The sulphide, ThS₂, is obtained, together with the oxysulphide, ThOS, according to Duboin,[480] by passing a current of sulphuretted hydrogen over a mixture of thorium chloride with excess of sodium chloride, at a red heat. The former forms large brown crystals, from which the small orange-yellow crystals of the oxysulphide may be separated by means of a sieve; the latter is purified by treatment with warm nitric acid, which dissolves the sulphide very readily. The oxysulphide is also obtained when the anhydrous sulphate is heated in sulphuretted hydrogen.[481]

The carbide, ThC₂, is obtained by the action of carbon on the oxide in the electric furnace; it is a yellow crystalline mass, decomposed slowly by water, energetically by dilute acids in the cold, with evolution of a complex mixture of hydrogen and hydrocarbons, in which many members of the paraffin, olefine and acetylene series have been observed.[482] Hydrogen constitutes over 50 per cent. of the mixture, the next most important constituents being the acetylenic hydrocarbons, followed by ethane.

Thorium fluoride, ThF₄, is obtained anhydrous by passing hydrogen fluoride over the anhydrous chloride or bromide at a temperature of 350°-400°. The tetrahydrate, ThF₄,4H₂O, is precipitated by addition of hydrofluoric acid to a solution of a thorium salt, or by the action of the acid on the hydroxide. Hydrofluosilicic acid also throws down the fluoride, even in the cold, from solutions of thorium salts. The fluoride is insoluble in water and mineral acids, as well as in excess of precipitant; this behaviour allows of a complete and easy separation of thorium from titanium and zirconium. The rare earth fluorides are also much more easily soluble in concentrated mineral acids than thorium fluoride, so that this compound may also be used in the separation from the rare earths. When heated in a stream of the acid to 800°, the hydrated salt yields the oxyfluoride, ThOF₂; ignited in the air, it leaves the dioxide. Precipitation with potassium fluoride gives the double fluoride, KThF₅,H₂O, which may be obtained anhydrous by fusion of the mixed fluorides; it is insoluble. An amorphous insoluble compound, K₂ThF₆,4H₂O, is obtained by boiling the hydroxide with a mixture of potassium hydrogen fluoride and hydrofluoric acid. Sodium and ammonium fluorides throw down the simple fluoride.

Thorium chloride, ThCl₄, is obtained in the anhydrous form by all the usual methods, the most convenient being perhaps the action of chlorine and sulphur monochloride on the heated dioxide. It almost invariably contains small quantities of oxychloride. When pure, it forms colourless needles fairly stable in dry air; the impure product gradually darkens in colour. It dissolves in water with considerable evolution of heat, and is soluble also in alcohol and moist ether. It melts at about 820°, and sublimes unchanged at somewhat higher temperatures; the vapour begins to dissociate at about 1050°, the dissociation increasing rapidly as the temperature rises. It resembles zirconium chloride in the ease with which it forms additive compounds with ammonia and organic bases, and addition and condensation products with organic oxygen-compounds; many double and complex chlorides are also known, among which the platinum compounds ThPtCl₈,12H₂O and Th₂Pt₃Cl₁₄,24H₂O, and the pyridine salt (C₅H₅NH)₂ThCl₆ may be mentioned.

From aqueous solution the octohydrate, ThCl₄,8H₂O, separates at ordinary temperatures; a heptahydrate and an enneahydrate have been described as precipitated from the alcoholic solution by addition of water. The basic salts, Th(OH)Cl₃,7H₂O and Th(OH)₂Cl₂,5H₂O, have been obtained by addition of the hydroxide to alcoholic hydrogen chloride. The oxychloride, ThOCl₂, may be obtained by the carefully regulated action of carbon tetrachloride on the dioxide, according to the equation:

ThO₂ + CCl₄ = ThOCl₂ + COCl₂

It is a colourless crystalline solid, which takes up moisture from the air, forming the hexahydrate.

Thorium bromide, ThBr₄, is a volatile solid which boils at 725°; it closely resembles the chloride. The iodide and a basic iodide, Th(OH)I₃,10H₂O, are known.

