The solubilities of the oxalates in mineral acids of various concentrations have been examined by Hauser and Wirth.[167] Whilst the solubilities in water are exceedingly slight, and increase with increasing atomic weight of the elements, i.e. from the cerium to the yttrium group, in mineral acids of concentration 3-4N the solubility becomes noticeable, and is greatest for the oxalates of the most positive elements. The solubility is greatly lessened, however, if considerable excess of oxalic acid be present.

Double oxalates with the alkali oxalates can be obtained with the salts of the yttrium elements only, the oxalates of the cerium elements being almost insoluble in excess of alkali oxalate in the cold. Of the alkali double oxalates, the potassium compounds are the most soluble, but the ammonium compounds show the greatest differences in solubility; von Welsbach has employed the method of fractional crystallisation of these salts from a saturated solution of ammonium oxalate for separations in the yttrium group. The sodium double oxalates are the least soluble of these double salts.

Since the rare earth elements are almost always separated in the form of the oxalates, the methods for transforming these into soluble compounds become important. They may be ignited to oxides, and these dissolved in nitric acid; if the content of ceria is very high, the oxide mixture may become insoluble, but this difficulty may be overcome by addition of a reducing agent—hydrogen peroxide is very convenient for this purpose. The oxalates may also be dissolved directly in fuming nitric acid, care being taken to avoid loss; if the mixture contains cerium, the oxidation is hastened, ceric salts having the property of acting as oxygen carriers. By boiling for a short time with potash, the oxalates may be easily transformed into the hydroxides, which can be dissolved in dilute acids.

Formates.

—On account of the considerable differences in solubility by which they are characterised, these salts have been employed for separations. The formates of the cerium group are considerably less soluble than those of the yttrium group. They may be partly precipitated from solutions of rare earth salts by addition of alkali formate—formic acid itself causes precipitation only with salts of weak acids, e.g. the acetates—but are best prepared by dissolving the oxides in formic acid; on concentration of the solution, the formates of the cerium and terbium elements successively separate, the salts of the yttrium group remaining in solution. The separation of the terbium earths by this method was attempted by Delafontaine; his ‘new’ element, Philippium, obtained from the mother-liquors, was in reality a mixture of the terbium and yttrium elements, which cannot be completely separated by the formate method.[168]

The acetates are readily soluble in water, the yttrium salts being rather less easily soluble than those of the cerium group. They are therefore obtained by dissolving the oxides in acetic acid; addition of alkali acetate to a solution of a rare earth salt gives no precipitate, even on boiling, behaviour which is in marked contrast to the ease with which the salts of other trivalent metals are hydrolysed under these conditions. In this respect the rare earth elements differ also from the tetravalent elements zirconium and thorium (and from cerium in the tetravalent state); soluble salts of the latter, on boiling with sodium acetate, give insoluble basic acetates. Even sparingly soluble compounds of the rare earth elements are as a rule taken into solution by digestion with ammonium acetate.

Tartrates.—Addition of ammonium tartrate to a neutral solution of rare earth salts throws down an amorphous precipitate, which dissolves easily in acids, and in excess of the precipitant. In the presence of tartaric acid, precipitation of the earths by addition of sodium hydroxide is completely inhibited. Potassium hydroxide under these conditions gives a precipitate in the case of the yttrium elements, though only on boiling; ammonia gives a crystalline precipitate even in the cold with this group. These precipitates are alkali double tartrates of the yttrium metals; the cerium elements give no precipitate at all. In all cases, therefore, the precipitation of the hydroxides is inhibited by the presence of tartaric acid.

A very large number of organic salts of the rare earth elements has been prepared and examined during the past two decades, in the endeavour to find some class of compounds which will allow of an easy separation of the group. The benzoates, succinates, hippurates, citrates and similar relatively simple salts first received attention, but less common acids, as e.g. the hydroxynaphthalenesulphonic acids, have also been employed.[169] The use of various organic acids for the separation and estimation of thorium in presence of the rare earths is outlined in that connection (see p. 288). More recently, the glycollates and cacodylates have been prepared. The glycollates[170] of the cerium elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts; they are more soluble than the yttrium compounds, which have the formula R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The cacodylates,[171] R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have similar solubility relations.

