ELEMENTAL BODIES. With regard to the elements of matter, chemists have agreed amongst themselves to consider all those bodies as simple which have not yet been decomposed. As already mentioned, the simple bodies are fifty-four in number, and for the convenience of study they have been arranged into classes. One system of classification is dependent upon the elements being metallic or non metallic. The non-metallic elements are divided into gazo lytes, or bodies which are permanently gaseous; metalloids, or bodies which resemble the metals in their chemical relations; and halogens, or bodies which produce salts when in union with the metals. The nonmetallic elements are thirteen in number; namely, oxygen, hydrogen, nitrogen, chlorine, iodine, bromine, fluorine, carbon, boron, silicon, sulphur, selenium, and phosphorus. The three first are the gazolytes, the next four the halogens, and the remaining six the metalloids. The metallic elements are forty-one in number, namely, potassium, sodium, lithium, calcium, barium, strontium, magnesium, aluminum, thorium, glucinum, zirconium, yttrium, manganese, zinc, iron, tin, cadmium, cobalt, nickel, arsenic, chromium, vanadium, molybdenum, tungsten, columbium, antimony, uranium, cerium, bismuth, titanium, tellurium, copper, lead, mercury, silver, gold, platinum, paladium, rhodium, osmium, iridium. These metallic elements are again divided into three orders, the first twelve being the bases of the alkalies and earths; the next twenty-one being metals whose oxides are not reduced by heat alone; and the remaining eight, metals whose oxides are reduced by a red heat. From these fifty-four elementary substances is formed all the beautiful variety of terrestrial objects. Nor is there any thing either very wonderful or mysterious in this fact, since, as we have seen, any given two of them, if made to unite in different proportions, can be made to produce the most opposite substances. These, again, united with each other, give rise to new compounds, which are susceptible of being combined, and so on through an almost infinite rotation of chemical union.* HEAT OR CALORIC. In our investigations of the phenomena of the material universe, we perceive two kinds of motion, which result from the two principles attraction and repulsion. Of the former we have already spoken, and it only remains to say a few words upon the latter. Repulsion, like attraction, takes place both at sensible and insensible distances. The former is exemplified by the flying off of the same light bodies which have been first attracted, after they have been some time in contact with a piece of excited resin or glass, and also by the recession from each other of the two similar ends of two magnetised needles. Repulsion at insensible distances, which is chiefly excited by heat, or, as it is called in chemical language, caloric, is exhibited in a great variety of phenomena. The principal effects of heat are expansion, liquefaction, vaporisation, evaporation, and ignition. With few exceptions, bodies are capable of expansion by means of heat; the gases being the most expansive, fluids less so, and solids least of all. When the iron rim of a coach or cart wheel is to be put on, it must first be heated to a considerable degree. The reason of this is obvious when hot, the circle is larger than when cold, and thus slips easily upon the wheel; as it cools, the circle decreases, and thus firmly binds the woodwork together. The expansion of aëriform substances is illustrated by a bladder being partly filled with cold air, and held before the fire. The air will swell out with the heat, and become in some instances so ex * From recent experiments in chemistry, there is reason to believe that all substances whatsoever are but modifications of one primitive substance. The absolute truth of this startling theory remains to be practically demonstrated. panded as to burst the bladder. As regards fluid bodies, the same fact is illustrated in the cases of the thermometer and barometer. By the accession or loss of heat, the alcohol or mercury expands or contracts, as shown by the index attached. The general law, therefore, is, that the expansion and contraction of matter are, with a few exceptions, dependent upon the increase and diminution of heat. The quantity or condition of heat that is discoverable by the thermometer, or by the organs of sensation, is called temperature. We are unacquainted with the extremes of temperature relative either to heat or cold. It has been compared to a chain, the extremities of which are concealed from view, whilst only a few of the middle links are exposed to observation. Although the universal result of an increase of temperature is an increase of bulk to the body thus subjected to heat, yet all bodies are not alike expanded by the application of the same quantity of heat. It of course follows as a general law, that different bodies at equal temperatures do not contain the same quantities of caloric. This quality of matter is called the capacity of bodies for