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Fig. 3.—Sectional diagram of gas plant. The retorts and furnace are on the right; the gas rises through the upright pipe T into the hydraulic main B; from there it passes into the atmospheric condensers D, from the lower cistern of which the condensed tar flows into the tar-well, H. Passing up through K, the gas is conducted into the scrubber, O, and from there into the purifier, M. From there it emerges through K′ into the purifier, M, and then into the gas-holder for distribution. (From Schultz’sChemie des Steinkohlentheers.)
From the scrubbers the gas is sent through another series of vessels packed with trays of lime or oxide of iron, in order to remove sulphuretted hydrogen and other sulphur compounds as completely as possible. A small quantity of carbon dioxide is also removed by these “purifiers,” as the presence of this gas impairs the illuminating power of coal-gas. From the purifiers the gas passes into the gas-holders, where it is stored for distribution. It remains only to be stated that the distillation of coal is effected under a pressure somewhat less than that of the atmosphere, the products of distillation being pumped out of the retorts by means of a kind of air-pump, called an exhauster, which is interpolated between the hydraulic main and the condenser, or at some other part of the purifying system. The coke left in the retort is used as fuel for burning under the retorts or for other purposes. The oxide of iron used in the purifiers can be used over and over again for a certain number of times by exposing itto the air, and when it is finally exhausted, the sulphur can be burnt out of it and used for making that most important of all chemical products, sulphuric acid. Thus the small quantity of sulphur present in the original coal (probably in the form of iron pyrites) is rendered available for the manufacture of a useful product.
The necessarily brief description of this important industry will suffice for the general reader. Those who desire further information on points of detail will refer to special works. We are here rather concerned with the subsequent fate of the different products, four of which have to be dealt with, viz. the gas, watery liquor, tar, and coke. The first and last of these having already been accounted for—the one as an illuminating agent and the other as fuel—may now be dismissed.
No story of applied science is complete unless we can form some idea of the quantities of material used, and the amount of the products obtained. From one ton of Newcastle coal we get about 10,000 cubic feet of gas, 110 to 120 lbs. of tar, 20 to 25 gallons of watery liquor, and about 1500 lbs. of coke. Different coals of course give different quantities, and the latter vary also according to the heat of distillation; but the above estimate will furnish a good basis for forming our ideas with some approach to precision. It has been estimatedalso that we are now distilling coal at the rate of about ten million tons per annum, so that there is annually produced 100,000 million cubic feet of gas, and about 500,000 tons of tar, besides proportionate quantities of the other products. The great metropolitan companies alone are consuming nearly three million tons annually for the production of gas, a consumption corresponding to about 6000 cubic feet per head of the population. This of course takes no account of the coal used for other manufactures or for domestic purposes, but it is interesting to compare these estimates with the consumption of coal in London about a century ago, before the introduction of gas, when, as Bishop Watson tells us in his work already referred to, the annual consumption was 922,394 tons.
The enormous quantity of tar resulting from our gas manufacture furnishes the raw material for the production of a multitude of valuable substances—colouring-matters, medicines, perfumes, flavouring-matters, burning and lubricating oils, &c. Out of this unsavoury waste material of the gas-works, the researches of chemists have enabled a great industry to spring up which is of continually growing importance. It will be the object of the remaining portion of the present volume to set forth the achievements of science in this branch of its application. The foundation of the coal-tarindustry was laid in this country—the country where coal was first distilled on a large scale for the production of gas, and where the first of the coal-tar colouring-matters was sent forth into commerce. We are at the present time the largest tar producers in Europe; it has been stated that we produce more than double the whole quantity of tar made in the gas-using countries of Europe; but in spite of this, our manufacture of finished products is by no means in that flourishing condition which might be expected from our natural resources in the way of coal, and the facilities which we possess for manufacturing the raw materials out of it.
But we must now take a glance at some of the other uses to which coal is put in order to realize more completely the truth of the statement made some pages back, viz. that this mineral has been the chief source of our industrial prosperity. Great as is the consumption of coal by the gas manufacturer, there is an equal or even a greater demand for the carbonaceous residue left when the coal has been decomposed by destructive distillation or by partial combustion. This residue is coke—the substance left in the retorts after the gas manufacture. There is a great demand for coke for many purposes; it is used in most cases where a cheap smokeless fuel is required; it is burnt inthe furnaces of locomotives and other engines, and is very largely consumed by the iron smelter in the blast furnace.
