Chapter IV: Introduction (2)

The pressure at which the steam is used in the locomotive is sometimes very considerable. A pressure equal to 180 lbs. on each square inch of the surface of the boiler is quite usual. The greater economy obtained by the employment of high-pressure steam acting expansively in the cylinder, points to the probability of much higher pressures being adopted. There is practically no limit but the power of the materials to resist enormous strains, and there is no reason, in the nature of things, why steam of even 500 lbs. per square inch should not be employed, if it were found otherwise desirable. It need hardly be said that locomotives are invariably constructed of the very best materials, and with workmanship of the most perfect kind. The boilers are always tested, by hydraulic pressure, to several times the amount of the highest pressure the steam is required to have, and great care is bestowed upon the construction of the safety-valves, so that the steam may blow off when the due amount of pressure is exceeded. The explosion of a locomotive is, considering the number of engines in constant use, a very rare occurrence, and is probably in all cases owing to the sudden generation of a large quantity of steam, and not to an excessive pressure produced gradually. Among the causes capable of producing explosive generation of steam may be mentioned the deposition of a hard crust of stony matter, derived from the water; this crust allows the boiler to be over-heated, and if water should then find its way into contact with the heated metal, a large quantity of steam will be abruptly generated. Or should the water in the boiler become too low, parts of the boiler may become so heated that on the admission of fresh water it would be suddenly converted into steam. When an explosion does take place, the enormous force of the agent we are dealing with when we bottle up steam in an iron vessel, is shown by the effects produced. Fig. 11 is from a photograph taken from an exploded locomotive, where we may see how the thick plates of iron have been torn like paper, and the tubes, rods, and levers of the engine twisted in inextricable confusion.

FIG. 11.—_Locomotive after Explosion._
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FIG. 12.—_Hancock’s Steam Omnibus._
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Locomotive engines for propelling carriages on common roads were invented many years ago, by Gurney, Anderson, Scott Russell, Hancock, and others. One designed by Hancock is represented in Fig. 12. Such engines do not appear to have found much favour, though the idea has been successfully realized in the traction engines lately introduced. Probably the application of steam power to the propulsion of vehicles along common roads fell into neglect on account of the superior advantages of railways, but the common road locomotive is at present receiving some attention. In the tramways which are now laid along the main roads in most large cities we see one-half of the problem solved. It is not so much mechanical difficulties that stand in the way of this economical system of locomotion, as the prejudices and interests which have always to be overcome before the world can profit by new inventions. The engines can be made noiseless, emitting no visible steam or smoke, and they are under more perfect control than horses. But vestries and parochial authorities offer such objections as that horses would be frightened in the streets, if the engine made a noise; and if it did not, people would be liable to be run over, and the horses be as much startled as in the other case. But horses would soon become accustomed to the sight of a carriage moving without equine aid, however startling the matter might appear to them at first; and the objection urged against the noiseless engines might be alleged against wooden pavements, india-rubber tires, and many other improvements. It is highly probable that in the course of a few years the general adoption of steam-propelled vehicles will displace horses, at least upon tramways. The slowness with which inventions of undeniable utility and of proved advantage come into general use may be illustrated by the fact of some great English towns and centres of engineering industry not having made a single tramway until, in all the populous cities of the United States, and in almost every European capital, tramways had been in successful operation for many years. [1890.]

Some time has elapsed since the foregoing paragraph was written for an earlier edition of this work, and during that period there has been an advance in both practice and opinion; so that now it has become highly probable that before the century ends a great change may be witnessed in our modes of locomotion, even on ordinary roads. Already every town of importance throughout the United Kingdom has been provided with excellent tramways, along which, in not a few instances, horseless vehicles roll smoothly, to the great convenience of the general public, while not one of the difficulties and dangers to general street traffic has been experienced that were so confidently predicted by those who were unable to perceive that an innovation might be an improvement. The now universally-popular bicycle has been continually receiving improvements, of which there appears to be no end, and as the machine and all the contrivances connected with it are so familiar to everyone, there is no need here to do more than to refer to them, because they have led the way to great improvements in ordinary carriages.

The steam-propelled vehicle for common roads has just been mentioned as an invention belonging to the first half of the century, and the reasons it did not find favour have been alluded to. There exists in the United Kingdom a law concerning horseless carriages travelling on highways, which was passed to apply to traction engines, and enacts that other than horse vehicles are not to go along a road at a greater speed than four miles an hour, and only two miles an hour through a town, and moreover they are to be preceded by a man bearing a red flag, etc. But a bill has been introduced (1895) into the legislature to amend this law, and permit the British people to use on their common roads such light self-propelled carriages as are becoming popular in France, as may be seen from the following account:—

