_Magnesia._--Suppose lime and magnesia are present. You may first evaporate to a small bulk, adding a drop of hydrochloric acid if the liquid becomes muddy. Then add ammonia and ammonium oxalate, when lime alone is precipitated as the oxalate of lime. Filter through blotting paper, and to the clear filtrate add some phosphate of soda solution. A second precipitation proves the presence of magnesia.

_Sulphates._--A solution of barium chloride and dilute hydrochloric acid gives a white turbidity.

_Chlorides._--A solution of silver nitrate and nitric acid gives a white curdy precipitate.

_Test for Lead in Drinking Water._--I will, lastly, give you a test that will be useful in your own homes to detect minute quant.i.ties of lead in water running through lead pipes. Place a large quant.i.ty of the water in a gla.s.s on a piece of white paper, and add a solution of sulphuretted hydrogen and let stand for some time. A brown colour denotes lead. Of course copper would also yield a brown coloration, but I am supposing that the circ.u.mstances preclude the presence of copper.

I have already said that rain water is the purest of natural waters; it is so soft, and free from dissolved mineral matters because it is a distilled water. In distilling water to purify it, we must be very careful what material we use for condensing the steam in, since it is a fact probably not sufficiently well known, that the softer and purer a water is, the more liable it is to attack lead pipes. Hence a coil of lead pipe to serve as condensing worm would be inadmissible. Such water as Manchester water, and Glasgow water from Loch Katrine still more so, are more liable to attack lead pipes than the hard London waters. To ill.u.s.trate this fact, we will distil some water and condense in a leaden worm, then, on testing the water with our reagent, the sulphuretted hydrogen water, a brown colour is produced, showing the presence of lead. On condensing in a block tin worm, however, no tin is dissolved, so tin is safer and better as the material for such a purpose than lead.

_Filtration._--We hear a great deal about filtration or filters as universal means of purifying water. Filtration, we must remember, will, as a rule, only remove solid or suspended impurities in water. For example, if we take some ivory black or bone black, and mix it with water and afterwards filter the black liquid through blotting-paper, the bone black remains on the paper, and clear, pure water comes through.

Filtering is effective here. If we take some indigo solution, however, and pour it on to the filter, the liquid runs through as blue as it was when poured upon the filter. Filtering is ineffective here, and is so generally with liquids containing matters dissolved in them. But I said "generally," and so the question is suggested--Will filtration of any kind remove matters in solution? This question I will, in conclusion, try to answer. Bone charcoal, or bone black, has a wonderful attraction for many organic matters such as colours, dyes, and coloured impurities like those in peat water, raw sugar solutions, etc. For example, if we place on a paper filter some bone black, and filter through it some indigo solution, after first warming the latter with some more of the bone black, the liquid comes through clear, all the indigo being absorbed in some peculiar way, difficult to explain, by the bone black, and remaining on the filter. This power of charcoal also extends to gases, and to certain noxious dissolved organic impurities, but it is never safe to rely too much on such filters, since the charcoal can at length become charged with impurities, and gradually cease to act. These filters need cleaning and renewing from time to time.

LECTURE V

ACIDS AND ALKALIS

_Properties of Acids and Alkalis._--The name acids is given to a cla.s.s of substances, mostly soluble in water, having an acid or sour taste, and capable of turning blue litmus solution red. All acids contain one or more atoms of hydrogen capable of being replaced by metals, and when such hydrogen atoms are completely replaced by metals, there result so-called neutral or normal salts, that is, neutral substances having no action on litmus solution. These salts can also be produced by the union of acids with equivalent quant.i.ties of certain metallic oxides or hydroxides, called bases, of which those soluble in water are termed alkalis. Alkalis have a caustic taste, and turn red litmus solution blue.

