with his death in a courageous defence against the consular army. His confederates were put to a violent death in prison.* THE FIRST TRIUMVIRATE.-Two men had attained great prominence in Rome. Caius Julius Cæsar, 100-44 B. C., a man of a thoroughly cultivated mind and great energy; and Marcus Licinius Crassus, who had defeated the gladiators under Spartacus. Cicero acted as a mediator between Pompey and Cæsar, and they united with Crassus, forming what is known as the first Triumvirate, and agreed to allow no political measure displeasing to either of them to be adopted. Many citizens were put to death, and Cicero, who had offended his friend Cæsar, was banished. Gaul was assigned to the special attention of Cæsar, Spain to Pompey, and Syria to Crassus.+ CESAR IN GAUL.-In the first year of his command Cæsar delivered Gaul from the invasions of the Helvetii and Suevi, and effected the subjugation of the South. In the second he imposed his yoke upon the fiercest nations of the North; in the third he subdued the West. The campaigns of the fourth and fifth years daunted the Germans and the Britons on their own soil. The subjugation of the vast region between the Alps, the Rhine, the Pyrenees, and the ocean was finally completed in the eighth year of Cæsar's proconsulship. In eight campaigns he is said-but the boast is Plutarch's, not his own-to have taken more than 800 cities, worsted 300 nations, and encountered 3,000,000 of men in arms, of whom he had slain 1,000,000, and made prisoners of an equal number. The general severity enhanced the favor of his indulgence. Among Cæsar's contemporaries it was remarked with admiration that throughout his Gallic campaigns his soldiers never mutinied. The toils and privations they endured more dismayed the enemy than their well known prowess on the field. Nothing could induce them when captured, to turn their arms against him, while Pompeius and Lucullus had been constantly confronted by renegades from their own ranks. Gaul had been conquered under Cæsar by the Gauls themselves, and it was the Gauls who were now about to conquer the empire of Rome. THE SECOND CIVIL WAR.-The death of Crassus left the * Dr. George Weber. +Gilman's General History. Merivale's History of Rome. supreme power to be struggled for by his two associates. Pompey, full of confidence, neglected ordinary precautions. Cæsar crossed the Rubicon, exclaiming, "The die is cast," and marched on Rome. Pompey fled, but afterwards collected a force with which he confronted Cæsar near Pharsalus, in Thessaly, and was defeated, August 9, B. C. 48. Pompey escaped to Egypt where he was assassinated. The most extraordinary honors were now heaped upon Cæsar, and he became sole master of the Roman world.* TRIUMPHS OF CESAR.-Cæsar shed tears of compassion over Pompey's death, and refused the instigator of the murder his promised reward. When he was chosen umpire between Ptolemy (king of Egypt) and his beautiful sister, Cleopatra, in a dispute concerning the throne, he decided: in favor of the latter, and by this means got involved in a war with the king and the people of Egypt, that retained him for nine months in Alexandria, and reduced him to great peril. It was only when fresh troops had arrived, and Ptolemy had been drowned after an unsuccessful engagement on the Nile, that he could place the government in the hands of Cleopatra (by whose charms he had been enchained), and proceed to fresh conquests. The rapid victory that he gained by the terror of his name over the son of Mithradates, had been rendered immortal by the memorable letter that announced the event; "I came, saw, conquered!" After a short delay in Rome, he passed over into Africa, where the friends of republican government, and the adherents of Pompey, had collected a vast army. Here Cæsar gained the bloody battle of Thapsus, where the hopes of the republicans were destroyed. A magnificent triumph of four days awaited the victor on his return to Rome. The last remnants of the friends of Pompey and the republic were destroyed in the frightful battle near Munda, where they fought with the courage of desperation.+ C.ESAR'S DEATH.-Cæsar now returned, as chief and ruler of the Roman empire, to the capital, where he was saluted as "Father of the Country," and elected dictator for life. He sought to win the soldiers and people by liberality, and the nobles by offices; he encouraged trade and agriculture, embellished the city with temples, theatres, and public places, improved the calendar, and forwarded all sorts of good and useful projects; but his evident attempts to gain the title and dignity of king induced some fanatical friends of liberty to engage in a conspiracy. His friend and flatterer, Marc Antony, offered him the kingly diadem during a feast, and despite the feigned distaste with which Cæsar rejected it, his secret satisfaction was discernible. At the head of the conspiracy stood the high-minded enthusiast for liberty, M. Junius Brutus, the friend of Cæsar, and the severe republican, Caius Cassius. In despite of every warning, Cæsar held a meeting of the senate on the Ides of March, in the Hall of Pompey. It was here that, with the exclamation, "Et tu, Brute," he fell, pierced by twentythree daggers, at the feet of the statue of his former opponent.