Part 12
One of the successful applications of Electro-Metallurgy is founded on the original application of it by Mr. Spencer. As already stated, he covered metal plates with wax, and after scratching through the coating, and exposing the metal, he submitted the plate to voltaic action in a solution of sulphate of copper, and thus obtained a representation, in relief, of the figures cut through the wax; but he does not seem to have thought of the application of this mode of deposition, since adopted, by which engravings in relief are obtained, and printed from with the ordinary letter-press, in the same manner as woodcuts. The name given to this new art is "Glyphography," and it is used with great advantage when the effect of copper-plate engraving is required; for cross lines, which are difficult to cut in wood, can be worked by this method with as great facility as in copper-plate etching.
Another application of Electro-Metallurgy that promises to be useful, is the coating of glass and earthenware vessels with copper, so as to enable them to be placed over the fire without being cracked. A glass sauce-pan might thus be made, which, protected by metal covering, would neither break nor crack when placed upon the fire, because the metal would diffuse the heat over the whole surface, and prevent the unequal expansion of the vessel, which is the cause of the cracking of glass and earthenware when placed upon the fire. A patent was granted last year for a mode of coating earthenware vessels with copper or iron by electro-chemical deposition. The earthenware is first covered either with copper leaf or with bronze powder, to obtain an electrical conducting surface on which the copper can be deposited, and the vessel is then placed in a solution of sulphate of copper, and put in connection with the negative pole of a voltaic battery.
The electrotype is frequently applied with advantage to the preservation and multiplication of objects of art and natural productions, for even the forms of flowers may be in this manner rendered durable; but the most important use that has been made of the process is in plating and gilding. To effect that object, it is necessary to employ a voltaic battery separated from the vessel in which the decomposition takes place. The annexed diagram shows an arrangement of this kind. A single cell of a Daniell's battery, _a_, is connected by wires from its positive and negative poles, with metal rods placed across the decomposition cell, _b_. The articles to be plated are suspended by wires from the metal rod, _f_, and a plate of silver is attached to the rod, _e_. Thus, when the vessel is filled with the silvering liquid, a voltaic current is established, and the deposition is effected on the articles connected with the negative pole.
The menstruum best adapted for electro-plating is a solution of silver in cyanide of potassium. During the process of deposition, the same quantity of metal that is deposited from the liquid on the objects connected with the negative pole of the battery is restored to it, by dissolving an equal quantity from the silver plate connected with the positive pole, and in this manner the solution is maintained at the same strength. Any thickness of silver may be deposited by continuing the process, but about one ounce and a half to a square foot of surface is considered a full quantity. Those parts on which no silver is required to be deposited are covered with varnish or wax, which protects them from the voltaic action.
Where the operation of electro-plating is carried on extensively, decomposing troughs, holding nearly 300 gallons, are employed, and the silver plates in a single trough expose a surface of thirty square feet to the dissolving action of the menstruum under the influence of the voltaic battery.
By the aid of electro-plating the most elaborate designs of the artist in metal can be covered with silver on every part; and a group, finely engraved in copper, may be made to resemble in every particular a work cut out of solid silver. The metal is usually deposited in a granulated state, resembling "frosted" silver, and the parts required to be bright are subsequently burnished; but by the addition of a few drops of the sulphuret of carbon to the solution, the silver may be precipitated perfectly bright.
GAS LIGHTING.
The invention of Gas Lighting had its origin in the earliest times of history; not, indeed, as we now see it, burning at the end of a pipe supplied with gas made artificially, and stored in gas-holders, but blazing from fissures in the ground, supplied from natural sources in the bowels of the earth. The Greek fire-altars are supposed to have been supplied with gas, either issuing from bituminous beds, or made artificially by the priests, by pouring oil on heated stones placed in cavities beneath. Fountains of naphtha, and fires issuing from the earth at Ecbatana, in Media, are mentioned by Plutarch in his life of Alexander, and many other ancient historians record the knowledge of similar instances of natural gas lighting.
In later times, the inflammable gas issuing into the galleries of coal mines, and either exploding when mixed with atmospheric air, or blazing as it issued from fissures in the coal, afforded instances of the natural production of gas, the ignition of which too frequently proved fatal to those in the mines.
A remarkable instance of the issue of inflammable gas into the shaft of a coal mine at Whitehaven, which produced a blaze about 3 feet diameter and 6 feet long, is noticed in the "Philosophical Transactions" of 1733. The part whence the gas issued was vaulted off, and a tube was inserted into the cavity and carried to the top of the pit, where it escaped in undiminished quantity for years, and lighted the country for a distance of several miles. Many experiments were made with this large issue of gas, and it was proposed to conduct it in pipes to Whitehaven, to light the streets of that town, but the proposition was rejected by the local authorities.