heat, and the quantity of heat which is necessary to raise any particular body to a certain temperature, is called its specific caloric. Heat, however, in some cases causes contraction instead of expansion. Thus water is of greater bulk at a temperature of 32° (the freezing-point) than it is at 394°. Some solids, also, as iron, antimony, bismuth, and many salts, contract when melted and expand as they become solid. Vaporisation is the rapid production of a thin vapour, as when water is converted into steam. The boiling. point of water, in a vessel exposed to the ordinary atmospheric pressure, is 212°, and although more heat be applied to the vessel in which it is contained, the temperature of the water is not increased. If this degree of heat be continued, the watery particles separate from each other and become steam or vapour. Steam is colourless, transparent, and invisible, resembling the atmosphere, and is 1696 times greater in bulk than water. Steam may be condensed, or its particles brought nearer to each other, either by removing the heat which is the cause of the repulsion, or by mechanical pressure, and the result is its return to the form of water. Water can be made to boil at a lower temperature than 212° by removing the pressure of the air. If a flask be half filled with water, the water made to boil, and as the steam escapes, a cork be put into the mouth of the flask, upon the heat being removed, the water will continue to boil, the heat in it being sufficient for that purpose when there is no pressure from the air. If the flask be put into cold water, the boiling will increase, from the steam being more effectually condensed; whereas, if the flask be put into boiling water, so as to prevent the condensation of the steam, the ebullition will immediately cease. Steam, as is well known, from its great force, and the manner in which it can be applied to propelling machinery, is of the greatest usefulness in manufactures. Distillation is the converting of a liquid into vapour, which is afterwards carried off through a pipe and condensed in what is called a refrigerator. This is a vessel filled with cold water, round the inside of which the pipe is wound; and as the vapour passes through the pipe, it is condensed by the lower temperature of the water in the vessel. Liquid substances give off vapour from their surface at temperatures below the boiling-point, which is termed evaporation. It is called spontaneous evapora tion when this takes place at the ordinary temperature of the atmosphere. A large quantity of vapour is given off from the surface of the earth and sea, which eventually forms clouds, or is condensed into rain and dew. Evaporation always produces cold when heat is not applied; the heat necessary for it being derived from surrounding objects. A current of air or a higher temperature tends greatly to quicken evaporation, as may be observed in the rapidity with which the surface of the earth dries when a brisk wind passes over it. All substances become luminous when heated to 800* processes. In vegetation it is indispensable, as without it plants do not acquire their due elementary constitution. They are weakly, inodorous, and fail to exhibit their natural colour. Vegetables which grow in the dark have a blanched appearance. The power of light to dispel vegetable colours is manifest in bleaching, where a dingy web becomes pure and white by exposure to the sun's rays. Its energy is still more decisively seen in the influence which it exerts in promoting chemical combination and decomposition, and the latter effect has been made use of as a measure of its power. Light enters into a kind of transitory union with certain substances, rendering them visible in the dark. Bodies which possess this property are called phosphorescent; such are the shells of fish, the bones of land animals, marble, lime-stone, and the like. The glow-worm is a remarkable instance of phosphorescence in living animals. in the dark and 1000° in daylight, unless they are con- | way is conspicuous in a variety of natural and artificial verted into vapour at a less elevated temperature. The light is red at first, and in this state a body is said to be in a state of ignition. If more heat is applied, the body becomes white, when it is said to be incandescent. When a body changes from the solid to the fluid state, there is a quantity of heat absorbed, which has no effect in raising the temperature. This has been called latent heat, a discovery effected by Dr Black, and which we shall shortly explain. For a demonstration of this doctrine, we may have recourse to water. If ice at a temperature below 32° be exposed to a warmer atmosphere, it receives caloric, and gradually rises to that point of the thermometrical scale. But as soon as it reaches it, the rise of temperature ceases, the ice begins to melt, and during the whole period of its liquefaction, its temperature, as also that of the water flowing from it, remains stationary at 32°. It is evident that, as caloric has continued to be