On Tuesday, 11th June, 1895, a race was started from Versailles to Bordeaux and back, a distance of 727 miles or more for the double journey. The first prize was the substantial sum of 40,000 francs (£1,600), to which was attached the condition of the carriage seating four persons, and other prizes were also to be awarded to various kinds of automatic vehicles. No fewer than sixty-six vehicles were entered for competition, and these were variously supplied with motive power from steam, electricity, or petroleum spirit. The starting place was Versailles at 12·9 p.m., and at 10·32 on Wednesday morning MM. Panhard & Levassor’s petroleum carriage arrived at Bordeaux, whence, after a stop of only four minutes, the return journey was begun, but shortly afterward an accident caused a delay of one hour, but the carriage made the whole distance at the average of 14·9 miles per hour. In this and three other carriages belonging to the same firm, the propeller was the Daimler motor. Though this carriage was the first to accomplish the trip it received only the second prize, the condition of seating _four_ persons not having been complied with. The first prize fell to a four-seated vehicle by Les Fils de Peugeot Frères, a firm who carried off besides the third and fourth prizes. These carriages were also driven by so-called petroleum motors. These motors are really gas engines on the principle to be presently mentioned, but the gas is produced by the vapourisation of a volatile constituent of petroleum (benzoline). The Daimler motor is a compact combination of two cylinders connected with a chamber containing the explosive mixture of gas and air. The pistons perform their in and out strokes simultaneously, but their working strokes alternately.

_PORTABLE ENGINES._

The application of steam power to agricultural operations has led to the construction of engines specially adapted by their simplicity and portability for the end in view. The movable agricultural engines have, like the locomotives, a fire-box nearly surrounded by the water, and horizontal tubes, and are set on wheels, so that they may be drawn by horses from place to place. There is usually one cylinder placed horizontally on the top of the boiler; and the piston-rod, working in guides, is, as in the old locomotive, attached by a connecting-rod to the crank of a shaft, which carries a fly-wheel, eccentrics, and pulleys for belts to communicate the motion to the machines. Engines of this kind are also much used by contractors, for hoisting stones, mixing mortar, &c. These engines are made with endless diversities of details, though in most such simplicity of arrangement is secured, that a labourer of ordinary intelligence may, after a little instruction, be trusted with the charge of the engine; while their economy of fuel, efficiency, and cheapness are not exceeded in any other class of steam engine.

Besides the steam engines already described or alluded to, there are many interesting forms of the direct application of steam power. There are, for example, the steam roller and the steam fire-engine. The former is a kind of heavy locomotive, moving on ponderous rollers, which support the greater part of the weight of the engine. When this machine is made to pass slowly over roads newly laid with broken stones, a few repetitions of the process suffice to crush down the stones and consolidate the materials, so as at once to form a smooth road. Steam power is applied to the fire engine, not to propel it through the streets, but to work the pumps which force up the water. The boilers of these engines are so arranged that in a few minutes a pressure of steam can be obtained sufficient to throw an effective jet of water. The cut at the end of this chapter represents a very efficient engine of this kind, which will throw a jet 200 feet high, delivering 1,100 gallons of water per minute. It has two steam cylinders and two pumps, each making a stroke of two feet. These are placed horizontally, the pumps and the air reservoir occupying the front part of the engine, while the vertical boiler is placed behind. The steam cylinders, which are partly hid in the cut by the iron frame of the engine, are not attached to the boiler, which by this arrangement is saved from injurious strains produced by the action of the moving parts of the mechanism. There are seats for eight firemen, underneath which is a space where the hose is carried. A first-class steam fire-engine of this kind, completely fitted, costs upwards of £1,300.

A cheap and very convenient prime mover has lately come into use, which has certain advantages over even the steam engine. Where a moderate or a very small power is required, especially where it is used only at intervals, the _gas engine_ is found to be more convenient. It is small and compact, no boiler or furnace is required, and it can be started at any moment. As now made, it works smoothly and without noise. The piston is impelled, not by the expansive force of steam, but by that of heated air, the heat being generated by the explosion of a mixture of common coal gas and air within the cylinder itself. Thus a series of small explosions has the same effect as the admissions of steam through a valve. A due quantity of gas and air is introduced into the cylinder, and is ignited by the momentary opening of a communication with a lighted gas jet outside. But the machine is provided with a regulator or governor, which so acts on the valve mechanism that this communication is made at each stroke only when the speed of rotation falls below a certain assigned limit, and thus the number of the explosions is less than the number of strokes, unless its work absorbs the machine’s whole energy, which, according to the size of the engine, may be from that of a child up to 30–horse power.

