The Rare Earths: Their Occurrence, Chemistry, and Technology

Impregnation.

—The composition of the ‘Lighting Fluid,’ as the solution of salts used for impregnation is technically termed, varies slightly according to the nature of the mantle required, and the conditions of washing. It is of the greatest importance that the ratio of thoria to ceria should be constant and definite; the usual proportions are chosen so that the ratio of the oxides is 99 : 1. Fig. 11 shows at a glance to what a remarkable extent small variations in the percentage of ceria affect the luminosity of the finished product.[518] The thorium nitrate is made up with distilled water to a solution of 25-35 per cent. strength, and the calculated quantity of a standard solution of cerium nitrate is added. It is usual to add to the mixture a small quantity of another nitrate, which on ignition will leave an oxide of which the function is to strengthen the skeleton of ash. Beryllium, zirconium, magnesium, or aluminium nitrate is usually employed, in quantity calculated to leave an amount of oxide constituting about 0·5 per cent. of the total oxides; for ramie fabrics, beryllium nitrate is generally chosen.

After having been immersed for two to five minutes in the solution, the separate lengths are freed from excess of the lighting fluid by means of a small wringing machine. The pressure between the rollers must be regulated very exactly, since on the amount of solution taken up by the fabric will depend the mass of the oxide skeleton. The weight of oxides left after ignition should be 0·5-0·6 gm. for a ‘normal’ upright mantle of 9·5 cm. length, corresponding to 1·0-1·2 gms. of the nitrates, or, for a 30 per cent. solution, to 3·3-4·0 gms. of solution. The weight of the fabric before impregnation is approximately 5 gms. for cotton, 3 gms. for ramie, and 1·5 gms. for artificial silk. A cotton mantle-fabric, therefore, must be allowed to retain rather less, a ramie fabric rather more, than its own weight of solution, whilst an artificial silk fabric must take up 2-2¹⁄₂ times its own weight of the fluid. The weight of the oxide ash left from these quantities has been found by experience to be most suitable; if the mass is greater than this, the light-emission is diminished without a compensating gain in strength; if it is less, the light-emission is indeed greater, but the mantle becomes too fragile.

The impregnated fabric-lengths, after passing through the wringing machine, are drawn singly on to glass forms which are arranged on stands, and freed from moisture in a drying room by hot air, a temperature of about 30°C. being maintained. Three to four hours are required, under these conditions; if the drying be too rapid, considerable shrinkage occurs, and the mantles obtained are then extremely fragile.

The Mantle Head.

—The normal upright mantle is supported from a central rod of compressed magnesia—fused quartz has recently been suggested[519]—by means of an asbestos thread. The thread in the older patterns was supported by simply doubling over the fabric at the end which was to become the head; more generally, however, a strip of tulle or gauze is sewn to the head end before the impregnation. In order to strengthen the head, it is treated before ‘finishing’ with a hardening or ‘fixing’ fluid, which usually consists of a mixture of magnesium and aluminium nitrates in aqueous solution; the following may be cited as a typical mixture: Aluminium nitrate, 300 parts; magnesium nitrate, 300 parts; chromium nitrate, 3 parts; borax, 5 parts; distilled water, 1500 parts. In order to secure that this fluid is applied to the head only, a little organic colouring matter is generally added, so that it may be clearly seen. The solution is soaked on to the head from mechanically held felt pads, which are kept at a convenient degree of saturation with the fluid by means of an ingenious compressed-air device. The mantle is then rapidly dried in a hot-air chamber.

After the fixing and drying processes, the head is ‘finished.’ The ordinary upright mantle is sewn together, at the end which has been treated, with carefully selected asbestos threads, an opening of some ten millimetres being left, and the asbestos is threaded diametrically across this opening—these diametrical threads support the mantle on its rod during use. These operations were formerly done by hand, when mantles of good quality were required, but machine treatment is gradually coming into extended use. Several mantles now on the market are supported at the head by metal rings, made from thin sheets of iron which have been plated with aluminium. In petroleum lamps, the mantle is usually supported from both sides by means of asbestos threads.

In the case of fabrics from which ‘inverted’ mantles are to be made, fixing is carried out as usual at one end, to a depth of about 1·5 cm. After drying, a strip of about 0·5 cm. width is bent over and sewn down, and through this double band an asbestos thread is drawn, by which the mantle is secured to a magnesia ring. The lower end is drawn together in the shape of a hemisphere, by means of threads drawn through the meshes of the fabric; an opening of 6-8 mm. is sometimes left, but in the more modern patterns the end is drawn almost completely together, and after cutting is pressed out on a wooden shaper by a wooden mallet.