To meet these demands a large quantity of coal is converted into coke by being burnt in ovens with an insufficient supply of air for complete combustion, or in suitably constructed close furnaces. The tar and other products have in this country until recently been allowed to escape as waste, but the time is approaching when these must be utilized. It will give an idea of the industrial importance of coke when it is stated, that about twelve million tons of our coal annually undergo conversion into this form of fuel. Chemically considered, coke consists of carbon together with all the mineral constituents of the coal, and small quantities of hydrogen, oxygen, and nitrogen. The amount of carbon varies from 85 to 97, and the ash from 3 to 14 per cent.
The conversion of coal into coke is a very venerable branch of manufacture, which was first carried out on a large scale in this country about the middle of the seventeenth century. As an operation it may appear utterly devoid of romance, but as Goethe has described his visit to the earliest of coke-burners, this fragment of history is worth narrating. When the great German philosophical poet was a student at Strassburg (1771), he rodeover with some friends to visit the neighbourhood of Saarbrücken where he met an old “coal philosopher” named Stauf, who was there carrying on the industry. This “philosophus per ignem” was manager of some alum works, and the ruling spirit of the “burning hill” of Duttweiler. The hill no doubt owed its designation to the coke ovens at work upon it, and which had been in operation there for some six or seven years before Goethe’s visit,i.e.since 1764. The coke was wanted for iron smelting, and even at that early period Stauf had the wisdom to condense his volatile products, for we are told that he showed his visitors bitumen, burning-oil, lampblack, and even a cake of sal ammoniac resulting from his operations. Goethe has put upon record his visit to the little haggard old coke-burner, living in his lonely cottage in the forest (Aus meinem Leben: Wahrheit und Dichtung, Book X). It is probably Stauf’s ovens which are described by the French metallurgist, De Gensanne, in hisTraité de la fonte des Mines par le feu du Charbon de Terre, published in Paris in 1770. After long years of coke-making, without any regard to the value of the volatile products, we are now beginning to consider the advisability of doing that which has long been done on the Continent.
It is not unlikely that Bishop Watson in thelast century had heard of the attempt to recover the products from coke ovens, for he gives the following very sound advice in hisChemical Essays:—
“Those who are interested in the preparation of coak would do well to remember that every 96 ounces of coal would furnish four ounces at the least of oil, probably six ounces might be obtained; but if we put the product so low as five ounces from 100, and suppose a coak oven to work off only 100 tons of coal in a year, there would be a saving of five tons of oil, which would yield above four tons of tar; the requisite alteration in the structure of the coak ovens, so as to make them a kind of distilling vessels, might be made at a very trifling expense.”—5th ed., 1789, vol. ii. p. 351.
We have yet to chronicle another chapter in the history of coal philosophy before finishing with this part of the subject. There is a branch of manufacture carried on, especially in Scotland, which results in the production of burning and lubricating oils, and solid paraffin, a wax-like substance which is used for candle-making. The manufacture of candles out of coal will perhaps be a new revelation to many readers of this book. It must be admitted, however, that the term “coal” is here being extended to only partially fossilizedvegetation of younger geological age than true coal, and to bituminous shales of various ages. Shale, geologically considered, is hardened mud; it may be looked upon as clay altered by time and pressure. Now if the mud, at the period of its deposition, was much mixed up with vegetable matter, we should have in course of time a mixture of more or less carbonized woody fibre with mineral matter, and this would be called a carbonaceous or bituminous shale. Shales of this kind often contain as much as 80 to 90 per cent. of mineral matter, and seldom more than 20 per cent. of volatile matter,i.e.the portion lost on ignition, and consisting chiefly of the carbonaceous constituents.
The story of the shale-oil industry is soon told. About the year 1847 oil was “struck” in a coal mine at Alfreton in Derbyshire, and in the hands of Mr. James Young this supply furnished the market with burning-oil for nearly three years. Then the spring became exhausted, and Mr. Young and his associates had to look out for another source of oil. Be it remembered that this happened some nine years before the utilization of the great American petroleum deposits. Many kinds of vegetable matter were submitted to destructive distillation before a substance was found which could be profitably worked, but atlength Mr. Young tried a kind of cannel coal which had about that time been introduced for gas making. This substance was called Boghead gas coal or Torbane Hill mineral, from the place where it occurred, which is at Bathgate in Linlithgow. This mineral was found to yield a large amount of paraffin oil and solid paraffin on destructive distillation, and from that time (1850) to this, the industry has been carried on at Bathgate and other parts of Scotland, where similar carbonaceous deposits occur.