In order to explain what is called the law of equivalence, I will remind you of the experiment of the previous lecture, when a piece of bright iron, being placed in a solution of copper sulphate, became coated with metallic copper, an equivalent weight of iron meanwhile suffering solution as sulphate of iron. According to the same law, a certain weight of soda would always require a certain specific equivalent weight of an acid, say hydrochloric acid, to neutralise its alkaline or basic properties, producing a salt.

The specific gravities of acids and alkalis in solution are made use of in works, etc., as a means of ascertaining their strengths and commercial values. Tables have been carefully constructed, such that for every degree of specific gravity a corresponding percentage strength of acidity and alkalinity may be looked up. The best tables for this purpose are given in Lunge and Hurter's _Alkali-Makers' Pocket-Book_, but for ordinary purposes of calculation in the works or factory, a convenient relationship exists in the case of hydrochloric acid between specific gravity and percentage of real acid, such that specific gravity as indicated by Twaddell's hydrometer directly represents percentage of real acid in any sample of hydrochloric acid.

The point at which neutralisation of an acid by alkali or _vice versa_ just takes place is ascertained very accurately by the use of certain sensitive colours. At first litmus and cochineal tinctures were used, but in testing crude alkalis containing alumina and iron, it was found that lakes were formed with these colours, and they become precipitated in the solution, and so no longer sensitive. The chemist was then obliged to resort to certain sensitive coal-tar colours, which did not, as the dyer and printer knew, form lakes with alumina and iron, such as methyl orange, fluorescein, Congo red, phenolphthalein, and so forth.

For determining the alkalimetric strength of commercial sodas, a known weight of the sample is dissolved in water, and a few drops of a solution of methyl orange are added, which colour the solution yellow or orange. Into this solution is then run, from a burette or graduated tube, a standard solution of an acid, that is, a solution prepared by dissolving a known weight of an acid, say hydrochloric acid, in a known volume of water. The acid is run in gradually until the yellow colour changes to pink, at which point the volume of acid used is noted.

Knowing the weight of acid contained in this volume of standard acid, and having regard to the law of equivalence mentioned above, it is an easy matter to calculate the amount of alkali equivalent to the acid used, and from this the alkali contained in the sample.

_Sulphuric Acid._--The first process for manufacturing sulphuric acid or vitriol was by placing some burning sulphur in a closed vessel containing some water. The water absorbed the acid formed by the burning sulphur. It was next discovered that by mixing with the sulphur some nitre, much more sulphuric acid could be produced per given quant.i.ty of brimstone. At first large gla.s.s carboys were used, but in 1746 the carboys were replaced by chambers of lead containing water at the bottom, and in these lead chambers the mixture of sulphur and nitre was burnt on iron trays. Next, although gradually, the plant was divided into two portions--a furnace for burning the sulphur, and a chamber for receiving the vapours. The system was thus developed into the one followed at the present time. The sulphur, or, in most cases, cupreous iron pyrites (a combination of iron and copper with sulphur), is burned in specially constructed kilns or furnaces, and the hot gases, consisting essentially of sulphur dioxide with the excess of air, pa.s.s through flues in which are placed cast-iron "nitre pots" containing a mixture of nitre (sodium nitrate) and vitriol. The gases thus become mixed with nitrous fumes or gaseous oxides of nitrogen, and, after cooling, are ready for mixing with steam or water spray in the lead chambers in which the vitriol is produced. These oxides of nitrogen enable the formation of sulphuric acid to take place more quickly by playing the part of oxygen-carriers. Sulphuric acid is formed by the union of oxygen with sulphur dioxide and water; the oxides of nitrogen combine with the oxygen of the air present in the chambers, then give up this oxygen to the sulphur dioxide and water or steam to form sulphuric acid, again combine with more oxygen, and so on. The exact processes or reactions are of course much more complicated, but the above represents what is practically the ultimate result. It is evident that the gases leaving the last lead chamber in which the formation of vitriol is effected, must still contain nitrous fumes, and it becomes a matter of importance to recover them, so that they can be used over again. To effect this object, use is made of the solubility of nitrous fumes in strong vitriol. The gases from the last lead chamber of the series are pa.s.sed through what is called a Gay-Lussac tower (the process was invented by the eminent French chemist Gay-Lussac), which is a tower made of lead, supported by a wooden framework, and filled with c.o.ke or special stoneware packing, over which strong vitriol is caused to flow.