The phthalates of the yttrium group have been found to be very valuable for purposes of separation by Meyer and Wuorinen.[172] The salts are readily obtained in solution by shaking together cold aqueous suspensions of the rare earth hydroxides, and phthalic acid; the clear solutions when warmed become cloudy, the organic salts hydrolysing very easily, with separation of the hydroxides. The most positive elements naturally remain longest in the solution, the weakly basic oxides accumulating in the first precipitates.

An organic compound which has proved very useful in the treatment of the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic form, this substance forms salts with metals, which in the case of the rare earth elements are especially characterised by the ease with which they may be obtained, and their high crystallising power. They may be prepared by double decomposition of neutral solutions of rare earth salts with ammonium acetylacetone, and crystallise readily from dilute alcohol. They have been used by Urbain in the fractionation of the yttrium group, and for determination of molecular weights by the boiling point method; Biltz[174] has shown that in solution they generally have the double formula R₂(C₅H₇O₂)₆.

The Rare Earth Elements, and the Periodic Classification

At the time of the introduction of the periodic classification the rare earth elements were generally believed to be divalent. This belief, which has persisted until quite recently,[175] was based chiefly on the electropositive character of the metals, and their general chemical resemblance to the elements of the alkaline earths; the isomorphism of the tungstates of calcium and the cerium elements, and of the molybdates of lead and the cerium elements, also supports this view. The physical evidence in favour of Mendelejeff’s view, however, is quite overwhelming; the specific heats of the metals, the equivalent conductivities of the chlorides, and molecular weight determinations by means of vapour densities and the boiling point method, prove beyond doubt that the elements are in fact trivalent.

In deciding in favour of the trivalent nature of the rare earth metals, Mendelejeff was influenced chiefly by the fact that there was no room in the table for divalent elements with the equivalent weights then assigned to the cerium and yttrium elements. At that time, only the six oxides obtained by Mosander were known; of these the accepted equivalents and atomic weights were as follows:

the values for terbium being uncertain. If cerium be considered trivalent in the cerous salts, its atomic weight becomes 138, that of barium being 136. Mendelejeff placed cerium in Group IV, series 8, in the position which it still occupies; he pointed out that the accepted equivalent must be too low, and suggested that the atomic weight should be at least 140, almost exactly the value accepted to-day.

This choice left the positions in Group III, series 8, horizontally before cerium, and in Group IV, series 10, vertically below it (see figure), to be filled by the two elements, lanthanum and didymium. No chemical evidence being available to decide the choice, he provisionally assigned didymium to the first (Group III, series 8), and lanthanum to the second (Group IV, series 10) position, at the same time expressing the opinion that didymium was probably a mixture of closely related elements. Yttrium then fell into place in Group III, series 6, above didymium, and erbium in Group III, series 10, below it. To the vacant space above yttrium in Group III, series 4, he assigned the hypothetical element Eka-boron, with atomic weight 44; this space is now occupied by scandium, which corresponds almost exactly in properties to the metal described by the Russian chemist. A part of the table illustrating these positions is shown in Fig. 4.

The determination of the specific heats of the metals by Hillebrand and Norton in 1875, whilst confirming the trivalency of the elements, rendered it necessary to alter the position of lanthanum, which was placed in Group III, series 8, instead of didymium, which was thus left without a place. This first indication that all the rare earth elements could not be fitted into the table without difficulties was soon followed by the discovery of several other members of the group, for which places could not easily be found.

Fig. 4.—Part of the Periodic Table, showing the positions originally assigned to the Rare Earth Elements by Mendelejeff

It was first pointed out by Brauner in 1881 that, with the exception of scandium (44·1) and yttrium (89·0), the rare earth elements form a zone of increasing atomic weight between barium (137·37) and tantalum (181·5). In 1902 he proposed[176] to consider the rare earth metals as a kind of zone or belt among the elements, comparable to the asteroids in the solar system, extending from cerium in Group IV to tantalum in Group V in a continuous series. The suggestion seems at first sight contrary to the whole principle of periodic classification, but it accords very well with the anomalous position of the rare earth group among the other elements; it is very well illustrated in the accompanying Fig. 5, which shows an helical or space representation of the table.