No cyanide of thorium is known, addition of potassium cyanide merely causing separation of the hydroxide. A ferrocyanide, Th[Fe(CN)₆],4H₂O, is thrown down as a white powder by potassium ferrocyanide; with potassium ferricyanide no precipitate is obtained. The platinocyanide, Th[Pt(CN)₄]₂,16H₂O, is obtained by double decomposition in yellowish-brown prisms.

Among the halogen oxysalts, the perchlorate, chlorate, bromate, and iodate were prepared by Cleve. The iodate is of great importance for purposes of detection and estimation, from the fact that, in presence of a large excess of alkali iodate, it is insoluble in strong nitric acid, whilst the analogous compounds of the rare earth elements dissolve readily in that solvent.

The sulphate, Th(SO₄)₂, is obtained anhydrous by evaporating the excess of acid from a solution of the dioxide in oil of vitriol, or by heating the hydrates. It resembles the sulphates of the rare earth elements, in that it dissolves in water at 0° to form a highly supersaturated solution, from which the hydrated forms separate out almost quantitatively when the temperature is allowed to rise. The solubility relations of the various hydrates, on account of their commercial importance, are somewhat fully treated in Chapter XVIII. A dihydrate, Th(SO₄)₂,2H₂O, is obtained by keeping the tetrahydrate at 110°. The ennea- and octohydrates are isomorphous with the corresponding thorium selenate hydrates, and the ennea- and tetrahydrates with the analogous uranous sulphate hydrates. The hydrates yield the anhydrous salt when heated to 400°; the anhydrous sulphate has already a considerable dissociation tension (15 mm.) at 575°. By treatment with excess of acid, and subsequent heating to 130° in vacuo, the acid sulphate, Th(SO₄),H₂SO₄, is obtained. An insoluble basic salt, ThOSO₄,2H₂O, is formed by continued boiling of the tetrahydrate in dilute solution, or more quickly by heating the solution in a closed tube to 120°-125°; a monohydrate, ThOSO₄,H₂O, is also known. Halla[483] has recently obtained the hydrate, ThOSO₄,5H₂O, by boiling a solution of the neutral sulphate with magnesium sulphate, and also by treating the anhydrous sulphate with a little water in presence of magnesium carbonate.

By precipitation with potassium sulphate the double salt, Th(SO₄)₂,2K₂SO₄,2H₂O, is formed; this is soluble in water but insoluble in potassium sulphate solution. The analogous sodium and ammonium salts are soluble both in water and excess of the corresponding alkali sulphate.

The sulphite, Th(SO₃)₂,H₂O, is obtained as a white amorphous precipitate by warming a solution of a thorium salt with sulphurous acid. Basic sulphites and double sulphites are also known; the precipitates obtained by addition of alkali sulphite dissolve readily in excess. The hydroxide is almost insoluble in sulphurous acid, behaviour which distinguishes thorium (and zirconium) from all the trivalent metals. No thiosulphate is known, the hydroxide being thrown down from boiling solution by addition of sodium thiosulphate: this method of precipitation was formerly much used for purposes of estimation, but it is more tedious and less accurate than the modern methods.

Thorium nitrate, Th(NO₃)₄,12H₂O, crystallises at ordinary temperatures in large hygroscopic tablets, very soluble in water and alcohol. The hydrates, Th(NO₃)₄,6H₂O and Th(NO₃)₄,5H₂O, have been obtained from hot solution and from nitric acid solution respectively. Thorium is employed in commerce almost entirely in the form of this salt, which is dehydrated until it contains about 48 per cent. ThO₂, which approximates to the formula Th(NO₃)₄,4H₂O; the commercial product, however, is not a definite hydrate. Kolbe[484] has described the additive product with antipyrine, 2Th(NO₃)₄,5C₁₁H₁₂ON₂, which melts at 168°. The extent to which thorium salts are hydrolysed in solution is very considerable, as is evident from the fact that the nitrate may be titrated with standard potash in presence of phenolphthalein as indicator; the solution first becomes alkaline to this reagent when 3·5 molecules of potash have been added for each molecule of thorium nitrate present.[485] Of the large number of double nitrates which have been prepared, the general types R´₂Th(NO₃)₆, where R´ = NH₄,K,Rb,Cs, and R´´Th(NO₃)₆,8H₂O, where R´´ = Mg,Mn,Zn,Ni,Co, are the most important.