The vitriol dissolves the nitrogen oxides, and so-called "nitrous vitriol" flows out at the base of the tower. The recovery of the nitrogen compounds from the nitrous vitriol is effected in Glover towers (the invention of John Glover of Newcastle), which also serve to concentrate to some extent the weak acid produced in the lead chambers, and to cool the hot gases from the sulphur burners or pyrites kilns. The weak chamber acid is mixed with the nitrous vitriol from the Gay-Lussac tower, and the mixture is pumped to the top of the Glover tower, which is of similar construction to the Gay-Lussac tower, but is generally packed with flints. This Glover tower is placed between the sulphur burners or pyrites kilns and the first lead chamber. The nitrous vitriol pa.s.sing down the tower meets the hot gases from the kilns, and a threefold object is effected: (1) The nitrous fumes are expelled from the nitrous vitriol, and are carried into the chambers, to again play the part of oxygen-carriers; (2) the weak chamber acid which was mixed with the nitrous vitriol is concentrated by the hot kiln gases; and (3) the hot gases themselves are cooled. The acid from the Glover tower is purified by special treatment--for example, the a.r.s.enic may be removed, after precipitation with sulphuretted hydrogen, in the form of insoluble a.r.s.enic sulphide,--and the purified acid is concentrated by heating in gla.s.s or platinum vessels.

A considerable amount of sulphuric acid is now made by the so-called "contact process," in which sulphur dioxide and oxygen unite to form sulphuric acid in presence of a heated "contact" substance, usually some form of finely-divided platinum.

_Nitric Acid._--This acid is usually prepared by distilling a mixture of sodium nitrate and vitriol in cast-iron retorts or pots, the nitric acid being collected in stoneware vessels connected one with another, or, as is more generally the case at the present time, in condensing apparatus consisting of stoneware pipes or coils cooled by water. The effluent gases are pa.s.sed through a scrubber in order to free them from the last traces of acid before discharging them into the atmosphere.

_Hydrochloric Acid._--The greater part of the hydrochloric acid manufactured in Great Britain is obtained as an intermediate product in the Leblanc alkali process, which will presently be described, being produced by heating common salt with vitriol. A large quant.i.ty is, however, also produced by the so-called direct process of Hargreaves & Robinson, which is, in principle, the same method as that employed in the Leblanc process, except that the intermediate product, vitriol, is not separated. It consists essentially in pa.s.sing the hot gases from pyrites kilns, as used in the manufacture of vitriol, through large cast-iron vessels containing common salt heated to a high temperature.

Various physical conditions must be complied with in order to make the process a success. For example, the salt is used in the form of moulded hard porous cakes made from a damp mixture of common salt and rock salt.

The cast-iron vessels must be heated uniformly, and the hot pyrites kiln gases must be pa.s.sed downwards through the salt in order to ensure uniform distribution. The hydrochloric acid is condensed in stoneware pipes connected with towers packed with c.o.ke or stoneware.

_Alkali: Leblanc Process._--The manufacture of vitriol, as I have described it to you, is the first step in the Leblanc process. The next stage consists in the manufacture of sodium sulphate (salt-cake) and hydrochloric acid from the sulphuric acid and common salt; this is called the salt-cake process. The production of salt-cake or crude sodium sulphate is carried out in two stages. A large covered iron pan, called the decomposing pan or salt-cake pot, is mounted in one part of the salt-cake furnace, and alongside it is the hearth or bed on which the second stage of the process, the drying or roasting, is effected.