+ THE SECOND TRIUMVIRATE. Thus fell the man of whom Marc Antony said in an oration: "Here was a Caesar! when comes such another ?^' The words of Antony infuriated the people, forced the murderers to flee, and led to the formation, B. C. 43, of the second triumvirate by himself, Cæsar Octavius, and Marcus Emilius Lepidus.* *Gilman's General History. + Dr. George Weber. CHEMISTRY. INTRODUCTION.-Chemistry is that branch of sciencewhich treats of the nature, the properties, and the changes of matter. A comprehensive subject, including, as it does, everything in earth, and air, and sea;-everything indeed which occupies space and is possessed of weight. In the common language of science the terms "physical" and' "chemical" often occur, and therefore require some ex-planation. The physical properties of a body are those which refer to its "natural" condition, as the Greek word signifies from which physical is derived. Thus all bodies are either solid, line form, in color, transparency, and in the different effects of heat and electricity upon them. These are physical properties, and we observe that they do not notice the composition of bodies. A piece of pure gold may be so strongly heated that it melts; it may be divided into parts so small as almost to surpass belief, yet each part would consist only of gold. But, if we heat a piece of paper, it browns or blackens before it burns; i. e., turns to carbon. Charcoal may therefore be obtained, and thus a difference is established between the composition of gold and paper, which it is the province of Chemistry to study. The term "chemical" is given to the changes which permanently affect the character of bodies. THE LAST YEARS OF THE Republic.-New proscriptions now took place, which proved particularly fatal to the knightly and senatorial ranks. The most deserving and illustrious men fell beneath the blows of assassins, the dearest relations of friendship, of blood, of piety, were torn asunder. Among the victims of Antony was Cicero, who was killed during an attempt at flight. His head and his right hand were placed upon the rostrum. After the possessors of power in Italy had satiated their vengeance, they marched against the republicans, who had established their camp in Macedonia, under the command of Brutus and Cassius. It was here, in the plains of Philippi, that a decisive double engagement took place, in which Cassius was obliged to yield to Antony, whilst Brutus repulsed the legions of Oc-liquid, or gaseous. They may vary in hardness, in crystaltavius. But when Cassius, deceived by false intelligence, had overhastily fallen upon his own sword, and the triumvars twenty days afterwards renewed the fight with united forces, Brutus, "the last of the Romans," was forced to succumb, and fell, like Cassius, upon his own sword. Hence- | forth the contest was no longer for freedom but for empire. The victors divided the Roman territory between them; Antony chose the East, Octavius the West; the feeble Lepidus, who at first received the province of Africa, but who never possessed much influence, was soon robbed of his share. But whilst the luxurious Antony was leading a voluptuous life at Cleopatra's court, in Alexandria, the shrewd Augustus and his high-spirited admiral, Agrippa, were winning the affections of the Roman people by liberal douations and diversions, rewarding the soldiers by a diminution of lands, and keeping up the discipline of the fleet and army. At length when Antony lavished Roman blood and Roman honor in an unsuccessful campaign against the Parthians, married Cleopatra, and gave the province of Rome to her son, the senate, at the instigation of Octavius, deprived him of all his honors, and declared war against Cleopatra. East and West stood opposed in arms. But the sea fight of Actium, despite the superiority of the Egyptians, was decided in favor of Octavius. Antony and Cleopatra fled. But when the victors approached the gates of Alexandria, the former fell on his sword, and Cleopatra, finding that her charms produced no impressio on the new potentate, destroyed herself by the poison of an asp. Egypt became the first province of the Roman empire.* THE EMPIRE -It is with good reason that the empire is regarded as dating from the day of Actium. Though Antony existed, and resisted, for nearly a year longer in Egypt, it was only as a desperate man clinging to life till the last moment. From the day of Actium, Octavian was sole master of the Roman world.t The following sonnet was written near Lake Trasymene, where the rill called Sanguinetto commemorates the bloody battle between the Romans and the Carthaginians: When here with Carthage Rome to conflict came, * Dr. George Weber. + Rawlinson's Ancient History. --William Wordsworth. All bodies are either simple or compound. In compoundstwo or more simple bodies are so united together according to fixed rules, that their individual characters are, for the time, lost. A simple body is called an element. A simple body or element can not be turned into any other form of matter. As in the case of gold, which