Fig. 6.—Part of the Periodic Table,
showing the Positions assigned to the Rare Earth Elements by Brauner in 1908

Brauner’s conception is also in accord with the physical properties of the elements and their compounds. These vary continuously throughout the group, and show nowhere the sudden transitions which are characteristic of other series in the table. Benedicts[177] has collected all the data bearing on the atomic volumes, and finds that those also vary continuously, with rise in the atomic weights, within quite small limits, all lying between the values for barium and tantalum. In face of all the evidence furnished by physical and chemical properties, however, Brauner[178] has recently reverted to an idea which he put forward in 1881, according to which lanthanum and cerium are placed as usual in Groups III and IV, series 8, whilst the other elements are distributed in order throughout the remaining groups, as shown in Fig. 6.

In support of this arrangement, he quotes the fact that some of the elements appear to be able to form higher oxides in the presence of other oxides, which act as oxygen carriers (see pp. 174, 177-8), though these higher oxides are certainly not salt-forming. He also deduces, from the rates of hydrolysis of the sulphates, that the elements fall into two parallel series, according to the strengths of the hydroxides as bases, on which ground he justifies the distribution throughout series 8 and 9. There can be no doubt, however, that this disposition is far less in accordance with the behaviour and properties of the rare earth elements than is the first arrangement, which places them in a transition zone between barium and tantalum; it is impossible, for example, to reconcile the properties of praseodymium with those of columbium and tantalum, or to find the slightest analogy between neodymium and molybdenum or tungsten, as the second arrangement requires.

The analogy of the rare earth group to the elements of Group VIII has been pointed out by many authors.[179] On the ground that the rare earth elements cannot be spread over the table in series 8-10, Steele[180] favours the early classification of Thomsen, according to which the elements are divided into three groups. The first, corresponding to Groups I and II of Mendelejeff’s table, consists of two sub-groups, each containing seven elements[181]; the second, corresponding to the first two long series of the periodic table, has two sub-groups, each of seventeen elements, of which the first and last seven are analogous—these elements fall into the same groups in the periodic table—whilst the middle three are interperiodic. These interperiodic elements are those which Mendelejeff places in Group VIII. The third division consists of one (or two) group(s) of thirty-one elements; here again, the first and last seven are analogous, whilst the interperiodic elements, which are seventeen in number, include the rare earth metals.

Steele’s idea has been extended by Werner,[182] who has drawn up a table to illustrate it. In this classification, the elements are arranged in order of atomic weight, but arbitrary gaps are left in such a way that similar elements may fall into the same vertical columns, as in the periodic table. The arrangement has the advantage that the interperiodic elements, consisting of the rare earth elements and the elements placed in Group VIII of the periodic table, here do fall in the middle of their respective periods, but it has several drawbacks, and does not represent the transition of properties from element to element so well as the helical representation of the periodic table, which brings out most clearly the true relations between the elements, and the anomalous position of the rare earth metals.

Mention must be made at this point of the theory of ‘Meta-elements’ put forward in 1888 by Sir William Crookes.[183] From his work on the cathode luminescence of some of the oxides (see next chapter), that author was led to the conclusion that several of the then-accepted rare earth elements, notably samarium and yttrium, were in reality heterogeneous, consisting of large numbers of very closely related bodies, differing so very slightly in properties that only the most refined methods could perceive the variations; for these he proposed the name Meta-elements. Though it has been proved that the differences observed by Crookes in the luminescence spectra were really due to the presence of very small quantities of impurities, his paper is of great interest, in that it contains a theory of evolution of the elements, and postulates the possibility of their decay. Modern developments in radioactivity have not only lent a curious force to these speculations, but even support his contention that a chemical element, in the ordinary sense of the word, is not necessarily homogeneous.[184] In the field of the rare earths, also, the homogeneity of elements is even now continually being called into question (see Thulium, p. 204). In any case, we have in the rare earth elements a series of bodies in which the change of properties from one member to another—and the consequent possibility of easy separation—is so very slight, and so far without parallel in the whole field of chemistry, that we are at least justified in asking whether some extension of our ordinary conception of an element is not required.