Thorium phosphates.—The precipitates obtained by addition of phosphoric acid or alkali phosphates to solutions of thorium salts are gelatinous solids of doubtful composition; they dissolve in mineral acids and in alkali carbonates, and their behaviour is of great importance in the technical treatment of monazite. Various phosphates and double phosphates are obtained by fusion methods, but none of these are important. The phosphite, Th(HPO₃)₂,3H₂O, and hypophosphite, Th(H₂PO₂)₄, are insoluble solids obtained by double decomposition. The hypophosphate ThP₂O₆,11H₂O, is of great importance for purposes of detection and estimation, since it is thrown down quantitatively from strongly acid solutions; under these conditions the rare earths remain in solution.

No neutral carbonate of thorium is known. Alkali carbonates precipitate a basic salt, which dissolves readily in excess; this fact is of very great importance in the commercial extraction of thorium, the sodium and ammonium double carbonates of the cerium elements being almost insoluble in alkali carbonates. Addition of alcohol to the solution throws down double carbonates, which can be washed with ice water. The salts K₆Th(CO₃)₅,10H₂O, Na₆Th(CO₃)₅,12H₂O, and (NH₄)₂Th(CO₃)₃,6H₂O have been obtained in this way; they dissolve readily in water or dilute alkali carbonate, though on warming or diluting the solution, the hydroxide separates. The thallium compound, Tl₆Th(CO₃)₅, is sparingly soluble, and is thrown down from a solution of the ammonium compound on addition of a thallium salt; it has been proposed for the microchemical detection of thorium. The quantitative separation of thorium by means of pure moist lead carbonate has been proposed for the purpose of estimation (see p. 288).

Thorium oxalate, Th(C₂O₄)₂,6H₂O, is precipitated quantitatively by means of oxalic acid, even in presence of considerable quantities of mineral acids. It is less soluble in sulphuric acid than any of the rare earth oxalates,[486] and is not attacked, as are the latter compounds, by concentrated nitric acid. In hydrochloric acid the solubility first increases rapidly with the concentration of the acid, and then suddenly decreases; this behaviour is due to the formation of an oxalochloride, 3Th(C₂O₄)₂,ThCl₄,20H₂O. When the amorphous oxalate obtained by precipitation is allowed to remain for a considerable time in contact with acids, it forms characteristic tetragonal prisms of the more stable form. The dihydrate, Th(C₂O₄)₂,2H₂O, is obtained when the hexahydrate is dried over sulphuric acid, or heated to 100°. The salt dissolves easily in excess of alkali oxalate, but is precipitated from the solutions by mineral acids, a fact which allows of another means of separation from zirconium, the double oxalates of which are much more stable towards acids. The solubility of the oxalate in alkali oxalate allows of separation from the rare earth elements, whilst its insolubility in excess of oxalic acid can be used for the separation from zirconium.

The formate and acetate can be obtained in the form of neutral salts by the action of the acids on the hydroxide; by double decomposition, amorphous precipitates of basic salts are obtained. With tartaric acid stable complex compounds are formed, as shown by the fact that alkalies will not precipitate the hydroxide from a solution in presence of that reagent, and by the elevation of the specific rotatory power. Many complex salts are known, the simplest having the composition ThO(C₄H₄O₆R´)₂,8H₂O, where R´ = K,Na,NH₄; these are obtained by dissolving thorium hydroxide in concentrated solutions of alkali hydrogen tartrates. Thorium acetylacetone, Th(C₅H₇O₂)₄, is precipitated by addition of ammonia to an aqueous solution of the nitrate mixed with acetylacetone dissolved in ammonia; the solid is recrystallised from alcohol, and melts at 171°.

Atomic Weight of Thorium.