It may seem a matter of unimportance at the present time whether this Torbane Hill mineral is a true coal or not. About forty years ago, however, the decision of this question involved a costly law-suit in Edinburgh. The proprietor of the estate had granted a lease to a firm, conveying to the latter the right to work coal, limestone, ironstone, and certain other minerals found thereon, but excluding copper and all other minerals not mentioned in the contract. The lessees then found that this particular carbonaceous mineral was of very great value, both on account of the high quality of the gas, and afterwards on account of the paraffin which it furnished by Young’s process of distillation. Thereupon the lessor brought an action against the lessees, claiming £10,000 damages, on the ground that the latter had broken thecontract by removing a mineral which was not coal. Experts gave evidence on both sides; some declared in favour of the substance being coal, others said it was a bituminous shale, while others called it bituminated clay, or refused to give it a name at all. Judgment was finally given for the defendants, so that in the eye of the law the mineral was considered a true coal. As a matter of fact, it is impossible to draw a hard and fast line between coal and bituminous shale, as the one is connected with the other by a series of intermediate minerals, and the Torbane Hill mineral happens to form one of the links. It contains about 69 per cent. of volatile matter, and leaves 31 per cent. of residue, consisting of 12 parts of carbon and 19 of ash.
The manufacture started by Young has developed into an important industry, in spite of the fact that the original Torbane Hill coal has become exhausted, and that enormous natural deposits of petroleum are worked in America, Russia, and elsewhere. There are now some fifteen companies at work in Scotland, representing an aggregate capital of about two and a half million pounds sterling. Bituminous shales of different kinds are distilled at a low red heat in iron retorts, and from the volatile portions there are separated those valuable products which have already been alluded to, viz. burning and lubricating oils, solvent mineraloil, paraffin wax for candles, and ammonia. We may fairly claim these as coal products, although the shales used contain much mineral matter, the carbon averaging about 20 per cent., the hydrogen three per cent., the nitrogen 0·7, and the ash about 67 per cent. The shales worked are approximately of the same age as true coal,i.e.Carboniferous. The Scotch companies are distilling about two million tons of shale per annum, this quantity producing about sixty million gallons of crude oil, and giving employment to over 10,000 hands.
It is not the province of the present work to enter into the chemical nature of the products of destructive distillation in any greater detail than is necessary to enable the general reader to know something of the recent discoveries in the utilization of these products. We shall, however, have occasion later on to make ourselves acquainted with the names of some of the more important raw materials which are derived from this source, and certain preliminary explanations are indispensable. In the first place then, let us start from the fact that coal—including carbonaceous shale and lignite—when heated in a closed vessel gives gas, tar, coke, and a watery liquor. A clear understanding must be arrived at concerning the manner in which these products arise.
There is a widely-spread notion that thesubstances derived from coal and utilized for industrial purposes are present in the mineral itself, and that the art of the chemist has been exercised in separating the said substances by various processes. This idea must be at once dispelled. It is true that there is a small quantity of water and a certain amount of gas already present in most coals, but these are quite insignificant as compared with the total yield of gas and watery liquor. So also with respect to the tar; it is possible that in some highly bituminous minerals we might dissolve out a small quantity of tarry matter by the use of appropriate solvents, but in the coals mostly used for gas-making not a trace of tar exists ready formed, and still less can it be said that the coal contains coke. All these products are formedby the chemical decomposition of the coalunder the influence of heat, and their nature and quantity can be made to vary within certain limits by modifying the temperature of distillation.