is an element, no treatment can bring from it anything but gold. The Ancients looked upon air, earth, fire, and water as elements. But, as it is easy to obtain simpler forms of matter out of air, earth, and water, modern chemists do not look upon them as elements. As for fire, it is merely the result of the action of high temperature on certain bodies. Many of the elements are well known. Gold, silver, copper, lead, zinc, tin, iron, and mercury are household words; these are all elements. But many elements are so very rare as to be almost unknown. Others again, like oxygen, hydrogen, and nitrogen, are less known, because they are unseen and' exist only as gases. Only two of the elements are met with naturally in the liquid state, bromine and mercury. The recently discovered element Gallium is said to be a liquid, but has only been obtained in small quantities. Although Physics is a study quite apart from Chemistry, there is no possibility of studying Chemistry without reference to Physics. Chemistry relates essentially to the actions of bodies upon one another, and to the lasting changeswhich they either produce or experience. PHYSICAL AND CHEMICAL PROPERTIES OF WATER.—It has been already stated that the ancients considered water to be an element or simple body; the contrary, however, may be easily proved. It is a compound of two elements, one of which is called Oxygen, the other Hydrogen. In chemical language, water is Hydrogen Oxide. The difference between a mixture and a compound is well illustrated by water. The two elements, oxygen and hydrogen, which, when united in fixed proportions, form liquid water, exist naturally, and in the separate state, as gases. In a mixture, bodies retain their individual characters; in a compound, they are either lost or disguised. Water is a colorless liquid, without taste or smell. But water can exist in three states-as a liquid, a solid, and a vapor. At temperatures below the freezing-point we are familiar with it in the form of ice, and we all know how readily it rises in vapor. The atmosphere always contains a certain amount of water in the condition of vapor, the quantity of which depends upon temperature. A very homely illustration of these two facts-the existence of watery vapor in the air and its variation with the temperature-is daily seen. If a vessel filled with cold water is brought into a warm room, its surface is immediately covered with moisture. The air in contact with the vessel, being cooled down by the water, is no longer able to contain the vapor present in it, and it therefore condenses. A still more familiar illustration of the kind is the condensation of the moisture in the air breathed out from the lungs. In winter, when the outer air is much colder than the breath, the moisture is seen as a cloud; but in summer, when the temperature of the air and that from the lungs are more nearly equal, the moisture from the breath is not visible. Indeed, the effects of heat are very remarkable and worthy of attentive study. Water boils at a temperature of 1000 Centigrade, and, in the form of steam so enlarges its bulk, that it occupies a space 1,700 times as great as in the liquid state. At the moment of freezing, water expands so much that ice is lighter than water, and therefore floats. In the act of freezing, water is one of the most powerful agents in the formation of soil from the hardest rocks; these are broken up through the formation of ice, and by degrees rendered sufficiently small to allow of the growth of plants. During a severe winter the water contained in the pores of the soil freezes, and in expanding causes a breaking up of the hard clods. The force exerted by freezing water is well illustrated by the bursting of pipes and vessels which contain it, when exposed to a low temperature. If a vessel filled with broken ice be placed in a warm room, the ice will gradually melt; but a thermometer placed in it will continue to show the same temperature as long as any ice remains unmelted. The heat of the room is simply employed in overcoming the solidity of the ice, and in enabling the particles to move readily. This constant temperature of melting ice has been adopted as one of the fixed points of thermometers. Water is said to freeze at 0°, or zero on the scale of Centigrade. All degrees of cold below zero are indicated by the minus sign placed before the figure. Thus,-5° C. would show five degrees of cold on the Centigrade scale. Water expands for all degrees above 4o C., and at the moment of becoming solid it increases considerably in volume, 916 volumes of water becoming 1000 volumes of ice. This temperature of 4° C. indicates the point at which the density of water is greatest. This singular fact has the most important consequences in nature; on it depends, in some degree, the very life of aquatic plants and animals in the temperate and colder regions of the earth; for if the contraction of water by cold continued down to its freezing-point, each river and lake would become one mass of solid ice during every frost. There would be a continual fall of cold water to the bottom and rise of warm water to the top, till the whole was frozen, and