The mixture of common salt and vitriol is charged into the salt-cake pot, which is heated by a fire below. When from two-thirds to three-quarters of the hydrochloric acid has been expelled from the charge, the ma.s.s acquires the consistence of thick dough, and at this stage it is raked out of the pan on to the roasting hearth alongside, where the decomposition is completed by means of flames playing directly on to the top of the charge. The hydrochloric acid evolved during the process is condensed in much the same manner as in the process of Hargreaves & Robinson previously described. It is a curious fact that in the earlier years of the Leblanc process, hydrochloric acid, or "spirits of salt," as it is frequently called, was a by-product that required all the vigilance of the alkali-works inspectors to prevent it being allowed to escape from the chimneys in more than a certain small regulated amount. Now, it is the princ.i.p.al product; indeed, the Leblanc alkali maker may be said to subsist on that hydrochloric acid, as his chief instrument for producing chloride of lime or bleaching powder.

Mechanical furnaces are now used to a large extent for the salt-cake process. They consist broadly of a large revolving furnace-hearth or bed, on to which the mixture of salt and vitriol is charged, and on which it is continuously agitated, and gradually moved to the place of discharge, by rakes or the like, operated by suitable machinery.

The next stage of the Leblanc process is the manufacture of "black ash,"

or crude sodium carbonate. This is usually done in large cylindrical revolving furnaces, through, which flames from a fire-grate, or from the burning of gaseous fuel, pa.s.s; the waste heat is utilised for boiling down "black ash" liquor, obtained by lixiviating the black ash. A mixture of salt-cake, limestone or chalk (calcium carbonate), and powdered coal or coal slack is charged into the revolving cylinder; during the process the ma.s.s becomes agglomerated, and the final product is what is known as a "black-ash ball," consisting chiefly of crude sodium carbonate and calcium sulphide, but containing smaller quant.i.ties of many other substances. The soda ash or sodium carbonate is obtained from the black ash by lixiviating with water, and after various purification processes, the solution is boiled down, as previously stated, by the waste heat of the black-ash furnace. The alkali is sold in various forms as soda ash, soda crystals, washing soda, etc.

Caustic soda is manufactured from solution of carbonate of soda by causticising, that is, treatment with caustic lime or quicklime.

It will have been noticed that one of the chief reagents in the Leblanc process is the sulphur used in the form of brimstone or as pyrites for making vitriol in the first stage; this sulphur goes through the entire process; from the vitriol it goes to form a const.i.tuent of the salt-cake, and afterwards of the calcium sulphide contained in the black ash. This calcium sulphide remains as an insoluble ma.s.s when the carbonate of soda is extracted from the black ash, and forms the chief const.i.tuent of the alkali waste, which until the year 1880 could be seen in large heaps around chemical works. Now, however, by means of treatment with kiln gases containing carbonic acid, the sulphur is extracted from the waste in the form of hydrogen sulphide, which is burnt to form vitriol, or is used for making pure sulphur; and so what was once waste is now a source of profit.

_Ammonia-Soda Process of Alkali Manufacture._--This process depends upon the fact that when carbonic acid is forced, under pressure, into a saturated solution of ammonia and common salt, sodium bicarbonate is precipitated, whilst ammonium chloride or "sal-ammoniac" remains dissolved in the solution. The reaction was discovered in 1836 by a Scotch chemist named John Thom, and small quant.i.ties of ammonia-soda were made at that time by the firm of McNaughton & Thom. The successful carrying out of the process on the large scale depends princ.i.p.ally upon the complete recovery of the expensive reagent, ammonia, and this problem was only solved within comparatively recent years by Solvay. The process has been perfected and worked with great success in England by Messrs. Brunner, Mond, & Co., and has proved a successful rival to the Leblanc process.

Alkali is also produced to some extent by electrolytic processes, depending upon the splitting up of a solution of common salt into caustic soda and chlorine by the use of an electric current.