CHAPTER X
GENERAL METHODS OF SEPARATION

The chemist who sets out to prepare a pure compound of a rare earth element is faced by a great difficulty. The rare earth compounds occur in nature, as one might expect from their great similarity, as mixtures of very complex composition. After the relatively simple separation from foreign elements has been accomplished, the enormously greater difficulty of separating the elements from one another has to be encountered. So great is this difficulty, by reason of the fact that, with the sole exception of cerium, the elements show no variation in property sufficient to allow of the use of ordinary analytical methods, that even at the present day it is extremely doubtful if all the elements in the yttrium group are known to us.

The methods which can be adopted in attempting a separation are of two kinds. The first includes those processes which take advantage of the gradual variation in basic strength of the hydroxides as the atomic weight changes; the most important of these are fractional precipitation of the hydroxides, and fractional decomposition of the nitrates. Fractional precipitation of the hydroxides is generally effected by gradual addition of ammonia, soda, magnesia, or other base, to a solution of the mixed salts; such a solution may also be digested with the oxides obtained by ignition of another fraction of the rare earth compounds. If the digestion be sufficiently complete, the precipitate in each case will be richer in the less basic hydroxides, whilst the solution will be richer in the salts of the more electropositive elements.

The fractional decomposition of the nitrates is based on the fact that when a mixture of the salts is heated gradually, the nitrate of the least positive element begins to decompose first. The temperature is maintained for some time at the point at which decomposition begins; when nitrous fumes cease to be evolved the mixture is cooled, and extracted with water or dilute acids. The insoluble portion—basic or superbasic nitrate (see p. 128)—will then be richer in the less electropositive elements; the solution is evaporated, and the solid so obtained subjected to a somewhat higher temperature, and the process repeated several times. In this way, a series of fractions is obtained, in which the elements tend to distribute themselves in order of electropositive character. By a sufficient number of systematic repetitions of such steps, the elements may eventually be obtained in the form of compounds of approximate purity, which may then be refined by one of the methods of the second kind described below. Experience has shown, however, that a quicker and more complete separation may generally be effected by combining two or more methods of separation; one method will give the best separation up to certain limits, but then becomes much less valuable; the separation at this point is therefore taken up by another process. A process depending on differences of basic strength of the hydroxides is generally supplemented by a method of the second class, i.e. a process of fractional crystallisation; where the basicity method is not used (as, for example, in most of the recent processes for separation of the cerium elements), two or more different methods of fractional crystallisation will supplement one another.

The methods of the second class, which are processes of fractional crystallisation, depend on the differences in solubility which are observed in analogous compounds in passing from one member of the group to another. The value of these methods, as opposed to the methods depending on differences in basic strength, was clearly shown by Auer von Welsbach, who in 1885 succeeded in resolving Mosander’s ‘Didymium’ into two new elements, praseodymium and neodymium, by fractional crystallisation of the ammonium double nitrates; since that date, much attention has been devoted to the task of finding rare earth compounds which will lend themselves to such processes. The method is extremely laborious, and may involve several thousand recrystallisations, in consequence of the generally very slight differences of solubility, and the ease with which the rare earth compounds, being almost always isomorphous with one another, form mixed crystals.

Whilst the method of fractional crystallisation has come into general use for the separation of one element from another only within the last thirty years, processes for the separation of the cerium group from the yttrium elements, depending on differences of solubility, have long been known and used. The most important of these, the double sulphate method, depends on the fact that the potassium double sulphates of the cerium metals are almost insoluble, whilst those of the terbium group are sparingly, and of the yttrium group readily soluble in a concentrated solution of potassium sulphate. The cerium elements may be thus completely removed from a solution of mixed salts by addition of a crust of potassium sulphate crystals, or of an hot concentrated solution of the same reagent. In other cases, e.g. in the double carbonate and double oxalate processes, separation is effected by taking advantage of the greater tendency to the formation of double salts possessed by the yttrium metals.