every living thing in it must die. Water is not a good conductor of heat. Conduction consists in each particle taking up the motion of its neighbor and passing it on. The metals are good conductors. If a metal be heated at the top, the heat is rapidly communicated to the bottom. But water may be boiled at its surface without communicating heat downward. Although water (like air and gases generally) is not affected by conduction, its physical property of fluidity enables it to be readily heated by convection. By convection we mean the passage of heat from one portion of the fluid to another by a transfer of the particles themselves. When a liquid is heated from below, it expands, becomes lighter, and moves upward; a fresh portion of the fluid descends, becomes heated in its turn, and also rises. By this means a circulation of the particles is established, bringing about by degrees a uniform temperature in the whole mass of liquid. Heat applied to the surface of liquids can produce little or no movement in its particles; being expanded by heat, and thus rendered lighter, they remain upon the surface. A very simple experiment will prove this. If a glass jar be filled nearly to the brim and a sensitive thermometer inserted with its bulb upwards, and just below the surface, a layer of alcohol upon the water may be inflamed without imparting any heat that can be recognized by the thermometer. By performing the converse of the experiment, and laying a piece of ice upon the surface, the effect is immediately shown by the fall in the thermometer. The particles of water, being cooled, become heavier, and sink rapidly downwards. If the heating of water be continued, it boils. And here another fact meets us which is well worthy of observation. It does not matter how fierce the fire may be under the kettle, or other vessel, in which the water may be heated, tne water can only boil. It is said to boil when the steam has an elastic force just sufficient to balance and overcome the pressure of air upon its surface. Water boils at 100° C. when the air exerts its usual pressure; and the temperature of boiling water is so equally a fixed point, that it has been adopted in all thermometers. In the open air the boiling point can not be raised, because the pressure can not be increased. But, if water be boiled in a close vessel, as in the boiler of a steam engine, the steam which is generated having no escape until it can lift a certain weight, and continually increasing in quantity, gradually raises the pressure more and more to any point that may be desired; and, with the pressure, the temperature. The formation of steam, and its subsequent condensation, is the only mode of obtaining perfectly pure water. No water, except that specially prepared for chemical purposes, is absolutely hydrogen oxide. The purest natural water is rain; but even that contains not only gases in solution, which it has washed from the air in its descent, but also many mechanical impurities derived from the same source. And if the rain-water has once touched the soil, its impurity is of course increased by the solution of certain salts. All these impurities may be removed by distillation, the only process by which it is possible to separate a liquid from the solids which it has dissolved. The ordinary apparatus for distilling is the still and worm. The worm or coiled tube which receives the steam must be passed through a large quantity of cold water. The heat being greatest in the upper coils, the upper layers of water in the worm tub become first heated, so that if a supply of cold water is allowed to enter the bottom of the tub the heated portions overflow at the top, and are carried away. The distilled water is collected in vessels placed for its reception. Even salt water, by distillation, will yield distilled water, and such water is prepared on board ship for the general use of seamen. Although distilled, or chemically pure, water is of great value to the chemist in studying the varied properties of this important liquid, it would in its pure state answer comparatively but few of the purposes for which it was so plentifully provided by our Maker. Its great power of dissolving solids, liquids, and gases, a power which it possesses in a higher degree than any other liquid, adapts it to its varied uses, and gives distinctive characters to rivers and seas, and indeed to water of every description. Rain water contains varying quantities of oxygen, carbonic acid, minute quantities of carbonate of ammonium, and, during storms, of hydrogen nitrate and nitrate of ammonium. By the help of these, the rain assists in fertilizing the soil. The rain-water of towns is further rendered impure by the various exhalations, by the products of combustion of coal, by decay, and by the chemical processes there carried on. The washing power of rain is exerted as strongly upon solids as upon gases. The character of the water of a river, brook, or spring, will therefore entirely depend on the nature and solubility of the soils through which its waters have passed. A limestone rock will give rise to a calcareous water. Some springs flow through beds of salt, and are called