LECTURE VI

BORIC ACID, BORAX, SOAP

_Boric Acid._--At ordinary temperatures and under ordinary conditions boric acid is a very weak acid, but like silicic and some other acids, its relative powers of affinity and combination become very much changed at high temperatures; thus, fused and strongly heated boric acid can decompose carbonates and even sulphates, and yet a current of so weak an acid as hydrogen sulphide, pa.s.sed through a strong solution of borax, will decompose it and set free boric acid. Boric acid is obtained chiefly from Italy. In a tract of country called the Maremma of Tuscany, embracing an area of about forty square miles, are numerous chasms and crevices, from which hot vapour and heated gases and springs of water spurt. The steam issuing from these hot springs contains small quant.i.ties of boric acid, that acid being one of those solid substances distilling to some extent in a current of steam. The steam vapours thus bursting forth, owing to some kind of constant volcanic disturbance, are also more or less laden with sulphuretted hydrogen gas, communicating a very ill odour to the neighbourhood. These phenomena were at first looked upon by the people as the work of the devil, and priestly exorcisms were in considerable request in the hope of quelling them, very much as a great deal of the mere speech-making at the present time in England on foreign compet.i.tion and its evils, and the dulness of trade, the artificial combinations to keep up prices, to reduce wages, general lamentation, etc., are essayed in the attempt to charm away bad trade. At length a kind of prophet arose of a very practical character in the form of the late Count Lardarel, who, mindful of the fact that the chemist Hoffer, in the time of the Grand Duke Leopold I., had discovered boric acid in the volcanic steam jets, looked hopefully beyond the exorcisms of the priests and the superst.i.tions of the people to a possible blessing contained in what appeared to be an unholy confusion of Nature. He constructed tanks of from 100 to 1000 ft. in diameter and 7 to 20 ft. in depth, of such a kind that the steam jets were surrounded by or contained in them, and thus the liquors formed by condensation became more and more concentrated. These tanks were arranged at different levels, so that the liquors could be run off from one to the other, and finally to settling cisterns. Subsequently the strong liquors were run to lead-lined, wooden vats, in which the boric acid was crystallised out. Had the industry depended on the use of fuel it could never have developed, but Count Lardarel ingeniously utilised the heat of the steam for all the purposes, and neither coal nor wood was required. Where would that Tuscan boric acid industry have been now had merely the lamentations of landowners, fears of the people, and exorcisms of the priests been continued? Instead of being the work of the arch-enemy of mankind, was not it rather an incitement to a somewhat high and difficult step in an upward direction towards the attainment, on a higher platform of knowledge and skill, of a blessing for the whole province of Tuscany? What was true in the history of that industry and its development is every whit as true of the much-lamented slackening of trade through foreign compet.i.tion or other causes now in this country, and coming home to yourselves in the hat-manufacturing industry. The higher platform to which it was somewhat difficult to step up, but upon which the battle must be fought and the victory won, was one of a higher scientific and technological education and training. The chemist Hoffer made the discovery of boric acid in the vapours, they would no doubt take note; but Hoffer went no further; and it needed the man of both educated and practical mind like Count Lardarel to turn the discovery to account and extract the blessing. In like manner it was clear that in our educational schemes for the benefit of the people, there must not only be the scientific investigator of abstract truth, but also the scientific technologist to point the way to the practical realisation of tangible profit. Moreover, and a still more important truth, it is the scientific education of the proprietors and heads we want--educated capital rather than educated workmen.