The product is now ready for burning off; if it is to be marked, it is stamped at this stage with a solution of didymium nitrate and methylene blue; the former being only faintly coloured, the organic dye is added to give a definite impression. On ignition, the nitrate is converted into the oxide, which is deeply coloured, and, of course, permanent.

Burning off and Shaping.

—For the production of mantles of the best quality, these processes are usually carried out by skilled operators, each mantle being treated separately. Very frequently, however, mechanical arrangements are employed. The great objection to machine treatment of such a product lies in the fact that it must be identical for every mantle; whereas it is exceedingly difficult to ensure that the original fabric, and the processes of washing, impregnating, wringing, and drying have been absolutely uniform. The operation of shaping and hardening is a very delicate one, and on the care with which it is carried out, the quality of the mantle finally depends. Until quite recently, only the cheaper kinds of mantles were machine-treated; but as the uniformity of the fabric becomes more assured, and the earlier operations more exact, employment of machines at this stage will undoubtedly increase.

The prepared fabric is shaped on a suitable form, and removed by a holder, which supports it from the asbestos thread; a flame is then applied to the head. The burning-off proceeds readily, once started; when the upper half has been incinerated, the flame is removed. The weight of the unburnt portion prevents too rapid contraction taking place at first; when the flame is removed, the glow spreads slowly downwards, and the shrinking is thus kept as uniform as possible. The operation must be carried out under a ventilating hood. The organic material of the fabric is completely oxidised, and the nitrates are converted into oxides, which retain the exact shape of the original fibres. The skeleton now undergoes the process of shaping and hardening, for which a ‘radial’ blowpipe flame is used. The burnt-off product is placed over this; the gas is supplied at an initial pressure of only a few inches of water, which is increased towards the end of the operation. The process commences at the head, the mantle being slowly lifted and rotated so that it is shaped and hardened along the whole length. By this means the oxide skeleton is not only suitably shaped, but is rendered considerably more elastic and resistant. For inverted mantles, of course, specially shaped burners are required. The eyes of the operators must be protected from the glare by shades of green glass. Recently the processes of burning-off and hardening have been carried out by means of the same burner.

Where machines are employed, the prepared fabrics are burnt off on wire shapers, usually in rows of ten; mechanical arrangements for continuous ignition and motion and, in the hardening, for continuous elevation of the ash-skeleton, are in use, but the finished mantles maintain a uniform good quality only when the structure of the fabric and the earlier processes have been absolutely uniform.

Collodinisation.

—The burnt-off mantle is now ready for use, but is far too fragile for transport. A method has therefore to be found by which the finished product can be protected for a time without detriment to its use for illumination. Mantles of artificial silk, particularly those for use in high-pressure lamps, are sometimes sent out without having undergone the final processes of burning off and shaping, which, in this case, must be carried out on the consumer’s burner. ‘Inverted’ mantles also were formerly sent out after impregnation and drying. In this condition, of course, the mantles are readily packed and transported, and there is the additional advantage that the duty on the unburnt product is very much less than that on the finished mantle.

One of the earliest of Auer’s patents (vide supra, p. 271) protected the process of collodinisation, which is now extensively employed. The oxide skeleton is dipped into a solution of collodion (the mixed lower nitro-derivatives of cellulose, or cellulose nitrates) in a mixture of alcohol and ether, to which, to prevent shrinkage on drying, a little camphor is added. On account of the inflammability of the mixture, the ethyl alcohol and ether are occasionally replaced by a mixture of methyl alcohol and acetone, but with this less volatile mixture, drying of course is slower. After dipping, the solvents are removed in a current of air, leaving the mantle coated with an exceedingly thin film of collodion, which increases enormously its power of resisting shock and vibration. This film is not removed until the mantle is placed on the consumer’s burner, when on the application of a match it ignites instantly and burns away, leaving the oxide skeleton in the condition to which it was brought in the final stage of hardening and shaping in the factory. The process is now used for almost all kinds of mantles, having been successfully applied in Germany in recent years to those made from artificial silk. The addition of small quantities of various inorganic salts, e.g. nitrates of zirconium, magnesium, platinum, thorium, etc., to the collodion solution, has been proposed; these salts make the collodinised product extremely resistant, but have a very harmful effect on the oxide ash when the collodion has been burnt off.

The collodinised mantles are cut to length on a trimming machine, and are then ready for packing.