salt springs. Others, chalybeate, from the presence of carbonate of iron. Sulphurous waters contain sulphuretted hydrogen, and are known by the offensive smell of rotten eggs. The sea is the great reservoir of all matters soluble in water, while rivers, as regards the salts they contain, are but very dilute sea-water. The water of rivers is rarely as pure as spring-water, as it always contains more or less of animal and vegetable matter washed from the land. Rivers, and especially those whose current is strong, are, however, provided with a natural means of self-purification, in the oxidation of vegetable and animal matters through the agency of the air with which they are constantly being brought into contact; and also by the action of the various aquatic plants which give off a perpetual supply of oxygen. Spring waters are generally harder than river waters, owing to the presence of a greater quantity of salts of lime and magnesia. The rocks constituting the earth's crust are, in a thousand places, rent and torn into crevices and hollows, which form natural basins for the reception of water, which accumulates in them, till, at some point or ́other, it finds vent as springs, very often at great distances from the places where the water entered the ground. When water has dissolved as much of a gas, or of a solid as it can take up at a given temperature, it is said to be "saturated." If a salt dissolves in water without change of properties, and with a lowering of the temperature, simple solution is accomplished; but, if the temperature rises with a corresponding alteration in the properties of the salt, the salt is said to be chemically dissolved. An illustration of simple solution is afforded by sugar in water; of chemical solution, by sulphate of sodium, free from its water of crystallization. THE ATMOSPHERE.-The atmosphere was formerly regarded as an element, in the same way as water. But if we remember the present definition of an element, as a body which can not by any known means be resolved into simpler forms, the idea can not be entertained for a moment. It is now well known to be a mixture of several substances, in very different proportions, and of very distinctive properties. What, then, is this atmosphere, without which neither life, nor that very important source of heat and light-combustion — can be supported? Of what is it made, that it can furnish so many constituents to the animal and vegetable worlds? We know it to be a transparent vapor or gas, a material substance possessed of weight, held by the attraction of gravitation to the surface of the globe, with which it revolves, and which it surrounds. That the atmosphere does not consist wholly of one kind of matter is readily shown by the following experiment: A float made of cork, upon which is placed a small porcelain basin, containing a piece of phosphorus about the size of a pea, is placed in a basin full of water, and ignited, and over it is inverted a glass jar. The phosphorus burns brilliantly for a time, producing white vapors of phosphoric acid, (which must not be taken in with the breath as they are poisonous), which soon dis solve in the water, which then rises in the jar till it fills one-fifth of the space previously occupied by the air. By the combustion, therefore, of the phosphorus, something has been removed from the air in the jar, equal to one-fifth of its whole bulk. That it is not simply one-fifth of the volume of the air which has been removed is shown by the fact that what remains, although clear and colorless as the air, is possessed of different properties. A lighted taper in troduced into the jar is at once extinguished, proving that what is now there will not support combustion. In fact, that portion of the atmosphere forming one-fifth of its bulk, that will alone support combustion, has been consumed by the burning phosphorus, and has entered into union with it. In scientific language, the phosphorus has combined with the oxygen to form the compound, phosphoric acid. The matter so removed is Oxygen; what is left behind is called Nitrogen. Of these two invisible gases, oxygen and nitrogen, atmospheric air is mainly composed, in the exact proportions of 21 per cent. of oxygen, and 79 per cent. of nitrogen. Oxygen is a colorless, tasteless, and inodorous gas,. heavier than air, and very widely diffused throughout nature. It is a constituent of earth and water, and indeed of most created things. It is an element, and the most abun-dant of all the elements. All substances that burn well in air, burn with increased brilliancy in oxygen. So necessary is it to life, that oxygen used to be called "vital air." Without oxygen there would be no decay; none of that change by which dead matter is destroyed. Heaps of fallen leaves and other refuse would collect upon the earth, were it not for this busy, active oxygen; under its magic touch they assume new and simpler forms of existence, being for the most part changed into invisible gases. In those grand processes of nature-decay, respiration, and combustionoxygen plays the chief part. It enters into combinations with the decaying matters and the living body, and is either