Having once realized this principle with respect to coal itself, it is easy to extend it to the products of its destructive distillation. The tar, for instance, is a complicated mixture of various substances, among which hydrocarbons—i.e.compounds of carbon with hydrogen—largely predominate. The different components of coal-tar can be separated by processes which we shall have to considersubsequently. Of the compounds thus isolated some few are immediately applicable for industrial purposes, but the majority only form the raw materials for the manufacture of other products, such as colouring-matters and medicines. Now these colouring-matters and other finished products no more exist in the tar than the latter exists in the coal. They are produced from the hydrocarbons, &c., present in the tarby chemical processes, and bear much about the same relationship to their parent substances that a steam-engine bears to the iron ore out of which its metallic parts are primarily constructed. Just as the mechanical skill of the engineer enables him to construct an engine out of the raw material iron, which is extracted from its ore, and converted into steel by chemical processes, so the skill of the chemist enables him to build up complex colouring-matters, &c., out of the raw materials furnished by tar, which is obtained from coal by chemical decomposition.
The illuminating gas which is obtained from coal by destructive distillation consists chiefly of hydrogen and gaseous hydrocarbons, the most abundant of the latter being marsh gas. There are also present in smaller quantities the two oxides of carbon, the monoxide and the dioxide, which are gaseous at ordinary temperatures, together with other impurities. Coal-gas is burntjust as it is delivered from the mains—it is not at present utilized as a source of raw material in the sense that the tar is thus made use of. In some cases gas is used as fuel, as in gas-stoves and gas-engines, and in the so-called “gas-producers,” in which the coal, instead of being used as a direct source of heat, is partially burnt in suitable furnaces, and the combustible gas thus arising, consisting chiefly of carbon monoxide, is conveyed to the place where it undergoes complete combustion, and is thus utilized as a source of heat.
Summing up the uses of coal thus far considered, we see that this mineral is being consumed as fuel, for the production of coke, for the manufacture of gas, and in many other ways. Lavishly as Nature has provided us with this source of power and wealth, the idea naturally suggests itself whether we are not drawing too liberally upon our capital. The question of coal supply crops up from time to time, and the public mind is periodically agitated about the prospects of its continuance. How long we have been draining our coal resources it is difficult to ascertain. There is some evidence that coal-mining was carried on during the Roman occupation. In the reign of Richard I. there is distinct evidence of coal having been dug in the diocese of Durham. The oldest charters take us back to the early part of the thirteenth century for Scotland, and to theyear 1239 for England, when King Henry III. granted a right of sale to the townsmen of Newcastle. With respect to the metropolis, Bishop Watson, on the authority of Anderson’sHistory of Commerce, states that coal was introduced as fuel at the beginning of the fourteenth century. In these early days, when it was brought from the north by ships, it was known as “sea-coal”:—
“Go; and we’ll have a posset for ’t soon at night, in faith, at the latter end of a sea-coal fire.”—Merry Wives of Windsor, Act I., Sc. iv.
“Go; and we’ll have a posset for ’t soon at night, in faith, at the latter end of a sea-coal fire.”—Merry Wives of Windsor, Act I., Sc. iv.
That the fuel was received at first with disfavour appears from the fact that in the reign of Edward I. the nobility and gentry made a complaint to the king objecting to its use, on the ground of its being a public nuisance. By the middle of the seventeenth century the use of coal was becoming more general in London, chiefly owing to the scarcity of wood; and its effects upon the atmosphere of the town will be inferred from a proclamation issued in the reign of Elizabeth, prohibiting its use during the sitting of Parliament, for fear of injuring the health of the knights of the shire. About 1649 the citizens again petitioned Parliament against the use of this fuel on account of the stench; and about the beginning of that century “the nice dames of London would not come into any house or roomewhen sea-coales were burned, nor willingly eat of meat that was either sod or roasted with sea-coale fire” (Stow’s Annals).
For many centuries therefore we have been drawing upon our coal supplies, and using up the mineral at an increasing rate. According to a recent estimate by Professor Hull, from the beginning of the present century to 1875 the output has been more than doubled for each successive quarter century. The actual amount of coal raised in the United Kingdom between 1882 and the present time averages annually about 170 million tons, corresponding in money value to about £45,000,000 per annum. In 1860 the amount of coal raised in Great Britain was a little more than 80 million tons, and Professor Hull estimated that at that rate of consumption our supplies of workable coal would hold out for a thousand years. Since then the available stock has been diminished by some 3,650 million tons, and even this deduction, we are told on the same authority, has not materially affected our total supply. The possibility of a coal famine need, therefore, cause no immediate anxiety; but we cannot “eat our loaf and have it too,” and sooner or later the continuous drain upon our coal resources must make itself felt. The first effect will probably be an increase in price owing to the greater depth at which the coal will have to be worked.The whole question of our coal supply has, however, recently assumed a new aspect by the discovery (February 1890) of coal at a depth of 1,160 feet at Dover. To quote the words of Mr. W. Whitaker—“It may be indeed that the coal supply of the future will be largely derived from the South-East of England, and some day it may happen, from the exhaustion of our northern coal-fields, that we in the south may be able successfully to perform a task now proverbially unprofitable—we may carry coal to Newcastle.”