—The value adopted by the International Committee (1914) is 232·4, but most of the determinations carried out within the last thirty years show considerable discrepancies. The earlier work of Berzelius (1829) and Chydenius (1861) led to very widely varying results, and for the same reason little reliance can be placed on the results of Delafontaine (1863) and Hermann (1864). In 1874 Cleve determined the constant by ignition of the sulphate, obtaining the mean values 234·03 and 233·97; the figure 234 based on these results was for many years accepted as the true atomic weight. A series of determinations carried out by Nilson in 1882 led to much lower results. He employed the sulphate ennea- and octohydrates, first dehydrating these, and then igniting to oxide, and showed that Cleve’s value must be too high on account chiefly of the hygroscopic nature of the ignited oxide, which increases in weight when kept; but his own values show considerable discrepancies. The ratio Th(SO₄)₂,9H₂O-ThO₂ : ThO₂ (enneahydrate converted to oxide) gave the figure (corrected to vacuo) 232·51, whilst the ratio ThO₂ : 2SO₃ (anhydrous sulphate to oxide) gave 232·16; the ratio Th(SO₄)₂ : 9H₂O (hydrate to anhydrous salt) gave, however, 233·75. The value obtained for the ratio ThO₂ : 2SO₃ for anhydrous sulphate prepared from the octohydrate was 232·49 (corrected to vacuo). Five years later, Krüss and Nilson prepared the anhydrous sulphate from the pure octohydrate, and ignited this to the oxide. The ratio ThO₂ : 2SO₃ gave as a mean of very concordant results the figure 232·49.

Brauner criticises these values on the ground that no details are given as to the temperature required to obtain the anhydrous salt from the hydrates, and that probably some traces of sulphate must be decomposed at the temperatures required (450°-500°) to drive off all the water. The results obtained from the enneahydrate are to a great extent invalidated by the doubts as to the purity of the hydrate, completeness of dehydration, etc., which arise from the discrepancies in the values deduced from the three ratios. He accepts, however, the figure 232·49 obtained by Nilson and by Krüss and Nilson from material separated as octohydrate, with some uncertainty as to the second decimal figure.

Brauner himself employed the oxalate method in 1898; the purified hexahydrate was used, the percentage of thoria being determined by ignition, and of (C₂O₃) by titration with permanganate. The ratio ThO₂ : 2C₂O₃ gave results varying from 232·21 to 232·29, but as the value rose continuously as purification was carried further and further, he did not feel justified in taking a mean value. In 1900 Urbain determined the constant with material purified by the acetylacetone method. He prepared the octohydrate, heated it for ten hours in a bath of sulphur vapour at 440°, and ignited the anhydrous salt so obtained at a white heat. The ratio ThO₂ : 2SO₃ gave the result (corrected to vacuo) Th = 233·67. Brauner criticises the value on the ground that the hydrated salt was heated in a vessel open to the air, and that at the high temperature obtained, traces of moisture gaining access to the sulphate caused hydrolysis, with loss of sulphuric acid; this would cause the results to be too high. In 1905 Meyer and Gumperz employed the same method, and obtained values varying from 232·2 to 232·7, with the mean 232·47. Finally Brauner carried out an extended investigation to disprove the heterogeneity of thorium which had been ‘discovered’ by Baskerville (1904), in the course of which he showed the atomic weight of the element to lie between the limits 232·34 and 232·52.

Detection of Thorium.

—The element is best detected in a mixture of earths by the following reactions:

(1) Precipitation with hydrogen peroxide from warm, faintly acid solution.

(2) Precipitation with sodium hypophosphate, Na₂H₂P₂O₆, in concentrated hydrochloric acid solution. On boiling, a perceptible precipitate is obtained if only traces of thorium are present; but ceric and zirconium salts and titanium must be absent. The latter element gives no precipitate under these conditions if hydrogen peroxide is present; ceric salts may be decomposed by boiling. The possible presence of zirconium renders it necessary to boil the hypophosphate precipitate with nitric acid; on addition of oxalic acid to the clear solution, thorium is precipitated, whilst zirconium remains in solution, and may be detected.

(3) Potassium azide, KN₃, throws down thorium hydroxide from boiling neutral or faintly acid solutions. Ceric salts if present must be previously reduced; zirconium must be previously removed by oxalic acid.

(4) Precipitation may be effected with potassium iodate in strong nitric acid solution. Here also ceric salts must be reduced before applying the test. Zirconium also gives the test; the precipitate must therefore be washed and warmed with oxalic acid, in which thorium iodate is insoluble, whilst zirconium iodate is soluble.