The present chapter may be concluded with a bare mention of a few disconnected details, selected from the great mass of proposals, suggestions, and developments which have sprung up round the incandescent mantle industry.[520]

With regard to the composition of mantles, numerous proposals have been made. It is stated that thoria with 0·25 per cent. of uranic oxide, UO₃, gives a light almost equal to that of the Auer mantle. Zirconia with 0·40 per cent. of vanadium, in the form of the pentoxide, is said to give a splendid white light; the vanadium oxide slowly volatilises, but addition of an equivalent proportion of silica is said to prevent this. Langhans claims to have obtained a product equal in light-giving power to the Auer mantle, by using as impregnating fluid a solution of colloidal silica, obtained by the addition of nitric acid to a solution of sodium silicate, to which suitable quantities of rare earth nitrates are added. Bodies obtained by the use of very similar solutions give skeletons which are coming into extended employment for gas radiators. The ‘Sunlight’ mantles use a mixture of thoria (50 per cent.), alumina (40 per cent.), and chromium sesquioxide (10 per cent.).

A direction of development in which some success has been attained is the introduction of self-lighting devices. The catalytic action of finely divided metals has been proposed in innumerable patents,[521] but these devices are unreliable, and it seems doubtful if chemical methods will ever be successfully applied to the problem. For the lighting of streets, shops, etc., the ‘by-pass’ system is employed; a tiny jet of gas burns continuously from a pin-hole nozzle, which is momentarily increased, when the main supply is turned on, to such an extent that the gas issuing from the burner is ignited.[522]

An account of the innumerable forms of lamps and burners which have been introduced in the last twenty years would fill several volumes. The theoretical grounds on which improvements in this direction are based are outlined in an able article by Dr. H. Bunte, a recognised authority on incandescent lighting, which appeared recently;[523] for an account of some of the lamps which have been successfully applied, the reader is referred to a recent French publication.[524]

CHAPTER XX
ARTIFICIAL SILK—ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY

The history of the artificial silk industry, since its foundation about the year 1890, illustrates curiously the rapidity with which isolated facts, of apparently merely academic interest, are seized upon and adapted to the needs of modern civilisation. It is during this period, especially, that the bonds between science and industry, in a dozen different directions, have been drawn so close that to-day it is in many cases impossible to differentiate the two. The pure science of to-day is the technology of to-morrow—and not always even of to-morrow, but of to-day. But we have moved even beyond this; the industrial needs of the day are creating and extending our science at a rate which shows how relatively poor a stimulus has been the mere desire for knowledge. Such has been the history of the artificial silk industry. No sooner had Chardonnet shown that the preparation of a new fabric was not only possible but profitable, than a thousand aspects of the problem were taken up. Patents were taken out on all sides—the majority, as usual, valueless, one or two of great importance. Companies were formed, factories built, machines invented; numberless applications were proposed, mostly again worthless, whilst patient research and innumerable experiments have carried one or two suggestions to a successful place in practice. Among these has been the adaptation of artificial silk to the manufacture of mantles, which will be outlined in the present chapter. Before taking up this question, however, a short account of the manufacture of the fabric itself must be given.

Chardonnet Process.

—In the Chardonnet process, an account of which was published about 1890, continuous fibres are obtained by forcing through tiny jets a viscous solution of collodion, or nitrocellulose, as it has been misnamed, in a mixture of ethyl alcohol and ether. In the original form of the process, the solution was forced into water, which, by removing the alcohol and ether, caused an instantaneous coagulation of the surface, so that a filament was obtained which could be wound directly on to a spool. More generally, however, the jets deliver the solution into a chamber through which warm air is passed; this is equally effective in removing the solvents and causing surface coagulation, and the filaments are woven directly into threads of ten to forty strands, according to the purpose for which the fabric is required, fifteen to twenty being used for silk from which mantles are to be made. On account of its inflammability, the thread is denitrated by means of a solution of ammonium sulphide.

The raw material for the process is cellulose, usually in the form of cotton. Treatment of this with a suitable mixture of concentrated sulphuric and nitric acids replaces some of the hydroxyl groups by the ‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being formed, in which the so-called tetra-, penta-, and hexa-nitrates predominate.[525] The product, cellulose nitrate or collodion, very closely resembles the original cellulose in appearance and structure. It is washed thoroughly to free it from traces of acid—which render it liable to explode spontaneously—and after drying, dissolved in the minimum quantity of the mixed solvents;[526] the solution is filtered from insoluble impurities through wads of cotton, pressures of thirty to sixty atmospheres being required. This filtration purifies and thoroughly mixes the solution, so that perfect uniformity is obtained in the product. The glass jets through which the solution is now forced, under a pressure of forty to fifty atmospheres, have a diameter of 0·08 mm., but the threads obtained contract on the removal of the solvents, so that fibres of 0·01-0·02 mm. are formed.