_THE STEAM HAMMER._

Before the invention of the steam hammer, large forge hammers had been in use actuated by steam, but in an indirect manner, the hammer having been lifted by cams and other expedients, which rendered the apparatus cumbersome, costly, and very wasteful of power, on account of the indirect way in which the original source of the force, namely, the pressure of the steam, had to reach its point of application by giving the blow to the hammer. Not only did the necessary mechanism for communicating the force in this roundabout manner interfere with the space necessary for the proper handling of the article to be forged, but the range of the fall of the hammer being only about 18 in., caused a very rapid decrease in the energy of the blow when only a very moderate-sized piece of iron was introduced. For example, a piece of iron 9 in. in diameter reduced the fall of the mass forming the hammer to one-half, and the work it could accomplish was diminished in like proportion. Besides, as the hammer was attached to a lever working on a centre, the striking face of the hammer was parallel to the anvil only at one particular point of its fall; and again, as the hammer was always necessarily raised to the same height at each stroke, there was absolutely no means of controlling the force of the blow. When we reflect on the fact that the rectilinear motion of the piston in the cylinder of the engine had first to be converted into a rotary one, by beams, connecting-rod, crank, &c., and then this rotary movement transformed into a lifting one by the intervention of wheels, shafts, cams, &c., while all that is required in the hammer is a straight up-and-down movement, the wonder is that such an indirect and cumbersome application of power should have for so many years been contentedly used. But in November, 1839, Mr. Nasmyth, an eminent engineer of Manchester, received a letter from a correspondent, informing him of the difficulty he had found in carrying out an order received for the forging of a shaft for the paddle-wheels of a steamer, which shaft was required to be 3 ft. in diameter. There was in all England no forge hammer capable of executing such a piece of work. This caused Mr. Nasmyth to reflect on the construction of forge hammers, and in _a few minutes_ he had formed the conception of the steam hammer. He immediately sketched the design, and soon afterward the steam hammer was a _fait accompli_, for Mr. Nasmyth had one at once executed and erected at his works, where he invited all concerned to come and witness its performances. Will it be believed that four years elapsed before this admirable application of steam power found employment outside the walls of Mr. Nasmyth’s workshops? After a time he succeeded in making those best able to profit by such an invention aware of the new power—for such it has practically proved itself, having done more to revolutionize the manufacture of iron than any other inventions that can be named, except, perhaps, those of Cort and Bessemer. The usual prejudice attending the introduction of any new machine, however obvious its advantages are afterward admitted to be, at length cleared away, and the steam hammer is from henceforth an absolute necessity in every engineering workshop, and scarcely less so for some of the early stages of the process of manufacturing crude wrought iron. Whether blows of enormous energy or gentle taps are required, or strokes of every gradation and in any order, the steam hammer is ready to supply them.

FIG. 13.—_Nasmyth’s Steam Hammer._
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A steam hammer of the smaller kind is represented in Fig. 13. The general mode of action will easily be understood. The steam is admitted below the piston, which is thus raised to any required height within the limits of the stroke. When the communication with the boiler is shut off and the steam below the piston is allowed to escape, the piston, with the mass of iron forming the hammer attached to the piston-rod, falls by its own weight. This weight, in the large steam hammers, amounts to several tons; and the force of the blow will depend jointly upon the weight of the hammer, and upon the height from which it is allowed to fall. The steam is admitted and allowed to escape by valves, moved by a lever under the control of a workman. By allowing the hammer to be raised to a greater or less height, and by regulating the escape of the steam from beneath the piston, the operator has it in his power to vary the force of the blow. Men who are accustomed to work the valves can do this with great nicety. They sometimes exhibit their perfect control over the machine by cracking a nut on the anvil of a huge hammer; or a watch having been placed—face upwards—upon the anvil, and a moistened wafer laid on the glass, a practised operator will bring down the ponderous mass with such exactitude and delicacy that it will pick up the wafer, and the watch-glass will not even be cracked. The steam hammer has recently been improved in several ways, and its power has been more than doubled, by causing the steam, during the descent, to enter above the piston and add its pressure to the force of gravity. Probably one of the most powerful steam hammers ever constructed is that recently erected at the Royal Gun Factory at Woolwich, for the purpose of forging great guns for the British Navy. It has been made by Nasmyth & Co., and is in shape similar to their other steam hammers. Its height is upwards of 50 ft., and it is surrounded with furnaces and powerful cranes, carrying the huge iron tongs that are to grasp the glowing masses. The hammer descends not merely with its own weight of 30 tons; steam is injected behind the falling piston, which is thus driven down with vastly enhanced rapidity and impulse. Of the lower portion of this stupendous forge, nothing is visible but a flat table of iron—the anvil—level with the floor of the foundry. But more wonderful, perhaps, than anything seen aboveground, is the extraordinarily solid foundation beneath. Huge tablets of foot-thick castings alternate with concrete and enormous baulks of timber, and, lower down, beds of concrete, and piles driven deep into the solid earth, form a support for the uppermost plate, upon which the giant delivers his terrible stroke. Less than this would render it unsafe to work the hammer to its full power. As the monster works—soberly and obediently though he does it—the solid soil trembles, and everything movable shivers, far and near, as, with a scream of the steam, our ‘hammer of Thor’ came thundering down, mashing the hot iron into shape as easily as if it were crimson dough, squirting jets of scarlet and yellow yeast. The head of the hammer, which of course works vertically, is detachable, so that if the monster breaks his steel fist upon coil or anvil, another can be quickly supplied. These huge heads alone are as big as a sugar-hogshead, and come down upon the hot iron with an energy of more than a thousand foot-tons. By the courteous permission of Major E. Maitland, Superintendent of the Royal Gun Factories, we are enabled to present our readers with the view of the monster hammer which forms the Plate III.