_Borax._--A good deal of the Tuscan boric acid is used in France for the manufacture of borax, which is a sodium salt of boric acid. Borax is also manufactured from boronitrocalcite, a calcium salt of boric acid, which is found in Chili and other parts of South America. The crude boronitrocalcite or "tiza" is boiled with sodium carbonate solution, and, after settling, the borax is obtained by crystallisation. Borax itself is found in California and Nevada, U.S.A., and also in Peru, Ceylon, China, Persia, and Thibet. The commercial product is obtained from the native borax (known as "tincal") by dissolving in water and allowing the solution to crystallise. The Peruvian borax sometimes contains nitre. For testing the purity of refined borax the following simple tests will usually suffice. A solution of the borax is made containing 1 part of borax to 50 parts of water, and small portions of the solution are tested as follows: _Heavy metals_ (_lead_, _copper_, etc.).--On pa.s.sing sulphuretted hydrogen into the solution, no coloration or precipitate should be produced. _Calcium Salts._--The solution should not give a precipitate with ammonium oxalate solution.

_Carbonates._--The solution should not effervesce on addition of nitric or hydrochloric acid. _Chlorides._--No appreciable precipitate should be produced on addition of silver nitrate solution and nitric acid.

_Sulphates._--No appreciable precipitate should be produced on adding hydrochloric acid and barium chloride. _Iron._--50 c.c. of the solution should not immediately be coloured blue by 05 c.c. of pota.s.sium ferrocyanide solution.

_Soap._--Soap is a salt in the chemical sense, and this leads to a wider definition of the term "salt" or "saline" compound. Fats and oils, from which soaps are manufactured, are a kind of _quasi_ salts, composed of a fatty acid and a chemical constant, if I may use the term, in the shape of base, namely, glycerin. When these fats and oils, often called glycerides, are heated with alkali, soda, a true salt of the fatty acid and soda is formed, and this is the soap, whilst the glycerin remains behind in the "spent soap lye." Now glycerin is soluble in water containing dissolved salt (brine), whilst soap is insoluble, though soluble in pure water. The mixture of soap and glycerin produced from the fat and soda is therefore treated with brine, a process called "cutting the soap." The soap separates out in the solid form as a curdy ma.s.s, which can be easily separated. Certain soaps are able to absorb a large quant.i.ty of water, and yet appear quite solid, and in purchasing large quant.i.ties of soap it is necessary, therefore, to determine the amount of water present. This can be easily done by weighing out ten or twenty grams of the soap, cut in small pieces, into a porcelain dish and heating over a gas flame, whilst keeping the soap continually stirred, until a gla.s.s held over the dish no longer becomes blurred by escaping steam. After cooling, the dry soap is weighed, and the loss of weight represents the amount of moisture. I have known cases where soap containing about 83 per cent. of water has been sold at the full market price. Some soaps also contain more alkali than is actually combined with the fatty acids of the soap, and that excess alkali is injurious in washing silks and scouring wool, and is also not good for the skin. The presence of this free or excess alkali can be at once detected by rubbing a little phenolphthalein solution on to the freshly-cut surface of a piece of soap; if free alkali be present, a red colour will be produced.

LECTURE VII

Sh.e.l.lAC, WOOD SPIRIT, AND THE STIFFENING AND PROOFING PROCESS

_Sh.e.l.lac._--The resin tribe, of which sh.e.l.lac is a member, comprises vegetable products of a certain degree of similarity. They are mostly solid, gla.s.sy-looking substances insoluble in water, but soluble in alcohol and wood spirit. In many cases the alcoholic solutions show an acid reaction. The resins are partly soluble in alkalis, with formation of a kind of alkali salts which we may call resin-soaps.

Sh.e.l.lac is obtained from the resinous incrustation produced on the bark of the twigs and branches of various tropical trees by the puncture of the female "lac insect" (_Taccardia lacca_). The lac is removed from the twigs by "beating" in water; the woody matter floats to the surface, and the resin sinks to the bottom, and when removed forms what is known as "seed-lac." Formerly, the solution, which contains the colouring matter dissolved from the crude "stick-lac," was evaporated for recovery of the so-called "lac-dye," but the latter is no longer used technically. The seed-lac is bleached by boiling with sodium or pota.s.sium carbonate, alum, or borax, and then, if it is not pale enough, is further bleached by exposure to sunlight. It is now dried, melted, and mixed with a certain proportion of rosin or of orpiment (a sulphide of a.r.s.enic) according to the purpose for which it is desired. After further operations of melting and straining, the lac is melted and spread into thin sheets to form ordinary sh.e.l.lac, or is melted and dropped on to a smooth surface to form "b.u.t.ton-lac." Ordinary sh.e.l.lac almost invariably contains some rosin, but good b.u.t.ton-lac is free from this substance.