Chardonnet probably began his work about 1885. It is interesting to observe that an Englishman, Swan, had proposed in 1883 to use a solution of collodion in acetic acid, fabrics prepared by his process being shown at the London Exhibition of 1884.[527]

The Pauly or Cuprammonium Process.

[528]—It has long been known that a solution of copper hydroxide in ammonia solution—Schweitzer’s reagent—will dissolve cellulose. The use of this solvent for the production of artificial silk was proposed about 1900, and the method has become a serious rival of the older Chardonnet process. The solvent is prepared on a large scale by passing air through an ammonia solution to which copper turnings have been added. After addition of the cellulose, and filtration, the solution is forced through tiny jets into a bath of dilute acid, which removes the copper and precipitates the cellulose again.

The solution of cellulose by Schweitzer’s reagent is undoubtedly a chemical action. Cellulose is to be regarded as a polyhydric alcohol, with one or several atoms of hydrogen of the hydroxyl groups replaceable by metals. According to Piest (loc. cit.) a ‘Cupramine base’ is formed by the replacement of this hydrogen by copper and the amino-group, NH₂. The action of sodium hydroxide on cellulose, however, is generally regarded rather as an additive reaction, the product, ‘alkali cellulose,’ being usually written C₆H₁₀O₅,NaOH. A careful chemical investigation alone can reveal the actual nature of the compound formed; such an investigation, apart from its scientific interest, might yield results of considerable technical importance.

The Viscose Process.

—Shortly after the introduction of the Chardonnet process, patents were taken out which protected a very cheap and simple method of dissolving cellulose,[529] which had been discovered by two well-known English authorities. Cross and Bevan. They found that mercerisation, i.e. the action of the sodium hydroxide on cellulose, produces a swollen, transparent mass, which very readily takes up carbon disulphide. When exposed to the action of this liquid for three or four hours, at ordinary temperatures, the mass swells further, gelatinising and becoming soluble in water. On treatment with water, a yellowish, extremely slimy solution is obtained, from which cellulose is precipitated on prolonged standing, by heating, or by oxidation. The substance is apparently a cellulose xanthate, and may be written NaS·CS·O·C₆H₉O₄,NaOH.[530] On account of the extremely viscous nature of the aqueous solution, Cross and Bevan gave it the name Viscoid.

During the last few years this method of dissolving cellulose has been employed in the manufacture of artificial silk, under the name ‘Viscose Process.’ The product obtained is very suitable for the manufacture of incandescent mantles, and is considerably cheaper than either the Chardonnet or Pauly silk.

The Acetate Process.

—Quite recently numerous experiments have been carried out with the object of finding methods for employing the cellulose esters of organic acids in the preparation of fabrics. The acetate, which is generally used, gives solutions from which fibres can be obtained which are comparable to natural silk in strength, and which have the further advantage of being non-inflammable, and far less readily affected by water than artificial fabrics obtained by the above methods. It is prepared by treating cellulose with dilute acid, by which the so-called ‘hydrocellulose’ is obtained; this is treated with a mixture of glacial acetic acid and acetyl chloride, and the whole, after addition of a little concentrated sulphuric acid, warmed to 65°-70°C. As early as 1894, Cross and Bevan[531] had patented a process for this preparation by the action of acetyl chloride in the cold on an intimate mixture of cellulose and zinc chloride.

From the solution obtained, the acetates are precipitated by water, washed and dried. The mixture of esters dissolves in chloroform, nitromethane, acetic acid, phenol, pyridine, etc., and is re-precipitated by addition of alcohol, benzene, or ligroin (petroleum ether). On account of its non-inflammable character, cellulose acetate, as the product is called, is being used instead of the nitrate in the manufacture of celluloid; it is also used for non-inflammable cinematograph films. Fibres can be obtained by forcing the solutions through jets, and removing the solvent, as in the above processes; these are spun into threads which are coming into increasing use, on account of their extremely low conducting power, for the insulation of very fine electric leads. The product is at present too expensive, however, for use in the textile industries, or for the manufacture of mantles.

A solvent which had at one time some technical importance is zinc chloride.[532] The concentrated aqueous solution of the salt will take up cellulose in considerable quantity; and the solution has been used in the preparation of carbon filaments for glow lamps.