communicated, a quantity of it has disappeared, and become absorbed during the fusion. The same phenomenon takes place when a liquid is converted into vapour; and the inference drawn from is, that when a body passes from one state into another, a quantity of heat or caloric is lost, becomes latent, or passes into the body without raising its temperature. Dr Black was of opinion that this latent heat became chemically combined with the solid, and was the cause of fluidity. Dr Irvine, his pupil, took a different view of the subject. He supposed that the absorption of heat into the latent state is not the cause of liquefaction and vaporisation, but the effect. The absorption he attributed to what is called change of capacity for heat, or that quality of matter which causes one kind to be more or less heated than another, by the addition of the same quantity of heat. He concluded, as a general law, that the capacity of all solids for heat is increased by fusion, and that of all fluids by evaporisation. It is impossible to enter further into this interesting subject at present; but, before quitting it, we may mention an exception to the law of expansion by heat in the case of water. It is well known that water freezes at 32°, but it does not increase in density below 394. It is then at its maximum, and above or below that point its density diminishes. Hence ice is specifically lighter. The earth alumina, which will be afterwards described, also possesses the remarkable property of being contracted by heat. LIGHT. The nature of light, like that of heat, is still unknown to us. There are two theories respecting it: the first is, that light is a substance emanating from the sun and from all luminous bodies, from which it is projected in right lines with great velocity; the second is, that the sensation of light is produced by the vibration of a subtile fluid filling space-and is hence called the undulatory theory. Luminous bodies, according to this view of things, are merely stimuli, which excite these vibrations. An examination of these theories, however, cannot be here entered into. The connexion between light and heat is so obvious, that it is scarcely possible to examine the one independently of the other. If a mass of iron be put into a fire for some time, no change is produced except the expansion of the metal and the elevation of its temperature. Gradually, however, as the heat is communicated, a remarkable occurrence will be observed. The iron becomes ignited or red-hot; in other words, it emits light, and renders objects visible. The original sources of light are, first, the celestial bodies, as the sun and stars; and, secondly, terrestrial bodies, as a common fire or candle. Light passes freely through the atmosphere, and, striking upon objects, is reflected or thrown back by them; and thus they become visible. By means of a wedge of glass called a prism, light can be separated into seven colours, which are violet, indigo, blue, green, yellow, orange, and red. But it is only with the chemical agency of light that we have to do. Its influence in this A remarkable recent invention, the Daguerreotype, is wholly dependent for success upon the action of light. It consists in having a thin plate of silver, prepared with iodine, so placed that the rays of light reflected from an object to be sketched will fall upon it. This is done by putting the plate in a camera-lucida, and the action of the light upon the iodine and silver is such, that when the plate is subjected to the vapour of mercury a complete representation of the object is given. A beautiful illustration of the action of light may also be seen in photogenic drawing. Paper for this purpose is prepared by steeping it in a weak solution of nitrate of silver or bichromate of potassa. The paper must be kept from the light during the preparation; and if it is now exposed to the sun's rays with a leaf or other object upon it, a complete representation of the object will be obtained. The part exposed to the sun becomes darkened, while that covered by the leaf remains of a light colour. For a further definition of the principles of light, we refer to the article OPTICS. COMBUSTION. Combustion is a process not yet perfectly understood. It is usually described as the union of a combustible body with a supporter of combustion, attended with the evolution of light and heat. The combustible body is that which burns, but, in general, will neither support combustion, nor burn except in presence of a supporter of combustion. The supporter, again, does not itself burn, though necessary to the burning of a combustible. Oxygen gas, the ingredient which enables the air to support combustion, possesses, when pure, a high degree of the supporting quality. If a lighted taper, a combustible body, be plunged into this gas, the taper burns vividly, but the gas itself is not ignited. If, on the other hand, the taper be plunged into a combustible gas, such as pure coal gas, the gas is instantly ignited, but the taper is extinguished. These are general