The methods of estimating thorium are given in Chapter XVIII.

PART III
THE TECHNOLOGY OF THE ELEMENTS

CHAPTER XVII
THE INCANDESCENT MANTLE INDUSTRY—HISTORICAL AND GENERAL INTRODUCTION

The group of elements which we are considering can be divided, from the point of view of technical application, into two classes. The first of these contains one element only, titanium, which in its technology, as in its chemistry, stands apart from the others; it will, accordingly, be treated in a separate chapter. The second class contains the yttrium and cerium metals, with zirconium and thorium; the technical importance of these elements is due chiefly to the use of their oxides in illumination, to a small extent in Nernst lamps, and to a much greater extent in the so-called Incandescent Lighting. The manufacture of incandescent mantles[487] is a large and ever-extending industry, intimately bound up with the older process of coal-distillation, with its innumerable ramifications; indeed, it may be said that but for the ingenious invention of Dr. Auer, illumination by means of coal-gas would to-day have been almost obsolete. The discovery which resulted in the production of the familiar incandescent mantle of the present day may be regarded as the culmination of a century’s effort to increase the value of coal-gas as an illuminating agent. In the present chapter it is proposed to outline the history of these endeavours, and to give a short general account of Auer’s work and its results.

Soon after the introduction of gas as an illuminating agent it was realised that the luminosity of the flame is dependent on the presence of solid particles, which by the heat of combustion of the gas are raised to a temperature at which they emit radiations of wave-lengths corresponding to the ‘luminous rays’ of the spectrum. A non-luminous flame of sufficiently high temperature, therefore, can be rendered luminous by the introduction of suitable solids, and numberless investigators have striven, during the past century, to discover the most suitable method of increasing the luminosity of a flame in this way. The luminosity of the ordinary ‘bats-wing’ or ‘flat’ flame, now so rapidly going out of use, is due to the presence in the outer zone of the flame of heated particles of carbon, produced by the decomposition—or partial combustion—of ‘dense’ hydrocarbons, i.e. of hydrocarbons having a high percentage of carbon. Ordinary coal-gas consists largely of a mixture of hydrogen and methane, both of which burn with practically non-luminous flames, with small quantities of olefines, acetylenes, etc., to which the luminosity is chiefly due. It would appear, then, that by the introduction of dense hydrocarbons, a gas of poor illuminating power might be made much more valuable as a source of light. On the other hand, it is also apparent that the same end might be achieved by the introduction into a non-luminous or feebly luminous flame of an altogether foreign substance, introduced as such, and not continuously consumed, as is the carbon in the former method. Both these directions of improvement have been followed; since, however, the results achieved by the latter method have become recently of far greater importance, the applications of the first method will be dismissed quite briefly, and the history of the second will then be treated somewhat fully.

The first important attempt to increase the illuminating power of gases burning with feebly luminous flames was that of Faraday, who in the course of an investigation into the causes of the variations in luminosity of ‘portable gas,’ discovered benzene, or bicarburet of hydrogen, as he called it, in 1826. In 1830 an engineer named Dunnovan undertook to illuminate Dublin by means of water-gas[488] which he ‘carburised’ by addition of dense hydrocarbons. During the latter half of the nineteenth century this method became of some importance. It has been applied, in particular, to enrich the ‘natural gas’ of Ohio, North America. The dense hydrocarbons necessary for this purpose are obtained by the process known as ‘cracking.’ The viscous residues from the distillation of the mineral oil of the district are allowed to drop into a brick chamber, of which the walls are raised to a bright red heat, and the dense hydrocarbons which are evolved are removed by a current of the gas to be enriched. In this way a gas of relatively high illuminating power is obtained.

In the year previous to that in which Faraday first carburised water-gas, Berzelius had observed that thoria and zirconia, when heated in a non-luminous flame, emit an intense white light. Similar behaviour had long before been observed in the cases of magnesia, alumina, lime, zinc oxide, etc. The first practical application of this property of the oxides was that of Drummond, who in 1826 heated a pencil of lime in the oxy-hydrogen flame and obtained the intense white light which has since become so familiar as the Drummond or ‘lime-light.’ A further development in this direction was due to du Motay and Maréchal, who in 1867 illuminated the Place de Tuileries and the Hôtel de Ville in Paris by means of pencils of compressed zirconia—magnesia was also used—heated by means of oil vapour and oxygen.