In China, naturally produced gas is used on a large scale, and was so long before the knowledge of its application was acquired by Europeans. Beds of coal, lying at a great depth, are frequently pierced by the borers for salt water, and from these wells the inflammable gas springs up. It sometimes appears as a jet of fire from 20 to 30 feet high; and, in the neighbourhood of Thsee-Lieon-Teing, the salt works were formerly heated and lighted by means of these fountains of fire. Bamboo pipes carry the gas from the spring to the places where it is intended to be consumed. These canes are terminated by tubes of pipe-clay, to prevent their being burnt, and other bamboo canes conduct the gas intended for lighting the streets, and into large apartments and kitchens. Thus Nature presents in these positions a complete establishment of gas-light works.[9]
Though the production of illuminating gas from natural sources had been thus long known, the idea of distilling it artificially from coal, for the purpose of illumination, did not occur until the end of the last century. Dr. Clayton, indeed, nearly arrived at the practical application of carburretted hydrogen, in 1737, for he instituted experiments to prove that coal contains gas, nearly similar to that of the "fire damp" in coal mines, and that it burns with a bright flame. At that time, however, the nature of gases was so imperfectly known, that he was unable to do more than discover that coal possesses the property of giving out, when heated, gas that will burn with a bright light.
In the "Philosophical Transactions" of 1739, Dr. Clayton thus describes the effect of the "spirit of coal," obtained by destructive distillation in an iron retort. "I kept this spirit," he says, "in bladders for a considerable time, and endeavoured several ways to condense it, but in vain; and when I had a mind to divert strangers or friends, I have frequently taken one of these bladders, and pierced a hole in it with a pin, and, compressing gently the bladder near the flame of a candle till it once took fire, it would then continue flaming till all the spirit was compressed out of the bladder; which was the more surprising, because no one could discern any difference in the appearance between these bladders and those which were filled with common air."
The first known application of coal gas to illumination was made, in 1792, by Mr. William Murdoch, engineer at the Soho manufactory, to whose great ingenuity the world is also indebted for the invention of the first plan of a steam locomotive engine.[10] He was at that time occupied in superintending the fitting up of steam engines for the Cornish mines, and his attention having been directed to the properties of gas issuing from coal, he instituted a series of experiments on carburretted hydrogen, the practical result of which was the lighting of his house and offices, at Redruth, with coal gas. The mines at which Mr. Murdoch worked being some miles distant from his house, he was in the constant practice of filling a bladder with coal gas, in the neck of which he fixed a metallic tube with a small orifice, through which the gas issued. The lighted gas issuing through the tube served as a lantern to light his way; and as he thus proceeded along the road with the light issuing from the bladder, the country people looked upon him as a wizard.
The gas was generated by Mr. Murdoch in an iron retort, and collected in a common gasometer, from which it was conducted in pipes to the rooms to be lighted. He also, in an early stage of the invention, contrived a means for making the gas portable, by compressing it into strong vessels; a plan which, a few years since, was adopted by the Portable Gas Company in London.
Mr. Murdoch having proved the practicability of illumination by gas generated from coal, he continued his experiments to facilitate the manufacture of the gas on a large scale, and for its more perfect purification. The first public display of its illuminating power was made at the rejoicings for the peace of Amiens, in 1802, on which occasion part of the work-shops of Messrs. Boulton and Watt, at Soho, was brilliantly illuminated with coal gas by Mr. Murdoch. In 1805, Messrs. Phillips and Lee, of Manchester, had their extensive cotton mill fitted up with gas apparatus, under the superintendence of Mr. Murdoch, and the quantity of light given out by the burners in all parts of the cotton mill was equal to that of 3,000 candles.[11]
Notwithstanding these eminently successful trials of gas lighting, the prejudice against innovation prevented, for several years, the extensive adoption of the plan. As every establishment using gas had to make it, and as the apparatus was costly and imperfectly managed, the expense in the first instance, the trouble, and the noxious smell, presented great obstacles to the introduction of that mode of illumination. The popular notion, also, that streams of flame were rushing along the pipes produced an impression that gas lighting must be very dangerous, which it required time to disprove. It was not, therefore, till several years after the advantages and economy of gas had been practically established, that a public company was formed for laying down pipes to light the streets, and to convey the gas into houses for lighting shops.