The coal-fields of Great Britain and Ireland occupy, in round numbers, an area of 11,860 square miles, or about one-tenth of the whole area of the land surface of the country. Within this area, and down to a depth of 4,000 feet, lie the main deposits of our available wealth. Some idea of the amount of coal underlying this area will be gathered from the table[2]on the next page.
This supply, amounting to over 90,000 million tons, refers to the exposed coal-fields and to workable seams,i.e.those above one foot in thickness. But in addition to this, we have a large amount of coal at workable depths under formations of later geological age than the Carboniferous, such as the Permian formation of northern and central England. Adding the estimated quantity of coalfrom this source to that contained in the exposed coal-fields as given above, we arrive at the total available supply. This is estimated to be about 146,454 million tons. To this we may one day have to add the coal under the south-eastern part of England.
It is important to bear in mind, that out of the 170 million tons of coal now being raised annually we only use a small proportion, viz. from 5 to 6per cent. for gas-making. The largest amount (33 per cent.) is used for iron-smelting,[3]and about 15 per cent. is exported; the remainder is consumed in factories, dwelling-houses, for locomotion, and in the smaller industries.
The enormous advancement which has taken place of late years in the industrial applications of electricity has given rise to the belief that coal-gas will in time become superseded as an illuminating agent, and that the supply of tar may in consequence fall off. So far, however, the introduction of electric-lighting has had no appreciable effect upon the consumption of gas, and even when the time of general electric-lighting arrives there will arise as a consequence an increased demand for gas as a fuel in gas engines. Moreover, the use of gas for heating and cooking purposes is likely to go on increasing. Nor must it be forgotten that the quantity of tar produced in gas-works is now greater than is actually required by the colour-manufacturer, and much of this by-product is burnt as fuel, so that if the manufacture of gas were tosuffer to any considerable extent there would still be tar enough to meet our requirements at the present rate of consumption of the tar-products. Then again, the value of the tar, coke, and ammoniacal liquor is of such a proportion as compared with the cost of the raw material, coal, that there is a good margin for lowering the price of gas when the competition between the latter and electricity actually comes about. It will not then be only a struggle between the two illuminants, but it will be a question of electricityversusgas,plustar and ammonia.
While the electrician is pushing forward with rapid strides, the chemist is also moving onwards, and every year witnesses the discovery of new tar products, or the utilization of constituents which were formerly of little or no value. Thus if the cost of generating and distributing electricity is being lowered, on the other hand the value of coal tar is likely to go on advancing, and it would be rash to predict which will come out triumphant in the end. But even if electricity were to gain the day it would be worth while to distil coal at the pit’s mouth for the sake of the by-products, and there is, moreover, the tar from the coke ovens to fall back upon—a source which even before the use of coal-gas the wise Bishop of Llandaff advised us not to neglect.
The nature of the products obtained by the destructive distillation of coal varies according to the temperature of distillation, and the age or degree of carbonization of the coal. The watery liquor obtained by the dry distillation of wood is acid, and contains among other things acetic acid, which is sometimes prepared in this way, and from its origin is occasionally spoken of as “wood vinegar.” The older the wood, the more complete its degree of conversion into coal, and the smaller the quantity of oxygen it contains, the more alkaline does the watery liquid become. Thus the gas-liquor is distinctly alkaline, and contains a considerable quantity of ammonia, besides other volatile bases. The uses of ammonia are manifold, and nearly our whole supply of this valuable substance is now derived from gas-liquor. The presence of ammonia in this liquor is accounted for when it is known that this compound is a gascomposed of nitrogen and hydrogen. It has already been explained that coal contains from one to two per cent. of nitrogen, and during the process of distillation about one-fifth of this nitrogen is converted into ammonia, the remainder being converted partly into other bases, while a small quantity remains in the coke.