The use of non-luminous flames to secure illumination, by raising the temperature of solids suspended in them to the point of incandescence was proposed in 1839 by Cruickshank, who used a mantle of platinum wire, covered with lime and rare earths, which he heated by means of water-gas. In 1846 Gillard employed mantles of platinum wire, raised to incandescence in the flame of burning hydrogen, which he obtained by passing steam over heated iron wire; later he used water-gas (1848), his lamps with this modification being employed in Paris and in Philadelphia. Narbonne was later illuminated (1856-1865) by a similar device, but permanent success could hardly be obtained in view of the cost of the platinum mantles, which lasted only a few months. The same mantle was proposed in 1882 by Lewis, the ordinary Bunsen flame being suggested as the source of heat. In the same year Popp exhibited at the Crystal Palace lamps in which a platinum mantle was raised to incandescence by means of a flame of coal-gas and heated air. These attempts, however, served only to show that no permanent advance could be made in this direction.

A new development was made in 1880 by Clamond. He prepared a paste by grinding up calcined and powdered magnesia with a concentrated solution of magnesium acetate; by forcing this through a press he obtained a ribbon which was then wound crosswise on a wooden shaper, dried carefully, and ignited. In his later experiments twenty per cent. of zirconia was added to the magnesia. The mantle was supported in a platinum cage and heated in the flame of a mixture of coal-gas and heated air. This mantle gave an intense light, but was too fragile for extended use. In the following year, Lundgren patented a process by which lime, magnesia, and zirconia, made into a paste by the addition of gum, were forced through a press, and the resulting thread wound on a graphite-covered shaper. The mantle so obtained was stable, and gave an intense white light, but after having been heated for some time the oxides crumbled to powder. A modification of this process was introduced by Knöfler in 1894, in an attempt to use a cellulose solution containing rare earth salts; this was forced through jets, and the cellulose precipitated as a continuous thread from which the mantle was made. A further modification of Knöfler’s process by Plaisetty in 1901 was technically successful; but these developments must be taken up in a later chapter (vide p. 307).

In 1883 a process was patented by Fahnehjelm in Stockholm, by which for the first time a cheap and stable mantle of considerable efficiency was produced, and which, but for the advent of the Auer mantle, would undoubtedly have been commercially successful. Fahnehjelm’s mantle consisted of an arrangement of needles or lamellæ of magnesia, lime, zirconia, etc., suspended over a burner. The plates and needles were usually arranged in the form of a comb of suitable shape, and were found to give an intense light, and to be long-lived. In later forms the combs were made of rods of magnesia dipped into solutions of chromium salts. The great disadvantage of this invention lay in the fact that the combs required to be heated in the flame of water-gas, in order to secure a good incandescence; had it been possible to attain a sufficiently high temperature by the use of coal-gas, it is doubtful whether the Auer mantle would have ever been evolved.

The more important attempts to secure arrangements by which the radiations of heated solids could be used for illumination have now been outlined and the ground cleared for the consideration of the work of Baron von Welsbach. There remain yet to be mentioned, however, two attempts which are of especial interest in view of that work. The first is that of Frankenstein, who in 1849 made use of a ‘Light-multiplier’ obtained by impregnating gauze with a paste of chalk and magnesia ground with water. The second is that of Edison, who proposed (1878) to utilise the observations of Bahr and Bunsen (1864) and of Delafontaine (1874), of the remarkable incandescence exhibited by the yttria and erbia earths, and the terbia earths, respectively, when heated; he suggested the employment of a mantle of platinum wire covered with zirconia and the oxides of the rare earth metals, a proposal similar to that put forward nearly forty years earlier by Cruickshank.