Mr. Condie, in his form of steam hammer, utilizes the mass of the cylinder itself to serve as the hammer. The piston-rod is hollow, and forms a pipe, through which the steam is admitted and discharged, and the piston is stationary, the cylinder moving instead—between vertical guides. A hammer face is attached to the bottom of the cylinder by a kind of dovetail socket, so that if the striking surface becomes injured in any way, another can easily be substituted. The massive framework which supports the moving parts of Condie’s hammer has its supports placed very far apart, so as to leave ample space for the handling of large forgings.

FIG. 14.—_Merryweather’s Steam Fire-Engine._
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PLATE III.

THE GREAT STEAM HAMMER, ROYAL GUN FACTORY, WOOLWICH.
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FIG. 15.—_A Foundry._
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IRON.

“Iron and coal,” it has been well said, “are kings of the earth”; and this is true to such an extent that there is scarcely an invention claiming the reader’s attention in this book but what involves the indispensable use of these materials. Again, in their production on the large scale it will be seen that there is a mutual dependence, and that this is made possible only by means of the invention we have begun with; for without the steam engine the deep coal mines could not have the water pumped out of them,—it was indeed for this very purpose that the steam engine was originally contrived,—nor could the coal be efficiently raised without steam power. Before the steam engine came into use iron could not be produced or worked to anything like the extent attained even in the middle of the nineteenth century, for only by steam power could the blast be made effective and the rolling mill do its work. On the other hand, the steam engine required iron for its own construction, and this at once caused a notable increase in the demand for the metal. Once more, the engine itself supplies no force; for without the fuel which raises steam from the water in the boiler it is motionless and powerless, and that fuel is practically _coal_. In consequence of thus providing power, and also of supplying a requisite for the production of iron, coal has acquired supreme industrial importance, so that all our great trades and places of densest population are situated in or near coal-fields. But what we have further to say about coal may be conveniently deferred to a subsequent article, while we proceed to treat of iron, and of the contrivances in which it plays an essential part.

Iron has also been called “the mainspring of civilization,” and the significance of the phrase is obvious enough when we consider the enormous number and infinite variety of the things that are made of it: the sword and the ploughshare; all our weapons of war and all our implements of peace; the slender needle and the girders that span wide rivers; the delicate hair-spring of the tiny watch and the most tenacious of cables; the common utensils of domestic life and the huge battle-ships of our fleets; the smoothest roads, the loftiest towers, the most spacious pleasure palaces. Such extensive applications of iron for purposes so diverse have been rendered possible only by the greater facility and cheapness of production, together with the better knowledge of the properties of the substance and increased skill in its treatment, that have particularly distinguished our century. Apart again from the constructive uses of iron, it enters essentially into another class of inventions of which the age is justly proud, namely, those which utilize electricity in the production of light, mechanical power, and chemical action; for it is on a quality possessed by iron, and by _iron alone_, that the generation of current by the electric dynamo ultimately depends. This peculiar property of iron, which was first announced by Arago in 1820, and has since proved so fertile in practical applications, is that a bar of the metal can, under suitable conditions, be instantly converted into the most powerful of magnets, and as quickly demagnetized. What these conditions are will be explained when we come to treat of electricity.

FIG. 16.—_Aerolite in the British Museum._
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Besides the unique property of iron just referred to, and its superlative utility in arts and industries, there are other circumstances that give a peculiar interest to this metal. It is the chief constituent of many minerals, and traces or small quantities are found in most of the materials that make up the crust of the earth; it is present also in the organic kingdoms, being especially notable in the blood of vertebrate (_back-boned_) animals, of which it is an essential component. Notwithstanding its wide diffusion, iron is not found _native_, that is, as _metal_, but has to be extracted from its _ores_, which are usually dull stony-looking substances, as unlike the metal as can be conceived. In this respect it differs from gold, which is not met in any other than the metallic state, in the form of _nuggets_, minute crystals or branching filaments, and from metals such as silver, copper, and a few others which also are occasionally found native. It is true that rarely small quantities of metallic iron have been met with in the form of minute grains disseminated in volcanic rocks; but in contrast with the practical absence of metallic iron from terrestrial accessible materials is the fact that masses of iron, sometimes of nearly pure metal, occasionally descend upon the earth from interplanetary space. These are _aerolites_, of which there are several varieties, some consisting only of crystalline minerals without any metallic iron, others of a mixture of minerals and metals, but the most common are of iron, always alloyed with a small quantity of nickel, and usually containing also traces more or less of a few other metals and known chemical elements. The iron in some specimens has been found to amount to 93 per cent. of the whole. These _aerolites_, or _meteorites_, as they are also called, are of irregular shape and vary greatly in size, which however is sometimes very considerable: one found in South America was calculated to weigh 14 tons, another discovered in Mexico, 20 tons. There is in the British Museum a good specimen of an iron meteorite, which is represented in Fig. 16, where it will be observed that a portion has been cut off to form a plane surface, which when polished and etched by an acid, reveals a crystalline structure quite peculiar and distinctive, so that such meteorites can be recognized with certainty, even if they did not possess surface characters which are easily observed and identified when once a specimen has been examined. The fall of meteorites to the surface of the earth is comparatively rare, but it has been witnessed by even scientific observers; as when Gassendi, the French astronomer, saw in Provence the fall of a meteorite weighing 59 lbs. In the _Transactions of the Royal Society_ for 1802 may be found a detailed account of an instance in England of the fall which took place in Yorkshire, on the 13th December 1795, of a stone 56 lbs. in weight. Aerolites become ignited or incandescent by reason of the great velocity with which they pass through the atmosphere, whereby the air in front of them is condensed and heated, the heat often being sufficient to liquefy or even vaporize the solid matter. The so-called shooting stars are with good reason believed to be nothing but such incandescent aerolites, and the aerolites themselves are regarded as small asteroids, or scattered planetary dust, portions of which occasionally coming within the sphere of the earth’s attraction are drawn to its surface. Meteoric iron is too rare to be of any value as a source of iron, but certain specimens have been found in which the metal was malleable and of excellent quality. From such meteorites the natives of India and other places have, it is said, sometimes forged weapons of wonderful temper and keenness, and we may well imagine that when such weapons have been made from iron that had actually been observed to fall from the sky, they would be regarded as endowed with magical powers, so that we may perhaps ascribe to such circumstances the origin of some of the legends about enchanted swords, etc. It is significant also that in some Egyptian inscriptions of the very highest antiquity, the word indicating iron has for its literal meaning _stone of the sky_.