The presence of 5 per cent. of rosin in sh.e.l.lac can be detected by dissolving in a little alcohol, pouring the solution into water, and drying the fine impalpable powder which separates. This powder is extracted with petroleum spirit, and the solution shaken with water containing a trace of copper acetate. If rosin be present, the petroleum spirit will be coloured emerald-green.

Borax, soda crystals, and ammonia are all used to dissolve sh.e.l.lac, and it may be asked: Which of these is least injurious to wool? and why? How is their action modified by the presence of dilute sulphuric acid in the wool? I would say that soda crystals and ammonia are alkalis, and if used strong, are sure to do a certain amount of injury to the fibre of wool, and more if used hot than cold. Of the two, the ammonia will have the least effect, especially if dilute, but borax is better than either.

The influence of a little sulphuric acid in the wool would be in the direction of neutralising some of the ammonia or soda, and sh.e.l.lac, if dissolved in the alkalis, would be to some extent precipitated on the fibre, unless the alkali, soda or ammonia, were present in sufficient excess to neutralise that sulphuric acid and to leave a sufficient balance to keep the sh.e.l.lac in solution. Borax, which is a borate of soda, would be so acted on by the sulphuric acid that some boric acid would be set free, the sulphuric acid robbing some of that borax of its soda. This boric acid would not be nearly so injurious to wool as carbonate of soda or ammonia would.

The best solvent for sh.e.l.lac, however, in the preparation of the stiffening and proofing mixture for hats, is probably wood spirit or methylated spirit. A solution of sh.e.l.lac in wood spirit is indeed used for the spirit-proofing of silk hats, and to some extent of felt hats, and on the whole the best work, I believe, is done with it. Moreover, borax is not a cheap agent, and being non-volatile it is all left behind in the proofed material, whereas wood spirit or methylated spirit is a volatile liquid, _i.e._ a liquid easily driven off in vapour, and after application to the felt it may be almost all recovered again for re-use.

In this way I conceive the use of wood spirit would be both more effective and also cheaper than that of borax, besides being most suitable in the case of any kind of dyes and colours to be subsequently applied to the hats.

_Wood Spirit._--Wood spirit, the pure form of which is methyl alcohol, is one of the products of the destructive distillation of wood. The wood is distilled in large iron retorts connected to apparatus for condensing the distillation products. The heating is conducted slowly at first, so that the maximum yield of the valuable products--wood acid (acetic acid) and wood spirit--which distil at a low temperature, is obtained. When the condensed products are allowed to settle, they separate into two distinct layers, the lower one consisting of a thick, very dark tar, whilst the upper one, much larger in quant.i.ty, is the crude wood acid (containing also the wood spirit), and is reddish-yellow or reddish-brown in colour. This crude wood acid is distilled, and the wood spirit which distils off first is collected separately from the acetic acid which afterwards comes over. The acid is used for the preparation of alumina and iron mordants (see next lecture), or is neutralised with lime, forming grey acetate of lime, from which, subsequently, pure acetic acid or acetone is prepared. The crude wood spirit is mixed with milk of lime, and after standing for several hours is distilled in a rectifying still. The distillate is diluted with water, run off from any oily impurities which are separated, and re-distilled once or twice after treatment with quicklime.