removed from the atmosphere, or enters it again in some new form. Yet, necessary as it is, we could not live in an atmosphere of pure oxygen! If a rabbit is made to breathe pure oxygen, although it does not appear to suffer immediately, yet its respiration soon becomes very rapid, the circulation of the blood is quickened, and the animal shows signs of excitement. Symptoms of weakness appear,. which are followed by insensibility, and death takes place in a few hours. This result is owing to the rapid destruction of the muscular tissues, which exceeds their powers of repair. Mixed with oxygen, the atmosphere, when dry, contains 79 per cent. by measure of nitrogen. Like oxygen,. it is colorless, tasteless, and free from smell. But it does not support either respiration or combustion. Its presence in such large proportion in the atmosphere is a proof that it is not injurious to respiration, while it serves to moderate the activity of respiration as well as of combustion. From the difficulty with which it enters into combination, nitrogen is admirably adapted for this purpose; indeed, there is no other gas which could be made to take its place. Next to oxygen and nitrogen, the most important constituent of the air is watery vapor. Its amount varies greatly; the higher the temperature, the more vapor it contains. Whenever a body of air, saturated with moisture, is cooled, it must set free a portion of that moisture in some form or other, either as rain or snow, hail, mist or dew. The simplest form of deposit is dew. This deposit is most abundant on rough surfaces. So long as the sun shines upon soils and plants, they absorb more heat than they lose, and consequently become warmer. But, as soon as the sun sets, radiation continues from them without any sensible addition of heat; they cool rapidly, and, by lowering the temperature of the air in immediate contact with them, cause it to deposit on their surface a portion of its moisture in drops of dew. Dew is deposited most abundantly on cloudless nights; for, when the sky is hidden by clouds, these will stop, or radiate back to the earth, a portion of the heat received from it, and prevent any great lowering of the temperature of the air near the surface of the ground. Radiation at night frequently becomes most destructive to vegetation in spring and autumn by lowering the temperature of the air below the freezing point of water, when the dew is deposited in the form of frost. The temperature at which the atmosphere begins to deposit moisture is called the dew point. Rain has been already mentioned as a very pure form of water. In falling through the air it literally washes the latter, and frees it from those gaseous and other impurities which in places contaminate it. These substances it dissolves, and carries through the soil into the circulation of the plants. When the temperature in the upper regions of the air is below the freezing point (0° C.) the moisture freezes and falls as snow. Whether the snow-flakes are formed at once by the freezing of the moisture in the cloud, or whether the frozen particles unite with a thousand others in their fall, has not been fully ascertained. The exquisitely beautiful forms assumed by the snow-flakes must be closely examined to be fully appreciated. The effects of the physical changes of the vapor of the air into liquid water and solid snow, are of great importance. Whenever a solid melts or is made liquid, and whenever a liquid is changed into vapor, a certain amount of heat is required. And so also when a vapor turns into a liquid, or a liquid becomes solid, the same amount of heat is set free. In the act of melting, the water which flows from ice is found to have the same temperature as the ice; heat is absorbed; and again, when water is changed into ice or snow, the same amount of heat is set free as was necessary to thaw the ice or snow. Now as 79° C. are required to thaw one pound of ice, so as to change it into water having the same temperature as the ice, so likewise is the same amount of heat (viz., 79 C., or 79 units of heat) set free, when one pound of water is changed into one pound of ice. The formation of snow is consequently attended by a rise of temperature! A fall of snow produces also peculiar effects on vegetation. From its porous character the snow in its fall from the sky entangles much air, which assists by its nonconducting power for heat in protecting the tender shoots from the frost. A fall of snow one inch in depth affords in melting one-tenth of an inch of water. Far behind oxygen, nitrogen, and watery vapor in quantity, but always present in the air, is carbonic acid. There is one volume or measure of carbonic acid in 2,500 volumes or measures of air. Its sources are most numerous, the great processes of respiration, decay, combustion, germination, and fermentation, being each and all productive of carbonic acid gas. Carbonic acid is not an element, but a compound of two elements, carbon and oxygen. One of the commonest forms of carbon is known as charcoal, and whenever charcoal is fully burned, whether in air or in oxygen, carbonic acid results. As carbon is contained in all combustible matter, it