About the year 1880 Dr. Carl Auer began the study of the rare earth elements. The chemical aspect of his work has already been dealt with (vide p. 168); but the results obtained by the technical application of his observation that threads of cotton, impregnated with a solution of salts of the elements, leave after ignition a coherent ash of oxide, which glows brightly when heated, have been of far greater importance than the purely scientific aspect, valuable though that is. A series of experiments soon showed that a fabric of suitable shape, impregnated with a solution of nitrates or acetates of the rare earth elements, after being dried and drawn together at one end by means of a platinum wire, can be ignited in a Bunsen flame in such a way as to leave a coherent skeleton of the earth oxides, which can be formed and hardened by suitable manipulation with a high temperature burner; the mantle so prepared, when suspended from a lateral support in a Bunsen flame, gives a light of considerable intensity, the colour varying with the oxides employed from green to orange tints.

The earlier mantles, which were placed on the market about 1883, consisted chiefly of oxides of lanthanum and zirconium, with smaller quantities of the other oxides, selected according to the shade of light desired. These mantles were protected by patents taken out in France in 1884, and in Germany in 1885 and the following years. The process[489] was briefly the following: A vegetable fibre, of cylindrical form, woven from threads of about 0·22 mm. diameter, is washed with dilute hydrochloric acid, then with distilled water, and impregnated with a 30 per cent. solution of the selected salts. The fabric is then wrung out and dried, and cut into suitable lengths, allowance being made for subsequent shrinkage. One end of each cylinder is then drawn together by means of a platinum wire, and the mantle hung from a side support over a burner and incinerated. The head is then treated with a solution of aluminium and magnesium nitrates (beryllium nitrate and the corresponding phosphates are also specified) to strengthen it, and the mantle dried, and ‘formed’ by means of a very hot flame. This first patent protected several definite mixtures of salts, chosen so that the mantle should emit light of a definite known tint. The chief oxides employed were lanthana, yttria, magnesia, and zirconia. A German patent granted in 1886[490] protects the use of thorium salts, and a long list of salts of the elements with numerous acids; an important advance mentioned in this specification is the process of collodinisation of the finished mantle, by dipping in a solution of rubber in benzene or of collodion (cellulose nitrate) in ether and alcohol, which renders the product strong enough for transport. From 1885 to 1891 numerous improvements were effected; asbestos threads were substituted for platinum wire, central rods of magnesia replaced the lateral platinum support, and various mixtures of oxides were tried. None of the innumerable mixtures employed, however, was successful in establishing the struggling industry on a firm basis in face of the vigorous competition of the electric lamp, and it was not till 1891 that the introduction of the final ‘Auer Mixture,’ which is in use at the present day, gave the welcome assurance of a certain success to von Welsbach and his assistants. The discovery of this mixture was a result of the examination of a quantity of impure thoria; it was found that mantles made from the nitrate gave a light which steadily decreased in intensity as the impurities were removed. It needed only the observation that the impurities consisted chiefly of cerium compounds to turn the long and arduous investigation in the direction of final success, and our present mantles, which consist approximately of 99 per cent. thoria and 1 per cent. ceria, were placed on the market in 1891, the composition being announced by patent in 1893.[491]

The effect of increasing or decreasing the ratio of the two oxides, and the theories which have been advanced to account for the results, must be referred to in a later chapter (vide p. 294). It may be mentioned here, however, that practically no other known mixture gives such satisfactory results, though mantles have been manufactured of alumina with small quantities of chromic oxide, and ‘inverted’ mantles made of these oxides with zirconia have recently been advocated by Professor Lewes,[492] an authority on gas lighting. Mixtures of alumina and uranium oxide have also been patented, but no mantles appear to have been manufactured according to the specifications. In this connection, also, may be mentioned the various attempts to evade the Auer patents by taking advantage of the ‘discovery’ of ‘new’ elements. One enterprising firm, after having an account of a ‘new’ element, Lucium, inserted in a well-known scientific periodical, put salts on the market, and proceeded to manufacture mantles from what were proved by analysis to be cerium compounds. Similar ‘new’ elements were Russium, Kosmium, and Neo-kosmium, names which covered various mixtures of thorium and cerium compounds with other salts.