The fabrics prepared by the processes which have been mentioned above are of great technical value. In lustre they far surpass natural silk, and they take dyes very well, but owing to the ease with which they tear, they cannot be woven alone for textiles, but are always used in ‘mixed’ materials. The acetate silk, which approaches the natural fibre in strength, is not much less expensive. Whilst the price of natural silk is roughly 35 francs per kilo. (approx. 13s. 3d. per lb.), the costs of production of the artificial fabrics are—Chardonnet 15 frs., Pauly 12 frs., Viscose 7 frs. per kilo. (respectively 5s. 8d., 4s. 6d., and 2s. 8d. per lb.). Artificial silk, however, has uses distinct from the natural fibre, and is at present a competitor with it in one or two small fields only. Thus the production of natural silk is ten times that of artificial silk (50,000,000 kilos. per annum to 5,000,000 kilos.) in spite of the difference in price.

Artificial silk is very susceptible to the action of water, which weakens it very considerably. Its resistance is said to be greatly increased by the action of formaldehyde; the fabric is plunged into a bath containing an aqueous solution of the aldehyde, to which a little lactic acid has been added. The chemistry of the change is discussed at length by Beltzer (loc. cit.).

The threads of artificial silk far surpass in lightness those spun from vegetable fibres. A thread of twenty strands weighing one pound avoirdupois would be more than twenty miles long. At the same time the filaments have not the irregular tubular structure of vegetable fibres, but are solid cylinders. The fact that the filaments are continuous, so that there is relatively little torsion in threads spun from them, gives artificial silk its great advantage over the natural vegetable fibres for the manufacture of mantles. For this purpose the Pauly or Cuprammonium silk is most suitable, though Viscose silk is almost as good; the fibre obtained by the Chardonnet process is not quite so useful in this direction.

The Manufacture of Mantles from Artificial Silk.

—Whilst the fabrics made by the various processes outlined above are more expensive than the cotton and ramie formerly exclusively used in the mantle industry, they have the advantage, in addition to the fact that they produce better and more lasting mantles, that they do not need the laborious and troublesome process of washing which is so essential in the case of the vegetable fibres. From the nature of the methods used in its manufacture, artificial silk can contain no mineral residue; hence the fabric is immediately ready for impregnation.

As early as the year 1892 Schlumberger and Sinibaldi proposed the use of Chardonnet silk for the manufacture of mantles; but their patent, a Belgian one,[533] attracted little attention, although they stated clearly that the denitrated silk will readily take up the lighting fluid. Ignorance of this fact deferred the successful application of this fibre for ten years. In 1894 De Mare suggested the preparation of mantles by addition of the necessary salts to the collodion solution before squirting; in the following year Knöfler used the same process, recommending in addition the use of ammonium sulphide to denitrate the impregnated threads. These two attempts, which were found to be unworkable, owing to the difficulty of obtaining a homogeneous product before squirting, were merely efforts to compete against the Auer monopoly, resting on Welsbach’s patents, which covered impregnation of any natural fibre. In Knöfler’s process,[534] the salts were dissolved in alcohol and added to the collodion solution, which was then forced through jets into water, to which ammonia was added to prevent removal of the nitrates in solution; the threads were then denitrated with ammonium sulphide. The ammonia treatment of course converts the nitrates into the insoluble hydroxides, a departure which was followed in most of the numerous patents inspired by Knöfler’s process.

The first indications of the method which ultimately led to success are to be found in a patent taken out by Plaisetty, in 1901.[535] The specification protects the addition of thorium and cerium hydroxides to the cuprammonium solution of cellulose, but apparently without any inkling of the results that were to follow, and more or less incidentally, he includes in this patent the impregnation of the finished fabric and the subsequent treatment with ammonia. In the following year he applied for a German patent,[536] which was granted in May, 1903, in which he definitely protects the impregnation of the finished fabric, and the ammonia treatment, the fabric being then washed and dried, and burnt off as usual.

Impregnation.

—Since the filaments from which artificial silk is obtained are solid and rod-like in form, as opposed to the tubular structure of cotton and ramie filaments, it is rather surprising that the fabric should take up the lighting fluid in the necessary quantity (vide p. 295). It is found that a 50 per cent. solution of nitrates gives the best results, the impregnation requiring half an hour; a warm bath is usually employed. It is usual to add to the bath a quantity of thorium hydroxide, since the thorium nitrate of commerce generally contains nitric acid, which has a bad effect on the fabric.[537] The excess of solution is removed by means of a glass or porcelain centrifuge, not, as with cotton or ramie mantles, by use of a wringer; drying must be carried out very slowly. The fabric is not cut into lengths before impregnation, as in the case of cotton or ramie, but is immersed in the lighting fluid in long strips.