_Stiffening and Proofing Process._--Before proceeding to discuss the stiffening and proofing of hat forms or "bodies," it will be well to point out that it was in thoroughly grasping the importance of a rational and scientific method of carrying out this process that Continental hat manufacturers had been able to steal a march upon their English rivals in compet.i.tion as to a special kind of hat which sold well on the Continent. There are, or ought to be, three aims in the process of proofing and stiffening, all the three being of equal importance. These are: first, to waterproof the hat-forms; second, to stiffen them at the same time and by the same process; and the third, the one the importance of which I think English hat manufacturers have frequently overlooked, at least in the past, is to so proof and stiffen the hat-forms as to leave them in a suitable condition for the subsequent dyeing process. In proofing the felt, the fibres become varnished over with a kind of glaze which is insoluble in water, and this varnish or proof is but imperfectly removed from the ends of the fibres on the upper surface of the felt. The consequence is a too slight penetration of the dyestuff into the inner pores of the fibres; indeed, in the logwood black dyeing of such proofed felt a great deal of the colour becomes precipitated on the outside of the fibres--a kind of process of "smudging-on" of a black pigment taking place. The subsequent "greening" of the black hats after a short period of wear is simply due to the ease with which such badly fixed dye rubs off, washes off, or wears off, the brownish or yellowish substratum which gradually comes to light, causing a greenish shade to at length appear. If we examine under the microscope a pure unproofed fur fibre, its characteristic structure is quite visible. Examination of an unproofed fibre dyed with logwood black shows again the same characteristic structure with the dye inside the fibre, colouring it a beautiful bluish-grey tint, the inner cellular markings being black. A proofed fur fibre, on the other hand, when examined under the microscope, is seen to be covered with a kind of translucent glaze, which completely envelops it, and prevents the beautiful markings showing the scaly structure of the fibre from being seen. Finally, if we examine microscopically a proofed fibre which has been dyed, or which we have attempted to dye, with logwood black, we find that the fibre presents an appearance similar to that of rope which has been drawn through some black pigment or black mud, and then dried.

It is quite plain that no l.u.s.trous appearance or good "finish" can be expected from such material. Now how did the Continental hat manufacturers achieve their success, both as regards dyeing either with logwood black or with coal-tar colours, and also getting a high degree of "finish"? They attained their object by rubbing the proofing varnish on the inside of the hat bodies, in some cases first protecting the outside with a gum-varnish soluble in water but resisting the lac-varnish rubbed inside. Thus the proofing could never reach the outside. On throwing the hat bodies, thus proofed by a logical and scientific process, into the dye-bath, the gums on the outer surface are dissolved and removed, and the dye strikes into a pure, clean fibre, capable of a high degree of finish. This process, however, whilst very good for the softer hats used on the Continent, is not so satisfactory for the harder, stiffer headgear demanded in Great Britain. What was needed was a process which would allow of a through-and-through proofing and stiffening, and also of satisfactory dyeing of the stiffened and proofed felt. This was accomplished by a process patented in 1887 by Mr.

F.W. Cheetham, and called the "veneering" process. The hat bodies, proofed as hard as usual, are thrown into a "b.u.mping machine" containing hot water rendered faintly acid with sulphuric acid, and mixed with short-staple fur or wool, usually of a finer quality than that of which the hat bodies are composed. The hot acid water promotes in a high degree the felting powers of the short-staple wool or fur, and, to a lesser extent, the thinly proofed ends of the fibres projecting from the surfaces of the proofed hat-forms. Thus the short-staple wool or fur felts itself on to the fibres already forming part of the hat bodies, and a new layer of pure, unproofed wool or fur is gradually wrought on to the proofed surface. The hat-forms are then taken out and washed, and can be dyed with the greatest ease and with excellent results, as will be seen from the accompanying ill.u.s.tration (see Fig. 15). This successful invention emphasises the value of the microscope in the study of processes connected with textile fibres. I would strongly advise everyone interested in hat manufacturing or similar industries to make a collection of wool and fur fibres, and mount them on microscope slides so as to form a kind of index collection for reference.

[Ill.u.s.tration: FIG. 15.