After the introduction in 1891 of the final Auer mixture, progress became rapid. The original mantles, made from cotton, had many disadvantages; thus after being in use for some time they were found to shrink considerably, with marked decrease in strength and light-giving power. Once the success of the new form of lighting was assured, numberless investigations were undertaken to lengthen the life and increase the efficiency of the mantles. The most important of these were connected with the endeavour to replace cotton by some fabric which on ignition would leave the oxide skeleton in a harder, more coherent and more elastic condition. The first great advance in this connection was the introduction of Ramie fibre by Buhlmann in 1898. Ramie, China-grass, or grass-cloth, as it is sometimes termed, is a fabric made from the fibres of the tschuma plant of the Yang-tse-kiang valley and other parts of Asia; mantles made from it last longer and maintain their efficiency much better than the earlier cotton mantles, which they have very largely displaced. The use of artificial silk was patented by De Mare in 1894, but his process was unworkable; it was an effort to adapt to the purposes of incandescent lighting the nitro-cellulose process introduced by Chardonnet in 1890 for the manufacture of artificial silk. In 1897 De Lery and in 1900 Plaisetty made further efforts in this direction, and finally in 1902-1903 the latter worked out a process by which mantles were made directly from the spun fabric. These mantles are superior in every way to the earlier ramie or cotton kinds, and are rapidly coming into general use, especially for lamps using high-pressure gas. Numberless patents for the manufacture and improvement of this kind of mantle have been taken out during the last ten years; the most important of these will be dealt with in a later chapter.

Attempts have been made to secure greater strength and toughness in mantles in other directions also. The use of metallic wires in the fibre has been suggested; numerous patents deal with mantles ‘strengthened’ by doubling the thread at intervals, and by special methods of weaving the fibre. One method, which follows on the lines of Glamond and Lundgren, proposes[493] the use of mantles made from various oxides mixed with silica, the whole being worked into a paste by use of a gum or soap, from which threads are prepared by pressure; mantles made from these threads are said to be very strong and porous. Another patent[494] protects the manufacture of ‘incandescence bodies’ made from plates or combs prepared from a thread obtained in a rather similar way. A third of these innumerable suggestions recommends a preliminary impregnation of the fabric with an aluminium or magnesium salt,[495] from which the oxide is precipitated on the fabric by a suitable means, impregnation with the ordinary ‘lighting fluid’ being effected after drying. Quite an early patent[496] proposes the impregnation of the prepared mantle, either after or just before burning off, with an alcoholic solution of an organic silicon compound, so that when the mantle is in use a skeleton of silica is formed to ‘strengthen’ the oxide ash. No useful purpose can be served by extending the list of these proposals; enough has been said to indicate the various directions in which so many vain attempts at improvement have been made.

From the mechanical and physical side the recent developments have been very marked. The introduction of the ‘inverted’ lamp was a tremendous step forward, and paved the way to the second great improvement, the use of ‘high-pressure’ gas, with which such successful results are being obtained. The form of lamp now coming into use for street lighting gives 1500 candle-power per mantle, and usually carries three mantles; each lamp thus develops 4500 candle-power. The purely mechanical devices which are now used to secure ‘automatic’ lighting are rapidly bringing this form of lamp into favour for street illumination. A full account of these developments would be entirely beyond the scope of the present work. In the following chapters, therefore, no complete treatment of the incandescent lighting industry can be given; but whilst the chemical aspect is treated at some length, many points of more purely technical character, which are connected with this, have also been included.

CHAPTER XVIII
THE CHEMICAL TREATMENT OF MONAZITE

It has been stated in the previous chapter that the first Auer mantles were made of mixtures of various rare earth oxides, the mixture of thoria with 1 per cent. of ceria being first employed in October, 1891. The impetus given to the mantle industry by the success of the new mixture caused an immediate demand for thoria, which was at that time extracted from thorite (see p. 43). A ‘thorite-fever’ broke out along the coasts of Scandinavia, and the price of orangite rose to 600 marks per kilogram (about £13 10s. per pound avoirdupois), sinking again shortly to 80 marks[497] (about £1 16s. per lb). The discovery of the monazite sands of the Carolinas and Brazil, which at the present rate of consumption may be considered to be, for all practical purposes, inexhaustible, placed the industry on a firm basis, and the pure monazite, extracted from these deposits by the methods outlined in Chapter VII, is now almost the sole source of the thorium nitrate of commerce. Small quantities are obtained from thorianite, the separation of the pure material presenting, in this case, very little difficulty by reason of the solubility of the mineral in acids and the very high percentage of thoria.