But as nature has hardly provided man with the _metal_ iron, he has been obliged to find the art of extracting it from substances which are utterly unlike the metal itself. In this case, as in many others, the art has been discovered and practised ages before any scientific knowledge of the nature of the processes employed had been acquired. The idea prevails that there are such difficulties in extracting this metal; that elaborate and complex appliances, not unlike those in use in modern times, were requisite for the purpose; and therefore that the use of iron is compatible only with a somewhat late period in man’s history, and implies a comparatively advanced stage of civilization. Now there undoubtedly are facts which tend to confirm this view; for instance, the Spaniards who first colonized North America found the natives perfectly familiar with the use of copper, but without any acquaintance with iron, although the region abounded with the finest ferruginous minerals; and, again, the archæologists who have examined the relics of ancient civilizations and of pre-historic peoples about the shores of the Mediterranean, find in the earliest of these relics weapons and implements of rudely chipped stones, followed later by the use of better-shaped and polished stones; hence the periods represented by these, they have respectively designated by the terms _palæolithic_ and _neolithic_—the old and the new stone ages. At some later time the stone of these implements was gradually replaced by _bronze_, which is a mixture of copper and tin, while as yet iron does not occur in any form among the remains. In the latest layers, however, articles of iron are found, and it is inferred that this metal came into use only after bronze had been known for an indefinite period; hence these later pre-historic periods have come to be respectively called _the bronze age_ and _the iron age_. No doubt this succession really occurred in the localities where the observations were made, but it would not be justifiable to assume that the same was the case in every part of the world, for much would depend on such circumstances as the presence or absence of the essential minerals. We may also set against the supposed difficulty of obtaining iron from the ores, the still greater complexity of the methods required for the production of copper and of tin. Besides this is the fact that the ores of tin are found but in very few places in the world, and of these only the Cornwall mines, so well known in ancient times, would be likely to furnish a supply to the places where pre-historic bronzes are found; this implies that navigation and commerce must have already made considerable progress. On the other hand, iron has been produced and worked for untold ages by the negro races all over Central Africa, and the method of treating the ore has no doubt been that which is there still practised by certain scarcely civilized tribes, and it is as simple as any metallurgical operation can possibly be, requiring merely a hole dug in a clay bank, wherein the fuel and minerals are piled up, and the mere wind supplies sufficient blast to urge the fire to the needful temperature, or air is blown in from rude bellows made of a pair of skins alternately raised and compressed. These very primitive furnaces have in some places developed into permanent clay structures, seven or eight feet in height. The natives of Central Africa have therefore long known the method of extracting iron, as well as of forging and casting it.