rules, relating to supporters of combustion and combustible bodies. By examining the effects of combustion, in the case of a candle burning in the air of the atmosphere, it has been proved pretty clearly that a chemical action of the following kind takes place :-The combustible matter of the candle consists chiefly of two simple bodies, hydrogen gas and carbon, while oxygen is the supporter of combustion in the air. On burning a candle under a bell-shaped glass, filled with common air, a fluid gathers on the glass, which proves, on examination, to be pure water. The hydrogen of the burning body has here entered into combination with part of the oxygen of the air, forming water, a compound of the two. The carbon of the burning body also enters into union with a portion of the atmospheric oxygen, forming carbonic acid gas, which is left floating in place of the original quantity of oxygen. The presence of these can be proved, and the same process takes place in the case of coal, wood, &c. Thus it is seen that combustion only changes the forms of the burned bodies, and does not annihilate them. Plants, moreover, will soon extract the carbon again | brown by it. The liquid added previously has exactly from the carbonic acid, and the hydrogen from the water, leaving the oxygen once more in the atmosphere to support combustion, and fulfil its other uses; while the other principles render wood combustible anew. This round of changes goes on unceasingly, without any ingredient being destroyed. The phenomena of combustion are thus so far explicable, but unfortunately the source of the light and heat yet remains a mystery. It is unknown whether the chemical action is the cause of the light and heat being evolved, or the evolution of these the cause of the chemical action. Where all is doubt, it would be vain to dwell on this point. The laws stated respecting combustible bodies, and supporters of combustion, only apply generally, it is also to be observed, and under ordinary circumstances. Under the oxy-hydrogen blowpipe, the most incombustible bodies can be made combustible; and combustion can be shown to take place under an exhausted receiver, without the presence of any supporter, at least of a gaseous kind. We must wait in patience for a solution of these difficulties, until the genius of man has discovered more delicate instruments of philosophical investigation than any with which we are as yet acquainted. AIR AND WATER. Air-By the examinations of modern chemists, it has been shown that air is not an element, but is a compound body, consisting chiefly of two gases, oxygen and nitrogen. It also appears that the oxygen is the really active agent in relation to animal respiration, and that the nitrogen is a mere diluent in the mass, on the same principle as water may be made a diluent of spirits. We subjoin the exposition of Mr Hugo Reid (Chemistry of Science and Art) on this subject:-"The air consists mainly of nitrogen and oxygen, in the proportion, if these ingredients are alone regarded, of neutralised the carbonic acid; and in doing so has combined with an equivalent proportion of that substance, the quantity of which is thus indicated. The presence of watery vapour in the air may be demonstrated by exposing chloride of calcium or caustic potash. It absorbs the moisture, melts, and is found to have increased in weight. Strong sulphuric acid abstracts the moisture from air, increasing in bulk and becoming weaker. The dewpoint hygrometer also indicates the presence of moisture in air, and points out the precise quantity. The four bodies which enter into the com. position of the air are regarded as mechanically mixed, not chemically combined with each other. It is known from the nature of aërial bodies that they would mix thus, though not combined-that they would not separate and arrange themselves according to their respective specific gravities-but would each be diffused through the whole space to which it had access. The only two likely to be chemically combined are the nitrogen and oxygen; and the great facility with which the oxygen is separated from the nitrogen, as well as not being in equivalent proportions, shows that they are not in close chemical union. The oxygen is the chief agent in the important operation of breathing or respi ration of animals. Each individual is supposed, on an average, to breathe about twenty times every minuteto take in about sixteen cubic inches of air (12.8 nitrogen + 3.2 oxygen) at each inspiration-to return nearly the whole of the nitrogen (12-8 cubic inches), and 4-5ths of the oxygen (2.56 cubic inches), and to replace the remaining 5th of oxygen by an equal volume of carbonic acid gas (64 cubic inch)." The oxygen of the air is the great means of procuring heat and light, by its action with combustible bodies. Water. Water was also at one period believed to be a simple element in nature; but this supposition has given way before the examination of chemists. Water is now known to be composed of oxygen with hydrogen gas, in the relative proportions of 8 of oxygen to 1 of hydrogen. Into these substances can it be resolved by the action of electricity or fire, but at such a cost as to render the process unsuitable for economic purposes. Pure water, in chemistry, is called an oxide of hydrogen. It may be formed by exploding a mixture of oxy Sea-water (see article OCEAN) contains, in 1000 parts, about 46 of foreign matters, chiefly chloride of sodium. Its specific gravity is 1027.