Ammonia, the “volatile alkali” of the old chemists, and its salts are of importance in pharmacy, but the chief use of this compound is to supply nitrogen for the growth of plants. Plants must have nitrogen in some form or another, and as they cannot assimilate itdirectlyfrom the atmosphere where it exists in the free state, some suitable nitrogen compound must be supplied to the soil. It is possible that certain leguminous plants may derive their nitrogen from the atmosphere through the intervention of micro-organisms, which appear capable of fixing free nitrogen and of supplying it to the plant upon whose roots they flourish. But this is second-hand nitrogen so far as concerns the plant. It is true also that the atmosphere contains small traces of ammonia and acid oxides of nitrogen, which are dissolved by rain and snow, and thus get washed down into the soil. These are the natural sources of plant nitrogen. But in agricultural operations, where large crops have to be raised as rapidly as possible, someadditional source of nitrogen must be supplied, and this is the object of manuring the soil.
A manure, chemically considered, is a mixture of substances capable of supplying the necessary nitrogenous and mineral food for the nourishment of the growing plant. The ordinary farm or stable manure contains decomposing nitrogenous organic matter, in which the nitrogen is given off as ammonia, and thus furnishes the soil with which it is mixed with the necessary fertilizer. But the supply of this manure is limited, and we have to fall back upon gas-liquor and native nitrates to meet the existing wants of the agriculturist. Important as is ammonia for the growth of vegetation, it is not in this form that the majority of plants take up their nitrogen. Soluble nitrates are, in most cases, more efficient fertilizers than the salts of ammonia, and the ammonia which is supplied to the soil is converted into nitrates therein before the plant can assimilate the nitrogen. The oxidation of ammonia into nitric acid takes place by virtue of a process called “nitrification,” and there is very good reason for believing that this transformation is the work of a micro-organism present in the soil. The gas liquor thus supplies food to a minute organism which converts the ammonia into a form available for the higher plants. Some branches of agriculture—such as the cultivation of the beet forsugar manufacture—are so largely dependent upon an artificial source of nitrogen, that their very existence is bound up with the supply of ammonia salts or other nitrogenous manures. The relationship between the manufacture of beet-sugar and the distillation of coal for the production of gas is thus closer than many readers will have imagined; for while the supply of native guano or nitrate is uncertain, and its freight costly on account of the distance from which it has to be shipped, the sulphate of ammonia from gas-liquor is always at hand, and available for the purposes of fertilization.
Then again, there are other products of industrial value which are associated with ammonia, such, for example, as ammonia-alum and caustic soda. This last is one of the most important chemical compounds manufactured on a large scale, and is consumed in enormous quantities for the manufacture of paper and soap, and other purposes. Salts of this alkali are also essential for glass making. Of late years a method for the production of caustic soda has been introduced which depends upon the use of ammonia, and as this process is proving a formidable rival to the older method of alkali manufacture, it may be said that such indispensable articles as paper, soap, and glass are now to some extent dependent upon gas-liquor, andmay in course of time become still more intimately connected with the manufacture of coal-gas.
But quantitative statements must be given in order to bring home to general readers the actual value of the small percentage of nitrogen present in coal. Thus it has been estimated, that one ton of coal gives enough ammonia to furnish about 30 lbs. of the crude sulphate. The present value of this salt is roughly about £12 per ton. The ten million tons of coal distilled annually for gas making would thus give 133,929 tons of sulphate, equal in money value to £1,607,148, supposing the whole of the ammonia to be sold in this form. To this may be added the ammonia obtained during the distillation of shale and the carbonization of coal for coke, the former source furnishing about 22,000 tons, and the latter about 2500 tons annually. Small as is the legacy of nitrogen bequeathed to us from the Carboniferous period, we see that it sums up to a considerable annual addition to our industrial resources.