The nature and value of what has been done during the century in the treatment of iron would not be intelligible without some description of the ordinary processes of extracting the metal from the ores; and a scientific understanding of these implies some acquaintance with chemistry. Not because metallurgy has been developed from chemistry, for the fact is rather the reverse; indeed, as we have seen, the art of extracting iron from its ores was practised ages before chemistry as a science was dreamt of. Although we may assume that many of our readers have sufficient knowledge of chemistry to attach distinct ideas to such few chemical terms as we shall have occasion to use, yet it may be of advantage to others to have some preliminary notes of the character of the chemical actions, and of some properties of the substances that will have to be referred to. It is certainly the case that people in general, and even people very well informed in other subjects, have but the vaguest notions of the nature of chemical actions, and of the meaning of the terms belonging to that science. For example, one of our most popular and justly esteemed writers, treating of the very subject of iron extraction, calls the ore a _matrix_, thereby implying that the iron as metal is disseminated in detached fragments throughout the mass, which is a conception inconsistent with the facts. The reader will be in a more advantageous position for understanding the relation of the ores of iron to the metal, if he will follow in imagination, or still better in reality, a few observations and experiments like the following—of which, however, he is recommended not to attempt the chemical part unless he is himself practically familiar with the performance of chemical operations, or can obtain the personal assistance of someone who is. Taking, say, a few common iron nails, let him note some obvious properties they possess: they have weight—are hard and tough so that they cannot be crushed in a mortar—are opaque to light—if a smooth surface be produced on any part, it will show that peculiar shiny appearance which is called metallic lustre, in this case without any decided colour—they are not dissolved by water as sugar or salt is—and are attracted by a magnet. If several of the nails be heated to bright redness they may be hammered on an anvil into one mass, and this may be flattened out into a thin plate, or it may be shaped into a slender rod and then drawn out into wire; or otherwise the nails may be converted into the small fragments called iron filings. In these several forms the nails, as nails, will have ceased to exist; but the material of which they were formed will remain unchanged, and each and every part of it however large or small will continue to exhibit all the properties noted above as belonging to the substance of the nails, which in the cases supposed has undergone merely _physical_ change of shape. Treating our nails in yet another way, we may proceed to subject them to a _chemical_ change, by an experiment very simple in itself, but involving certain precautions, by neglect of which the tyro in chemical operations would incur some personal risks; these might however be obviated by using only very small quantities of the materials (a mere pinch of iron filings and a few drops of sulphuric acid), when the results would still be sufficiently observable. A few of the iron nails having been placed in a flask of thin glass, we pour upon them a mixture of oil of vitriol (sulphuric acid) and water, which has previously been prepared by gradually adding 1 measure of the acid to 5 measures of water. The action that takes place is greatly accelerated by heat, and indeed the contents should be heated to boiling by standing the flask on a layer of fine sand spread on an iron plate and gently heated from below. The nails will soon disappear, being completely dissolved by the acid liquid, and the turbid solution should be filtered through filtering paper as rapidly as possible and while still hot. This turbid and dirty looking condition is due to foreign matters in the nails, for these never consist of _pure_ iron. The filtered liquid is set aside to cool in a closed vessel, in which after a time will be found a deposit of crystals of a pale bluish-green colour. The liquor above these having been poured off, the crystals are to be rinsed with a very small quantity of _cold_ water, and then dried between folds of blotting-paper, after which they are ready for examination. The quantity of the diluted acid put into the flask should have a certain proportion to the weight of the nails; about 5 _fluid ounces_ to 1 ounce of iron will be found convenient, for if less is used the nails will not be entirely dissolved, and an excess will tend to keep the crystals in solution instead of depositing them when cold. The nails—as such—will now have passed out of existence: can we say that the iron that formed them exists in the crystals? Certainly not as the _metal_ iron, for every property of the metal will have disappeared. The crystals are brittle, can be crushed in a mortar—they are translucent—they show no metallic lustre, but only glassy surfaces—they are readily dissolved by water—they are not attracted by a magnet. The most powerful lens will fail to show the least particle of iron in them; they have in their properties no assignable relation to the metal of the nails, but are matter of quite another sort; and be it noted that this entire _otherness_ is the special and characteristic sign of _chemical change_. So complete is the transformation in the case we have been considering that it would never have been said that iron was contained in these crystals, but rather that the metal had for ever passed out of existence, but for one circumstance; and that is, that by subjecting the crystals to certain processes of _chemical analysis_ we can again obtain from them the iron in metallic state. Nay more, we should find the weight of metal so obtained to be exactly equal to that of the pure iron dissolved from the original nails, supposing of course that we operated upon the whole of the crystalline matter so produced. The inference therefore is that although every property of the iron appeared to be absent from the crystals, the iron entering in them retained there its original weight, and the correct statement of the change would be, that in the crystals the iron had lost all its original properties SAVE ONE, namely, its weight, or gravitating force, if we choose to call it so, a property belonging to it in common with every material substance. Chemical analysis can also separate from the crystals their other constituents and weigh them apart—so much water and so much sulphuric acid—and when to these weights that of the iron is added, the sum exactly makes up the weight of the crystals.