The person to whom the world is chiefly indebted for the practical application of gas lighting is Mr. Winsor, who had been a merchant in London. Being very sanguine as to the advantages to be derived from gas lighting, and possessing an ardent temperament which no opposition could quench, he undertook to introduce it to public notice, and to form a company for lighting the whole of England with gas. He hired the old Lyceum Theatre, which he lighted with coal gas, and he there delivered lectures and exhibited experiments to show the benefits that would arise from the universal use of gas light, and coke fuel. He published an extravagant prospectus of a company to be formed, with the following title:--"A National Light and Heat Company, for providing our streets and houses with light and heat, on similar principles as they are now (1816) supplied with water. Demonstrated by the patentee at No. 97, Pall Mall, where it is proved, by positive experiments and decisive calculation, that the destruction of smoke would open unto the empire of Great Britain new and unparalleled sources of inexhaustible wealth at this most portentous crisis of Europe. The serious perusal of this publication, and an attentive observation of the decisive experiments, will carry conviction to every mind."
In this prospectus Mr. Winsor attempted to make it appear that by adopting his plan there would be "a grand balance of profit for the whole realm of £115,000,000," and each shareholder of the company was promised, "at the lowest calculation, £570 for every £5 deposit." He entertained the notion of making the use of gas and coke compulsory, by levying a tax on all who obstinately refused to adopt what would be so much to their own advantage. This tax, he said, "cannot be oppressive in the least, because it falls on the obstinate only, who shall resist the use of a far superior, cheaper, and safer fuel." Not content with the language of prose, Mr. Winsor vented his thoughts and feelings in numerous poetical effusions. The flights of his Muse, however, were not into the regions of sublimity, as may be perceived by the following specimen:--
"Must Britons be condemned for ever to wallow In filthy soot, noxious smoke, train oil, and tallow, And their poisonous fumes for ever to swallow? For with sparky soot, snuffs and vapours, men have constant strife,-- Those who are not burned to death are smothered during life."
Mr. Winsor's absurd statements--in the truth of which he potently believed--and the wild, random manner of making them known, excited much ridicule and opposition. Among his opponents was Mr. Nicholson, the editor of the _Chemical Review_, who not only challenged Mr. Winsor's estimates, but the validity of his patent, on the ground that Mr. Murdoch was the original inventor. Mr. Winsor's plans and calculations were burlesqued in a cleverly written "Heroic Poem," published in a quarto volume, which, whilst professing to extol the virtues of gas and coke, quizzed its hero most unmercifully. The poem concluded with this address:--
"And when, ah, Winsor!--distant be the day!-- Life's flame no longer shall ignite thy clay, Thy phosphur nature, active still, and bright, Above us shall diffuse _post obit_ light. Perhaps, translated to another sphere, Thy spirit--like thy light, refined and clear-- Ballooned with purest hydrogen, shall rise, And add a PATENT PLANET to the skies. Then some sage Sidrophel, with Herschel eye, The bright _Winsorium Sidus_ shall descry; The _Vox Stellarum_ shall record thy name, And thine outlive another Winsor's fame."
"Though we may smile at Mr. Winsor's extravagant plans and calculations," observes the _Journal of Gas Lighting_, "we cannot but admire the enthusiasm with which he pursued his object, and ultimately succeeded in establishing the first gas company. The lighting of Pall Mall with gas, in the spring of 1807, gave increased stimulus to the project, and application was made to Parliament to carry it into effect. The bill was opposed by Mr. Murdoch and thrown out; but in the following year (1810) the application was successfully renewed. The scheme, however, as sanctioned by Parliament, was sadly shorn of its magnificent proportions; and, instead of a 'Grand National Light and Heat Company, for Lighting and Heating the Whole Kingdom,' its operations were limited to London, Westminster, and Southwark; nor were any special taxes imposed on those who should obstinately refuse to use the light and burn the coke. The Chartered Gas Company, established by Mr. Winsor's persevering efforts, has served as the guiding star to all other gas companies in the world."
The illuminating property of coal gas depends on the quantity of carbon it contains. Pure hydrogen gas burns with a pale blue flame that gives scarcely any light, though it possesses intense heating power. If, however, minute particles of a solid body--powdered charcoal, for instance--be thrown into the flame, they become white-hot, and the incandescence of those solid particles produces a brilliant light. The same effect is caused by the combustion of the carburretted hydrogen gas, and in a more perfect manner. That gas contains a large portion of carbon in a state of vapour, and when a light is applied to a jet of the gas the hydrogen immediately inflames, the carbon is deposited in the flame, and the minute particles into which it is disseminated become highly heated and ignite.
There are two distinct states of carbonization in illuminating gas. The commoner kind--the ordinary coal gas--consists of two measures of hydrogen mixed with one measure of carbon vapour. The specific gravity of such gas is about one-half that of atmospheric air, and it is eight times heavier than pure hydrogen.[12] The best kind of gas for illumination is obtained from the distillation of oil. It is called olefiant gas, and contains equal measures of hydrogen gas and carbon vapour; its specific gravity is 0.985, being about fifteen times heavier than pure hydrogen.