‘Fixing.’

—If the dried fabric, impregnated with the necessary salts, be finished and burnt off in the usual way, the oxide skeleton is extremely fragile, and soon falls to powder. The reason for this lies probably in the explosive decomposition of the nitrates, the weight of organic matter relative to the salts being very much less than in the case of ramie or cotton fabrics (vide p. 295). The additional ammonia bath advocated by Knöfler (vide supra) was therefore adopted by Plaisetty, and the nitrates in the dried impregnated fabric are converted into hydroxides by this treatment. For this process, to which the name ‘Fixing’ has been given by Böhm, numberless alternative proposals have been made. Plaisetty’s ammonia fixing gives a mantle which, after burning off, is exceedingly elastic and strong, but it is nevertheless open to serious objections. Thus the nitrates may be to some extent dissolved out by the fixing bath before precipitation of the hydroxides has occurred; to remedy this, the impregnated fabric must be very thoroughly dried before fixing, and in this case, apart from the trouble involved, the acid of the commercial nitrates will attack the fabric unless addition of thorium hydroxide has been made to the impregnating fluid. Again, the conversion into oxides is not complete, the outer layer first formed preventing free diffusion of the alkaline fluid. Finally, since ammonium nitrate is formed in the reaction, a very thorough washing is necessary to remove this salt.

It would be impossible to mention all of the numberless proposals which have been put forward for fixing; nor are the great majority worthy of mention.[538] One of the most important was that of Albrecht,[539] by which hydrogen peroxide is substituted for ammonia. This reagent, as is well known, precipitates from solutions of thorium salts ‘peroxide’ compounds (vide p. 255); from the fabric impregnated with the nitrate, free nitric acid is liberated in the reaction. Since the peroxide is soluble in nitric acid, two baths are used, the fabric being allowed to remain for a short time only in the first, which becomes strongly acid, and being then transferred to the second, in which the precipitation is completed. The burning off of the product so treated proceeds quite quietly, and leaves a very hard and elastic skeleton. The method, however, has the grave disadvantage that cerium salts are not precipitated under these conditions, but escape into the solution. To remedy this, arbitrary additions of cerium nitrate are made to the fixing bath, but the percentage of cerium, and therefore the lighting power (vide p. 293) of mantles fixed by this method is liable to fluctuate. A modification has been introduced[540] in which various salts are added to the hydrogen peroxide fixing baths to prevent the withdrawal of the cerium salt; these are chiefly acetates of the alkali metals and allied compounds. The fabric requires washing after this treatment.

A rather similar proposal[541] substitutes for ammonia an alkaline solution of hydrogen peroxide, obtained by dissolving sodium peroxide in water. After saturation for a minute or so, the fabric is wrung out and dried, there being no necessity, according to the patent, for any subsequent washing. The same company in an earlier patent[542] suggest a fixing bath of ‘an alkali or amine with an acid which can form insoluble basic double salts with the earth metals,’ the said salt to be precipitated on the fibre, whilst an alkali or amine nitrate goes into solution; acetic and benzoic acids and phenol are mentioned. Apparently this process did not give satisfactory results. The same may be said of the hypochlorite method of Visseaux.[543] Equally interesting, and doubtless equally practical is the proposal[544] to treat the dried impregnated mantle with ozone in a closed chamber, which possibly inspired the even more original suggestion[545] that the fabric be first washed in ozonised water, dried, then impregnated and dried, and finally treated with ozone. According to yet another French patent,[546] the impregnated and dried fabric is to be treated with an alcoholic solution of hydrofluoric acid, which will give a viscous insoluble mixture of thorium and cerium fluorides in the fabric, and at the same time will remove any impurities. Another patent[547] proposes the use of hydrazine and similar bases, cerium nitrate being added to the fixing solution.

Innumerable suggestions have been made for fixing by means of ammonia gas, or vapours of organic bases.[548] An English patent granted in February 1910[549] protects ‘various improvements,’ which consist in carrying out a preliminary treatment with steam, impregnation with the solution of nitrates, conversion of nitrates into oxides either by steam carrying ammonia, pyridine, etc., or by the action of these vapours without steam in a vacuum, all in one chamber, which can be exhausted or filled with various solutions or vapours as required.