A still simpler experiment, which may be performed by anyone with the greatest ease, may serve as a further illustration of the profound nature of the change in the properties of bodies brought about by chemical combination, and it will also serve as the occasion of directing attention to a remarkable circumstance that invariably characterizes such changes, and one that should always be present in our minds when we are considering them. A yard of flat _magnesium_ wire can be bought for a few pence, and after its metallic character has been observed in the silvery lustre disclosed by scraping the dull white surface, a few inches is to be held vertically by a pair of tongs, or by inserting one extremity in a cleft at the end of a stick, then the lower part is brought into contact with a candle or gas flame. The metal will instantly burn with a dazzlingly brilliant light, and some white smoke (really fine white solid particles) will float into the air; but if a plate be held under the burning metal, some of the smoke will settle upon it, together with white fragments that have preserved some shape of the metallic ribbon, but which a touch will reduce into a fine white powder, identical with the well-known domestic medicine called “calcined magnesia”—a substance totally different from the _metal magnesium_. The reader will scarcely require to be told that in this burning the metal is entering into combination with the oxygen of the air—by which that invisible gas somehow becomes fixed in these solid white particles, so entirely unlike itself. But this experiment might be so arranged that the quantities of magnesium and oxygen entering into the magnesia could be weighed. For this purpose special appliances would be required in order to ensure complete combustion of the metal, for in the experiment as just described some small particles are liable to be shielded from the oxygen by a covering of magnesia, and the arrangement would have to be such that the _whole_ of the white powder could be gathered up and weighed. In the absence of such appliances, and of a delicate balance, together with the skill requisite for their use, the reader must for the time be contented to take our word for what would be the result. In every experiment the magnesia would be found heavier than the metal burned in the proportion of 5 to 3; in other words, magnesia always contains (so the phrase runs) 3 parts of magnesium combined with 2 of oxygen: never more nor less. A definite proportion between the weights of the constituent substances characterizes every chemical combination, and when this is once determined in a single sample of the compound, it is determined for every portion of the same, wherever found or however produced. But each compound has its own particular proportion, that is, the quantitative relations are different for each. For example, the two constituents of water, hydrogen and oxygen, are combined in the ratio of 1 to 8, etc.; and oxygen combines with metals in a ratio different in each case. Then occasionally the same ratio of constituents occurs in compounds of different composition. The elementary student is apt to suppose that this is _because_ of the _law_ which he finds stated, probably in almost the first page of his text-book: “Every compound contains its elements in definite and invariable proportions”; and even well-educated people entertain the idea of the fact being “governed by” or “obeying” the law just quoted,—a misconception arising from the other use of the word “law,” as signifying an enactment. The real case however is the converse; namely, that a multitude of facts like that above stated have governed the law, and caused it to be what it is—the general statement of many observed facts.

We have assumed that the reader’s chemical knowledge had already made him aware that in every case of ordinary combustion the oxygen of the atmosphere is in the act of entering into combination with the burning body: as with the magnesium, so with a coal fire, a gas flame, or a burning candle; only in these last cases the products of the combustion pass away invisibly. The candle by burning disappears from sight, but its matter is not lost, and as in the case of magnesium, the compounds it forms weigh more than the unburnt candle. The experiment is commonly shown in courses of elementary lectures on chemistry, of so burning a candle that the invisible products are retained in the apparatus, instead of being dissipated in the atmosphere, and the increase of weight of the burnt candle over the original one is demonstrated by the balance. Important as is the part played by oxygen in all chemical actions on the earth, the composition of the atmosphere was not understood until the end of the eighteenth century, and it was well on into the nineteenth before the quantities of its constituents were accurately determined. Now everyone knows that air is mainly made up of a _mixture_ of the two gases _oxygen_ and _nitrogen_. A _mixture_ of two or more things is very different from a chemical _combination_ of them; for in the former each ingredient retains its own properties. (See _Air_ in Index.) Nitrogen being an inert gas that takes no part in combustion, or in the ordinary chemical actions of the air, acts therein simply as a diluent of the oxygen. It is necessary in relation to our present subject to bear this in mind, as well as the relative quantities of the two gases in air. For our immediate purpose we may neglect the minor constituents of air—such as watery vapour, carbonic acid, etc., of which the total weight does not exceed one hundredth part of the whole—and consider air as a mixture of 23 parts by weight of oxygen with 77 of nitrogen, or calculated in volumes, 21 measures of oxygen with 79 of nitrogen. Compounds of oxygen with nearly every one of the other seventy or more chemical elements are known, and these compounds, which are called _oxides_, are arranged by chemists under five or six classes, forming as they do _basic radicles_, _acid radicles_, _saline oxides_, etc. With some of these compounds belonging to different classes, we must make acquaintance after noticing the elementary substance with which the oxygen is united.