The three products resulting from the distillation of coal—viz. the gas, ammoniacal-liquor, and coke—having now been made to furnish their tale, we have next to deal with the tar. In the early days of gas manufacture this black, viscid, unsavoury substance was in every sense a waste product. No use had been found for it, and it was burnt, orotherwise disposed of. No demand for the tar existed which could enable the gas manufacturers to get rid of their ever-increasing accumulation. Wood-tar had previously been used as a cheap paint for wood and metal-work, and it was but a natural suggestion that coal-tar should be applied to the same purposes. It was found that the quality of the tar was improved by getting rid of the more volatile portions by boiling it in open pans; but this waste—to say nothing of the danger of fire—was checked by a suggestion made by Accum in 1815, who showed that by boiling down the tar in a still instead of in open pans the volatile portions could be condensed and collected, thus furnishing an oil which could be used by the varnish maker as a substitute for turpentine. A few years later, in 1822, the distillation of tar was carried on at Leith by Drs. Longstaff and Dalston, the “spirit” being used by Mackintosh of Glasgow for dissolving india-rubber for the preparation of that waterproof fabric which to this day bears the name of the original manufacturer. The residue in the still was burnt for lamp-black. Of such little value was the tar at this time that Dr. Longstaff tells us that the gas company gave them the tar on condition that they removed it at their own expense. It appears also that tar was distilled on a large scale near Manchester in 1834, the “spirit”being used for dissolving the residual pitch so as to make a black varnish.
But the production of gas went on increasing at a greater rate than the demand for tar for the above-mentioned purposes, and it was not till 1838 that a new branch of industry was inaugurated, which converted the distillation of this material from an insignificant into an important manufacture. In that year a patent was taken out by Bethell for preserving timber by impregnating it with the heavy oil from coal-tar. The use of tar for this purpose had been suggested by Lebon towards the end of the last century, and a patent had been granted in this country in 1836 to Franz Moll for this use of tar-products. But Bethell’s process was put into a working form by the great improvements in the apparatus introduced by Bréant and Burt, and to the latter is due the credit of having founded an industry which is still carried on by Messrs. Burt, Bolton and Haywood on a colossal scale. The “pickling” or “creosoting” of timber is effected in an iron cylindrical boiler, into which the timber is run; the cylinder being then closed the air is pumped out, and the air contained in the pores of the wood thus escapes. The creosoting oil, slightly warmed, is then allowed to flow into the boiler, and thus penetrates into the pores of the wood, the complete saturation of which isinsured by afterwards pumping air into the cylinder and leaving the timber in the oil for some hours under a pressure of 8 to 10 atmospheres.
All timber which is buried underground, or submerged in water, is impregnated with this antiseptic creosote in order to prevent decay. It will be evident that this application of tar-products must from the very commencement have had an enormous influence upon the distillation of tar as a branch of industry. Consider the miles of wooden sleepers over which our railways are laid, and the network of telegraph wires carried all over the country by wooden poles, of which the ends are buried in the earth. Consider also the many subaqueous works which necessitate the use of timber, and we shall gain an idea of the demand for heavy coal-tar oil created by the introduction of Bréant’s process. Under the treatment described a cubic foot of wood absorbs about a gallon of oil, and by far the largest quantity of the tar oils is consumed in this way at the present time. Now in the early days of timber-pickling the lighter oils of the tar, which first come over on distillation, and which are too volatile for the purpose of creosoting, were in much about the same industrial position as the tar itself before its application as a timber preservative. The light oil had a limited use as a solvent for waterproofing and varnishmaking, and a certain quantity was burnt as coal-tar naphtha in specially constructed lamps, the invention of the late Read Holliday of Huddersfield, whose first patent was taken out in 1848 (seeFig. 4). Up to this time, be it remembered, that chemists had not found out what this naphtha contained. But science soon laid hands on the materials furnished by the tar-distiller, and the naphtha was one of the first products which was made to reveal the secret of its hidden treasures to the scientific investigator. From this period science and industry became indissolubly united, and the researches of chemists were carried on hand-in-hand with the technical developments of coal-tar products.
Fig. 4.—Read Holliday’s lamp for burning light coal-tar oils. The oil is contained in the cisternc, from whence it flows down the pipe, when the stopcock is opened, into the burnera. Below the burner is a little cup, in which some of the oil is kept burning, and the heat from this flame volatilizes the oil as it flows down the pipe, the vapour thus generated issuing from the jets in the burner and there undergoing ignition. The burner and cup are shown on an enlarged scale atain the lower figure.