* Mineral waters, in a similar manner, contain various foreign bodies; as, for example, carbonated waters, which contain carbonic acid; sulphureous waters, which hold surphureted hydrogen; and chalybeate waters, which contain sulphate or carbonate of iron. Water may be impure, either by the chemical union of these and other foreign bodies, or by the mechanical mixture of substances. The latter may It also contains, as constant ingredients in every situation, a little carbonic acid gas and vapour of water. Ingen and hydrogen in a tube by the energy of electricity. volume, the carbonic acid forms about 1-2000th part; or 0.5 parts in 1000 by measure; which is equal to 0-75 parts in 1000 by weight. In some situations the carbonic acid is so much as 0.62 volumes in 1000-at other places, only 0.37 volumes in 1000. Its proportion is greater in summer than in winter, during night than in the day time, in elevated situations than on the plains. The watery vapour is more variable in proportion. The mean is supposed to be about 10 parts in 1000 by weight, 15 by volume. The quantity is determined by the temperature, heat being the sole cause which sustains the vapour in the aërial state. The various methods of *Specific gravity is the relative gravity or weight of any body or analysing atmospheric air proceed upon the principle of substance, compared with that of some other body which has been withdrawing the oxygen. This may be done by a stick fixed upon as a standard. By universal consent, pure distilled of phosphorus suspended over water or mercury in a water has been assumed as the standard; and it fortunately hapjar of air; or, which is the best mode, by the combus-pens that a cubic foot of pure water weighs exactly 1000 ounces tion of hydrogen mixed with the air to be examined. The presence of carbonic acid gas is shown by agitating a quantity of air with lime-water. The carbonic acid and lime unite, and form the insoluble carbonate of lime, which, diffused through the liquid, renders it milky and opaque. On exposing to the air a saucer of lime-water, a thin crust or pellicle of carbonate of lime will be soon found on the surface of the liquid, formed in the same manner. The quantity of carbonic acid may be judged of by passing a little solution of caustic potash into a vessel of air over mercury, and observing how much of the gas is withdrawn, this substance removing the carbonic acid; or, by adding water of baryta gradually to a large quantity of air in a bottle, and agitating. The carbonic acid neutralises the baryta; and the liquid is added until there is a slight excess of baryta, as indicated by a slip of turmeric paper being now rendered avoirdupois. Water is indicated by unity-thus, 1. When, therefore, it is expressed that any body has a specific gravity of 2, then, bulk for bulk, it is just twice the weight of water. If there be more figures than one, and there be a dot or point between them thus, 25-the unit is here divided into ten parts, and the body is twice and five-tenth times, or two and a half times, heavier than water. If two figures occur-thus, 10-40-the unit is supposed to be divided into an hundred parts, and the body is ten and figures, the unit is supposed to be divided into a thousand parts; forty-hundredth part times heavier than water. If there are three if four, into ten thousand parts, and so on; the number and value of the figures always indicating the exact specific gravity of the body according to the above principle. Common air is sometimes taken as a standard with which to compare gases, as in the instances mentioned in the text. It is a simpler and more intelligible way of comparing the relative weights or densities of aerial substances. But all the solids and fluids are estimated with regard to water. generally be removed by filtration, but when the union | The following are some of the characters which distin is chemical, distillation and other processes are requisite to produce a pure liquid. In nature, water is never altogether pure. When it contains a chemical compound of lime, it is said to be hard, and in this condition it decomposes the soap which is employed with it. ACIDS. Acids are a most important class of chemical compounds, and have the following characteristic properties:-The greater number of them have a sour taste, and are very corrosive. With few exceptions, they change vegetable blues to red, they are mostly soluble in water, and they unite with the alkalies, earths, and metallic oxides, forming what are called salts-an order of bodies of the highest importance in the arts, manufactures, &c. Some