We begin with _carbon_, which forms the chief constituent of all our combustibles. Some specimens of graphite, plumbago, or “blacklead” consist of almost pure carbon (98 per cent.), and some varieties of wood charcoal exceptionally contain 96 per cent.; but in ordinary charcoal the percentage is much less. Coal, the most familiar of our solid fuels, varies greatly in composition, carbon being the predominating constituent, in amount from 57 to 93 per cent. Coke, another fuel much used in metallurgical operations, is made by heating coal without access of air, when a large quantity of gaseous substances is expelled. Coke burns with an intense and steady heat without emitting any visible smoke, but it does not ignite as readily as coal. Carbon forms two different compounds with oxygen: both are invisible gases, but they differ in the proportions of the constituents, and present different properties. When carbon (coal, coke, or charcoal) is completely burnt, that is, with an abundant supply of air, the product is _carbonic acid_ gas, in which 3 parts of carbon are combined with 8 of oxygen: when, on the other hand, the carbon is burnt with a sufficiently restricted access of air, the result is _carbonic oxide_ gas, in which 3 parts of carbon are united with only 4 of oxygen. The reader will here observe that the former contains just twice as much oxygen as the latter for the same quantity of carbon. This fact and numberless others like it are expressed or summed up by another _law_ of chemical combination which states that when two elements combine in several different proportions these are invariably such that the ratios in the several compounds will be found to have exact and simple numerical relations; that is, such as may, when reduced to their lowest terms, be expressed by the simple integers 1, 2, 3, etc., as 1 : 2, 3 : 2; ... 8 : 9, etc. It comes to the same thing if we compare together the weights A and A´ which are united in each compound with any one identical weight of B, giving of course the ratio A : B ÷ A´ : B. For instance, in the case just given, of carbon and oxygen, 3 : 4 ÷ 3 : 8 = 2 : 1. This, which is simply stating the facts, is called the _law of multiple proportions_. On a later page will be found another illustration (see Index, _Nitrogen and Oxygen Compounds_), and its expression in terms of the _atomic theory_, which goes behind the facts (so to speak), but is extremely useful by comprehending many other groups of facts in chemistry and in other sciences. Carbonic acid gas is of course incombustible, but carbonic oxide gas burns by uniting with the additional proportion of oxygen and becoming carbonic acid. On the other hand, carbonic acid gas passing over red-hot coals takes up from them the additional proportion of carbon, and is, we may say, _unburnt_ into carbonic oxide. When we see a pale blue flame flickering over the bright embers in a fire grate, it is carbonic oxide burning back again by taking more oxygen from the air above the coals. Carbonic oxide combines directly with two or three of the metals, as, for instance, it forms a volatile compound with nickel, at a certain temperature, and this is decomposed again at a higher temperature. The like takes place with iron, although in very small quantities, but the observation throws some light on the processes of reduction. Carbonic oxide is neither acid nor basic, but carbonic acid is an acid oxide, and as such unites with oxides of the basic class to form another range of compounds. Thus, for example, the oxide of the metal _calcium_ is quicklime, which is strongly basic, and this directly combines with carbonic acid, forming a _neutral_ substance called in systematic chemistry _calcium carbonate_, or more commonly but less correctly, carbonate of lime, familiar to everyone in the compact state as limestone, and marble, and in a more or less pulverent condition as chalk. When any of these is heated to redness, carbonic acid is expelled and quicklime remains. Like most oxides, quicklime forms a compound with water, the combination being attended with the extrication of much heat, the compact quicklime swelling and crumbling into _slaked lime_. The chemist’s term for a compound of a _basic_ oxide with water is _hydrate_, while that of an _acid_ oxide with water is for him properly an _acid_, or in order to particularly distinguish this class, an _oxy-acid_. It was however the older practice to give the name of acid to the oxide alone, and this naming having found its way into popular language is much more familiar to the non-scientific reader. The systematic names of the two compounds of carbon and oxygen are _carbon monoxide_ and _carbon dioxide_, but we shall use here the more familiar terms carbonic oxide and carbonic acid.

We have now to call attention to a substance which contributes by far the largest part to the solid crust of our globe. It is called _silica_, from _silic-_, the Latin word for flint (without case suffix): it is seen in flint, and very pure in rock crystal, quartz, agate, and calcedony. It forms the essential part of every kind of sand and sandstone, and is the principal ingredient of clay, granite, slate, basalt, and many other minerals. Silica is the oxide of a quasi-metal called _silicon_, which can be obtained from silica with difficulty, and only by roundabout processes, presenting itself in different conditions according to the process used. Silica is an acid oxide, and it readily unites with most of the basic oxides when heated with them, forming a class of compounds of different properties which are much modified in admixtures containing two or more. Very few of these _silicates_ are soluble in water, most of them are not: they are all fusible at various temperatures, except silicate of alumina, of which fire-clay is chiefly constituted. Alumina, it should be stated, is the oxide of the metal aluminium. The silicates of lime and of magnesia fuse only with great difficulty; but the silicates of iron and of manganese are easily fused, and silicate of lead still more so. Glass is a mixture of silicates, often of lime, soda, and alumina; sometimes of lead and potash mainly; porcelain and pottery consist chiefly of silicate of alumina with varying proportions of silicates of iron, of lime, etc.

It now remains only to mention two non-metallic elements that are nearly always present in crude iron, but which the metallurgist strives to eliminate, as they are in general very injurious to the quality of the material even when their amount is very small. The first is _sulphur_, well known as _brimstone_, also as _flowers of sulphur_, a yellow coloured solid, which burns in the air. The product of the combustion is an invisible gas of a readily recognized pungent odour: this is an acid-forming oxide containing equal weights of sulphur and oxygen. There is another oxide in which the weight of oxygen is one and a half times that of the sulphur, and this is the radicle of the very active _sulphuric acid_ or _oil of vitriol_. Sulphur, like oxygen, unites with most of the other elements, forming compounds called _sulphides_. Of these the iron compound called _pyrites_ is the best known, and its occurrence in coal prevents the use of that material as fuel in contact with iron or other metals. _Phosphorus_ is an element that occurs naturally only in combination; in its separated state it is a very inflammable solid. It combines directly with other substances and is taken up by some fused metals in large quantities. In many cases a very small proportion of it existing in a metal greatly modifies the properties of that metal. Phosphorus forms several oxides, and these are radicles of powerful acids, among which is _phosphoric acid_ that combines with basic oxides to form _phosphates_.