In 1825 Michael Faraday discovered a hydrocarbon in the oil produced by the condensation of “oil gas”—an illuminating gas obtained by the destructive distillation of oleaginous materials. This hydrocarbon was analysed by its illustrious discoverer, and named in accordance with his results “bicarburet of hydrogen.” In 1834 the same hydrocarbon was obtained by Mitscherlich by heating benzoic acid with lime, and by Péligot by the dry distillation of calcium benzoate. For this reason the compound was named “benzin” by Mitscherlich, which name was changed into “benzol” by Liebig. In this country the hydrocarbon is known at the present time as benzene. Twenty years after Faraday’s discovery, viz. in 1845, Hofmann proved the existence of benzene in the light oils from coal-tar, and in 1848 Hofmann’s pupil, Mansfield, isolated considerable quantities of this hydrocarbon from the said light oils by fractional distillation. At the time of these investigations no great demand for benzene existed, but the work of Hofmann and Mansfield prepared the way for its manufacture on a large scale, when, a few years later, the first coal-tar colouring-matter was discovered by our countryman, W. H. Perkin.
It is always of interest to trace the influenceof scientific discovery upon different branches of industry. As soon as it had been shown that benzene could be obtained from coal-tar, the nitro-derivative of this hydrocarbon—i.e.the oily compound produced by the action of nitric acid upon benzene—was introduced as a substitute for bitter almond oil under the name of “essence of mirbane.” Nitrobenzene has an odour resembling that of bitter almond oil, and it is still used for certain purposes where the latter can be replaced by its cheaper substitute, such as for the scenting of soap. Although the isolation of benzene from coal-tar gave an impetus to the manufacture of nitrobenzene, no use existed for the latter beyond its very limited application as “essence of mirbane,” and the production of this compound was at that time too insignificant to take rank as an important branch of chemical industry.
The year 1856 marks an epoch in the history of the utilization of coal-tar products with which the name of Perkin will ever be associated. In the course of some experiments, having for their object the artificial production of quinine, this investigator was led to try the action of oxidizing agents upon a base known as aniline, and he thus obtained a violet colouring matter—the first dye from coal-tar—which was manufactured under a patent granted in 1858, and introduced into commerce under thename of mauve. A brief sketch of the history of aniline will serve to show how Perkin’s discovery gave a new value to the light oils from coal-tar and raised the manufacture of nitrobenzene into an important branch of industry.
Thirty years before Perkin’s experiments the Dutch chemist Unverdorben obtained (1826) a liquid base by the distillation of indigo, which had the property of forming beautifully crystalline salts, and which he named for this reason “crystallin.” In 1834 Runge discovered the same base in coal-tar, although its identity was not known to him at the time, and because it gave a bluish colour when acted upon by bleaching-powder, he called it “kyanol.” Again in 1840, by distilling a product obtained by the action of caustic alkalies upon indigo, Fritzsche prepared the same base, and gave it the name of aniline, from the Spanish designation of the indigo plant, “anīl,” derived from the native Indian word, by which name the base is known at the present time. That aniline could be obtained by the reduction of nitrobenzene was shown by Zinin in 1842, who used sulphide of ammonium for reducing the nitrobenzene, and named the resulting base “benzidam.” The following year Hofmann showed that crystallin, kyanol, aniline, and benzidam were all one and the same base. Thus when the discovery of mauve opened up ademand for aniline on the large scale, the labours of chemists, from Unverdorben in 1826 to Hofmann in 1843, had prepared the way for the manufacturer. It must be understood that although Runge had discovered aniline in coal-tar, this is not the source of our present supply, for the quantity is too small to make it worth extracting. A mere trace of aniline is present in the tar ready formed; from the time this base was wanted in large quantities it had to be made by nitrating benzene, and then reducing the nitrobenzene.
The light oils of tar distillation rejected by the timber-pickling industry now came to the front, imbued with new interest to the technologist as a source of benzene for the manufacture of aniline. The inauguration of this manufacture, like the introduction of steam locomotion, is connected with a sad catastrophe. Mansfield, who first showed manufacturers how to separate benzene and other hydrocarbons from the light oils of coal-tar, and who devised for this purpose apparatus similar in principle to that used on a large scale at the present time (seeFig. 5), met with an accident which resulted in his death. In the upper part of a house in Holborn in February 1856, this pioneer was carrying on his experiments, when the contents of a still boiled over and caught fire. In his endeavours to extinguish the flames he receivedthe injuries which terminated fatally. Applied science no less than pure science has had its martyrs, and among these Mansfield must be ranked.