acids are destitute of a sour taste, but their affinity for the three classes of bodies above named is a universal characteristic. Acids are all compound bodies, and some of them have more than one basis or radical. There are a number of acidifying principles, but oxygen (which shall be immediately described) is the most extensive one. The acid is distinguished by the name of its base, and its degree of oxidation, that is, the quantity of oxygen it contains, by the termination of that name in ous or ic, or the prefix hypo (under). The highest degree of oxygenation is marked by the termination ic, as nitric acid, and the salt which is formed from it is made to terminate in ate; the next by that of ous, as nitrous acid, and the salt which is formed from it is made to terminate in ite; and the lowest by hypo, as the hyponitrous acid. Sometimes oxygen combines in a greater quantity with the acidifiable radicals, in which case the product is said to be superoxygenated. All acids are not susceptible of these various degrees of oxygenation, some being limited to only one. There are a considerable number of acids, and the number is continually increasing by the discovery of new ones; but of the most important there are few, and these we shall notice as we come to treat of their bases. SALTS. This term has been usually employed to denote a compound, in definite proportions, of acid matter with an alkali, earth, or metallic oxide. When the proportions of the constituents are so adjusted that the resulting substance does not affect the colour of infusion of litmus or red cabbage, it is then called a neutral salt, because the peculiar powers of both bodies are suspended and concealed; they are rendered neutral or inactive. When bodies combine in such a way as to satisfy their mutual affinities, they are said to saturate each other. When the predominance of acid is evinced by the red of these infusions, the salt is said to be acidulous, and the prefix super or bi, is used to indicate this excess of acid. If, on the contrary, the acid matter is deficient, or short of the quantity necessary for neutralising the alkalinity of the base, the salt is then said to be with excess of base, and the prefix sub is attached to its name. These must be understood, however, only as general rules. There are exceptions to be found in the case of some salts, as the compounds formed by an acid and an alkali, an earth, or a metallic oxide, are denominated. For example, a certain salt formed by nitric acid and lead, though the acid be perfectly neutralised, reddens vegetable blues; and a salt formed by boracic acid with soda retains the powers of an alkali, in the respect in question, though with a double proportion of acid in it. METALS, OXIDES, EARTHS, AND ALKALIES. We arrange these classes of substances together, because, although they are to a certain extent distinct, yet they have all a very remarkable relationship, as we shall shortly see. Many of the metals, such as iron, lead, &c., are familiarly known to every one, but there are a great many others which are very rarely to be met with. guish metals from other bodies:-They are, for the most part, hard and heavy, and are all opaque; insoluble in water; they possess a peculiar lustre; admit of being so highly polished as to reflect light; are capable of being melted by heat, and of recovering their solidity by cooling; most of them may be extended by hammering, and all are rapid conductors of electricity. They are of various colours, and require different degrees of heat to fuse or melt them. They generally occur in the earth in what are called veins, and are seldom found in the pure metallic state, but generally in combination with some other substance, in which state they are called ores. The metals, which are all simple bodies, will be individually described afterwards. Most metals, when subjected to heat until they become melted, combine with the oxygen of the atmosphere, and form what are called oxides. Oxides are destitute of those properties which distinguish the metal from which they are formed. Instead of being bright, shining, elastic, and ductile substances, they are generally a dry, earthy-looking powder. Other substances besides metals, however, are capable of being converted into oxides; and it must be kept distinctly in view, that in every case there is not so much oxygen imparted as will produce acidification. Oxygen frequently combines in various proportions with a substance, rendering it an oxide, but without advancing it to the state of an acid. In order to distinguish each compound thus formed, the language of chemistry is very systematic. The first is called a protoxide; the second, a deutoxide; and the third, a peroxide. The term Earths was formerly, and is still, but in a modified sense, applied to several substances which compose all the various rocks, stones, gems, mountains, and soils, covering the surface of the globe. They are tasteless, inodorous, dry, uninflammable, sparingly soluble, difficult of fusion, and of