is easy to conceive how every variety of combustion will introduce a certain quantity of carbonic acid into the atmosphere. The blood also contains carbon in union with other elements. Every time a breath is drawn, a portion of the inspired air, but especially of its oxygen, is taken up by the blood circulating through the lungs, and the air which is returned by expiration is considerably changed. In the place of oxygen which it gave up, it contains a similar quantity of carbonic acid. Breathed air contains one hundred times as much carbonic acid as the fresh air before it is breathed. This breathed air will neither support combustion, nor serve any longer for respiration. If we breathe through a glass tube into a bottle containing lime water (made by shaking a little slaked lime with water and decanting the clear liquid) the latter becomes milky from the formation of chalk or carbonate of lime. Now air in its ordinary condition would not produce this change, on account of the exceedingly small quantity of carbonic acid contained in it That carbonic acid is evolved in the process of decay is easily proved. It is found that the air, in a confined space, over decaying matter, soon ceases to have the power of supporting combustion. If this were not so, and did not leaves and plants in their decay resolve themselves into gases, what would become of all the refuse vegetable and animal matter, the accumulation of ages of living and dead organized beings? This question at once suggests another. Since carbonic acid is always present in the air, being continually produced by respiration, combustion, and the like, at the expense of the oxygen, how does it happen that the oxygen does not unduly diminish, and the carbonic acid increase? It suggests an inquiry into the manner in which plants obtain the carbon which they require for their growth. A very simple experiment will prove the presence of carbon in every part of a plant. The charring of a leaf, stalk, or piece of wood is due to the presence of carbon, which separates as charcoal, and which is not burned (or oxidized) so readily as those other constituents of plants which contain hydrogen. As this carbon is an elementary substance, and therefore can not be produced by the plant itself, it must be derived from some external source. Now, the only sources from which the constituents of plants are derived, are the soil, water, and the air. From the soil come all its mineral ingredients, or all those which are already, so to speak, burned or oxidized, in fact, all that which remains in the solid state after the plant is completely burned-its ash. This mineral matter or ash, however, rarely exceeds 111⁄2 per cent. of its weight. Again, great as is the influence of water, or hydrogen oxide, on the plant, both as forming one of its chief constituents and as contributing largely in other ways to its growth, it can only furnish the two elements, hydrogen and oxygen. So, then, the principal part of the carbon must come from the air, and carbonic acid is the only compound in the air which contains it. The leaves have the power of decomposing carbonic acid under the influence of the sun's light; they keep the carbon for their own use, and restore the oxygen to the air. Owing to its solubility in water, some of the carbonic acid of the atmosphere is washed by every shower into the soil, and is thus conveyed to the plants by another channel, viz., through their roots. This solubility of carbonic acid enables aquatic plants to get their due supply of it, which they require as much as land plants. It is evident that the carbonic acid of the air would be rapidly exhausted by the constant processes of vegetation, were there not some equally continuous source of supply. Such a source, however, is found in the respiration of animals, and in the decay of both animal and vegetable matter. Thus there is a mutual interchange of kind offices between animals and vegetables, by which the purity and the uniformity of the air is maintained, the one supplying to the air what the other requires for its existence. Animals respire oxygen and exhale carbonic acid; plants absorb carbonic acid, keep back the carbon, and give out oxygen. To the constituents of the air already enumerated, viz., oxygen, nitrogen, watery vapor, and carbonic acid, must be added Ammonia, which, in combination with carbonic acid as Ammonium Carbonate, is always present, although only in minute quantities. One million parts of air contain only about one-quarter of one part by day, and about twice that quantity by night. The comparative smallness of the quantity present by day is due partly to its consumption in the nutrition of plants, and partly to the fact that the ammonia which accumulates in the day and night is dissolved and deposited by the morning mists. At all times, and from many sources, ammonia is given out to the air. There can be neither decay or putrefaction without its development, nor can any material containing nitrogen and hydrogen be burned without its formation. It is also given out in respiration and in perspiration. Not that it is ever generated in any of these processes in equal quantity with car |