The _rationale_ of the process of making coal gas consists in expelling the volatile matters from the coal by heat, in closed vessels or retorts, and then separating the gas and purifying it on its passage from the retort to the gas-holder, where it is stored for use.
The retorts are usually made of cast iron, and are about 7 feet long, 14 inches in depth, and the same in width; the shape being that of an arch. The retorts will hold two hundredweight of coal each, but they are never filled, because during the process of distillation the carbonaceous part of the coal expands, and occupies more space than the original quantity, the proportion of expansion being as one and a quarter to one. There is a large aperture for the admission of coal and the extraction of coke, which aperture is covered with a lid, and screwed to make it air-tight. A tube is inserted into the mouth of the retort, to carry off the products of the distillation. That tube rises about twelve feet, and then dips into a large horizontal pipe, one foot in diameter, called the hydraulic main, in which are collected the tar and ammoniacal liquor that distil from the coal. Ten or fourteen retorts are usually set back to back in brickwork, to be heated by one fire; but the plan has been recently introduced of employing long clay retorts, which are charged at both ends. These are found to possess considerable advantage over iron, not only from their lower price, but from the facility with which the carbonaceous deposit is removed, and the full charges worked off. Coke is generally burned in the furnaces, and the heat is continually maintained so as to keep the retorts red-hot.
As atmospheric air cannot gain access to the coal in the retorts, the gases expelled do not inflame, nor can the parts that are not volatile be consumed without a supply of air. It is entirely a process of distillation, in which all the products can be collected. The volatile parts are carried off by the pipe, and the solid carbonaceous matter, or coke, is left in the retort.
The first effect of heat on coal, after it is put into the retort, is to expel the moisture, which, in combination with the air, issues in the form of steam. Tar then distils, with some portions of gas, consisting of hydrogen and ammonia. When the retort has attained a bright cherry-red heat, the disengagement of the carburretted hydrogen is most active; and it is found that the more quickly the coal is heated, the greater is the quantity of illuminating gas generated.
The production of coal gas, and the development of its properties at different stages of distillation, may be readily shown by means of a common tobacco pipe. Fill the bowl of the pipe with small pieces of coal, cover it over with a lump of clay, and then put it into a hot fire, with the stalk of the pipe projecting through the bars. Presently steam will be seen to issue from the pipe, and afterwards smoke, and, if a light be applied, a jet of flame will issue forth, the brilliancy of which will increase as the bowl of the pipe becomes more heated, until the best part of the gas has been distilled from the coal.
The gas is mingled with various volatile products as it issues from the retort, and requires to be purified before it is fitted for illumination. The most abundant matter that passes over with it is tar. The vapour of that substance, however, condenses when cooled, and it may thus be separated from the gas very effectually. For that purpose the gas, after having deposited a large portion of the tar in the hydraulic main, is made to traverse through a number of vertical pipes, and in passing through them a further quantity of tar, accompanied by ammoniacal liquor, is deposited, and collected in a reservoir at the bottom. The next process is the purification of the gas from carbonic acid and sulphuretted hydrogen. This is commonly done by passing it through water and lime; the combination of the carbonic acid with the lime being facilitated by agitation. The method of purifying by lime was introduced by Mr. Clegg; and by a later process, oxide of iron is used to absorb the sulphuretted hydrogen. The gas, when purified, is conveyed to the gas-holder, whence it is forced by pressure into the mains and pipes.
An apparatus for generating coal gas on a small scale for private establishments, remote from sources of ordinary supply, is represented in the accompanying woodcut. The retort, A, is fitted in a small furnace. The coal is put in at F, and the products of distillation pass through the bent pipe, E. The more liquid portions of the tar pass at once through the tube, B, into the receiver, G; and as the gas passes along the bent tube, C, it becomes cooled, and a further deposit of tar and ammoniacal liquor is made. The gas is then conveyed along another tube into the purifier, H, filled with lime and water, and it thence passes into the gas-holder. Tubes are inserted into the latter for conveying the gas to the burners.
The quantity and the quality of the gas yielded by coal differ materially according to the kind employed. One ton of good Newcastle coal will yield 9,500 cubic feet of gas, which, when burnt in the best manner, gives a light equal to that of 422 lbs. of spermaceti candles. One ton of Wigan cannel coal yields 10,000 cubic feet, and gives a light equal to 747 lbs. of spermaceti candles.[13] The price, in London, of good gas from Newcastle coal, is 4s. 6d. per thousand cubic feet, which gives a light equal to 74½ lbs. of spermaceti, and equal to 89 lbs. of mould candles; therefore, when the latter are 8d. a pound, the burning of gas is twelve times more economical than the burning of candles. In Liverpool, gas from cannel coal is supplied at the low price of 3s. 9d. per thousand feet; and that gas gives at least one-third more light than the ordinary London gas.