moderate specific gravity. These bodies will be more particularly described when we come to treat of their metallic bases. Alkalies may be defined as bodies which combine with acids. so as to impair or neutralise their activity, and produce what are called salts. They are distinguished by properties the reverse of acids, and the two classes are generally looked upon as antagonist substances. Besides the power of neutralising acids, there are four alkalies, namely, potash, soda, ammonia, and lithia, which possess the following properties in a high degree:-They change vegetable blue to green, red to purple, and yellow to a reddish brown; they have an acrid and urinous taste; they are powerful corrosives of animal matter, with which they combine so as to produce neutrality; they also unite with oils and fats, forming the wellknown substance soap; they combine with water and alcohol in any proportion. Four of the earths, namely, lime, baryta, strontia, and magnesia, possess alkaline properties to a considerable extent, and are hence called alkaline earths. These bodies differ from the pure alkalies, inasmuch as they become insoluble in water when neutralised by carbonic acid. Moreover, alkalies possess the power of changing vegetable colours after being saturated with carbonic acid, and by this criterion they are distinguished from the alkaline earths. It was long observed that the properties of earths very nearly resemble those of the compounds of oxygen and metals called metallic oxides; but it remained for the brilliant genius of Sir Humphry Davy to show that both the earths and alkalies are metallic oxides. It thus appears, then, that the globe is one vast mass of various kinds of metals, disguised by various substances, but chiefly by oxygen. Earths and alkalies are simply metallic oxides; whilst a farther impregnation of these substances with oxygen produces an acid; and, lastly, the union of acids with alkalies, &c., gives rise to that very numerous and important class of substances called salts. Of the elemental substances at present known, six seem capable of combining with all the others. When combined with a certain portion of the other simple bodies, they form acids; and when with the rest, they constitute bases or alkaline bodies, which are capable of uniting with and neutralising the acids, as we have formerly observed. To these six bodies the name of supporters of combustion has been given. The eighteen bodies, which, when combined with the supporters, become acids, have been distinguished by the name of acidifiable bases. The thirty-one bodies, which, when united with the supporters, become alkalies, have been called alkalifiable bases. The simple supporters of combustion are as follow:-Oxygen, chlorine, bromine, iodine, fluorine, and sulphur. OXYGEN. Oxygen gas is a permanently elastic fluid, that is, one which no compressing force, or degree of cold, hitherto applied, has ever been able to reduce to a liquid or solid form. It forms, as we have already observed, one of the constituents of the atmosphere, is colourless, and destitute of taste and smell. Its specific gravity is 11111, that of common air being reckoned unity. Combustible bodies burn in it with more brilliancy, and more light and heat is evolved, than when combustion takes place in the atmosphere. If a candle, the wick of which is red-hot, be introduced into a vessel containing oxygen, the candle will instantly be lighted. Oxygen has the power of combining with every other simple body; the multifarious compounds which it thus forms, such as oxides, acids, and bases, or alkalies, we have already adverted to. In the act of respiration, oxygen, in the nice economy of the human body, is made to unite with it, and becomes a portion of the human frame. Vegetables also inhale and exhale it at certain seasons, so as admirably to supply what is absorbed by animals. It is the intensely rapid chemical union of oxygen with the combustible body, which gives rise to the light and heat in our common fires, candles, &c. It may be readily procured from a variety of substances, as, for instance, from saltpetre or the black oxide of manganese. These may be introduced into a gun barrel, with the touch-hole plugged up. From the orifice of the barrel let a tube be conducted into an inverted glass jar, filled with water. When the other extremity of the apparatus is subjected to heat, the oxygen gas is expelled from the manganese, and entering the glass jar, displaces the water and fills the vessel. This is a cheap and easy method of obtaining this remarkable aëriform body. Oxygen can be prepared by putting 1000 grains of binoxide of manganese into a retort with an equal weight of aqueous sulphuric acid. This is done by means of a retort fixed over a spirit-lamp. The bent tube of the retort enters a pneumatic trough, in which jars are placed for receiving the gas as it passes from the neck of the retort. HYDROGEN. Hydrogen gas is a permanently elastic fluid, transparent and colourless, and when pure, destitute of taste or smell. It can scarcely be said to exist in an isolated state, but it forms one of the constituents of water, from which it can be disengaged by various simple processes. It is the lightest body with which we are acquainted, and is employed in combination with other gases to inflate balloons. A bladder filled with this gas will ascend in the atmosphere, in the same manner as a piece of cork or wood plunged by force to the bottom of a vessel of water. Hydrogen will not support combustion, but is itself remarkably combustible. When one volume of oxygen is mixed with two of hydrogen, it burns with a loud explosion, by an electric spark, or the contact of a red-hot wire. The product of this experiment is water. It is said that a few cautious draughts of this gas may be taken, but it cannot be inspired for any length of time without occasioning death. Frogs live in it for a long time, showing these animals to be very tenacious of life. By far the most important mpound of hydrogen with any other substance is that with oxygen, forming the indispensable fluid which covers nearly two-thirds of our globe, water. It unites with the other supporters of combustion; but the compounds, except muriatic acid, already mentioned, are not of any great importance. Hydrogen may be prepared by putting 500 grains of zinc into a common beer bottle, and pouring upon the zine three ounces of water and five drachms of aqueous sulphuric acid. The hydrogen is disengaged as the acid, the oxygen in the water, and the metal combine. By means of a bent tube from the bottle, the gas can be conveyed into jars placed in a trough. AZOTE, OR NITROGEN. This gas is permanently elastic, transparent, colourless, and inodorous. It is a very little lighter than oxygen. When breathed, it destroys animal life; and a burning body, if immersed in a jar containing it, is instantly extinguished. It is not combustible; it enters extensively into combination; it is an abundant element in animal matter; and its existence in such large quantity is a chief distinction between the constitution of animal and vegetable life. Its existence in the atmosphere we have already adverted to. Whether it is chemically united with oxygen in that compound, or only mixed with it, is not precisely known. That it has the property of combining with all the supporters of combustion, there can be little doubt; but the subject has not yet been thoroughly investigated. With oxygen it unites in no fewer than five proportions; by far the most important being Nitric Acid, or Aquafortis.-This virulent substance is a compound of one volume azotic, and two and a half volumes of oxygen gas. Common nitric acid is of an orange colour, on account of its containing a little muriatic acid, as also a little sulphuric acid and water. Light has likewise an effect upon it. The specific gravity of the strongest procurable nitric acid is 155, and then it contains one-seventh of its weight of water; that of commerce is about 1·423, and contains two-fifths of its weight of water. Nitric acid has very remarkable effects upon water with regard to the production of heat. If diluted with half its weight of water, heat is evolved; but if the water be in the state of snow, intense cold is the result. Hence, this compound is employed to produce great degrees of cold. If nitric acid highly concentrated be thrown upon phosphorus, charcoal, or oil of turpentine, it inflames them. It is very extensively used in the arts, and forms a numerous and important class of salts, having the generic name of Nitrates, such as nitrate of silver, nitrate of potash, &c. Some of these we shall notice afterwards. Nitrous acid is a compound of the same kind, but with a lesser quantity of oxygen. Amongst the other compounds of azote and oxygen, that entitled the protoxide of azote, or, as it was formerly called, nitrous oxide, is the most remarkable. Davy discovered that we may breathe it for a short while without any effect being produced, except an exhilaration of the mind. Combustibles burn in it more brilliantly than in common air. There is also a deutoxide of azote and a hyponitrous acid; but these do not require minute detail. Azote combines likewise with chlorine and bromine. Nitric acid can be procured by filling a glass retort about one-third full of equal weights of aqueous sulphuric acid and common nitre. The retort is then subjected to heat, and a vapour is distilled over, which, condensed, is nitric acid. Ammonia, or Hartshorn.-This important substance is formed by the combination of azote with hydrogen, and is obtained in the state of gas, by means of the salt called sal ammoniac, which is a compound of muriatic acid and ammonia. This substance is to be introduced into a retort, along with quicklime, and then subjected to heat. Ammonia is driven off in the form of gas, and is to be collected in glass jars standing over mercury. Ammoniacal gas is colourless, has a strong pungent smell, an acrid caustic taste, and cannot be drawn into the lungs. Its specific gravity is 0-59027. Water ab |