In effecting a separation of closely related bodies by fractional processes, in which a large number of repetitions of the same operation are necessary, only the most careful and systematic procedure can avoid much waste of valuable material; in these processes, the object of the chemist is to obtain pure end fractions, whilst keeping the middle fractions as small as possible. One method of procedure generally adopted is illustrated in Fig. 7, which represents a fractional crystallisation of a mixture of four or five substances, α, β, ... φ; the separations being usually conducted in such a way that subgroups of three, four or five elements are first obtained, these being then further fractionated to obtain the pure elements. In the diagram, crops of crystals are represented by crosses, the mother-liquors by circles; for the sake of illustration, the process is made to appear as simple as possible.

The mixture is dissolved up, and allowed to crystallise; the crystals are filtered off, the filtrate concentrated, and a second crop obtained; this is repeated until five or six crops of crystals have been obtained. These, with the mother-liquor, constitute series A. The first fraction is now recrystallised; it yields a crop of crystals, fraction 1 of series B, and a mother-liquor, which is added to fraction 2 of series A, as indicated by the dotted arrow and circle; on recrystallisation of this mixture, a crop of crystals, fraction 2 of series B, is obtained, together with a mother-liquor, which is recrystallised with fraction 3 of series A. In this way, by continued repetition, series are obtained, of which each contains one fraction more than its predecessor; the least soluble constituent is thus concentrated in the fractions represented on the left of the diagram, whilst the most soluble accumulates in the mother-liquors. After a greater or smaller number of series have been traversed, according to the differences in solubility, the end fractions in each series will be pure. These are no longer fractionated, and the number of fractions in each series begins to diminish, as shown on the diagram. The middle fractions will contain the compounds of intermediate solubility; these may be separated by further fractionation on the same lines, or may perhaps be better treated by a different or modified process.

In a modification of the method, each fraction of series A is recrystallised separately, yielding a crop of crystals, and a mother-liquor; series B is then built up by adding to the crystals from fraction 2 the mother-liquor from fraction 1, to the crystals from fraction 3 the mother-liquor from fraction 2, and so on; the fractions in this series are then recrystallised separately, and the third series built up by the similar combination of the crystals and mother-liquors.

Similar systematic methods of procedure must be adopted in working out any method of fractional separation; it can at once be seen that where, as in the rare earth group, only small variations in properties exist, much time and care must be expended, if pure products are required.

Since the development of the methods of spectrum analysis, the difficulty of testing the efficiency of a method of separation, and of examining the purity of the products obtained, has been greatly lessened. The only reliable test at the disposal of the earlier chemists was the determination of the equivalent weight, which still constitutes an important check on the modern methods. Some account of the methods available for the control of the methods of separation is essential in a general account of the rare earths; but before describing these, it will be convenient to give a short description of the methods used in the extraction of the elements from the rare earth minerals.

Extraction of the Rare Earths from Minerals

With the exception of those containing large proportions of columbium, tantalum, and titanium, the rare earth minerals are easily decomposed by acids. The silicates, as a general rule, can be satisfactorily treated with hydrochloric acid in the ordinary way, but for large quantities, the use of sulphuric acid is more desirable. The more refractory minerals are completely decomposed by fused alkali hydrogen sulphate; sodium bisulphate is more suitable for this purpose than the potassium compound, the sodium double sulphates of the rare earth elements being more soluble than the potassium salts. Hydrofluoric acid also attacks the refractory minerals very readily; the rare earths, in this case, are left as the insoluble fluorides.

After decomposition with sulphuric acid or bisulphate, the cold residue is extracted with water, the rare earth sulphates or double sulphates being removed in solution. Digestion with nitric acid may be necessary at this stage, if titanium, columbium, etc., are present; after filtration, the solution is evaporated to dryness, and the residue extracted with dilute hydrochloric acid. The solution is saturated with sulphuretted hydrogen to remove lead, copper, bismuth, molybdenum, etc., and treated in the usual way with ammonium chloride and ammonia. The precipitate is washed, and dissolved in hydrochloric acid, the solution heated to about 60°, and the rare earths precipitated by addition of excess of oxalic acid, which holds in solution any zirconium which may be present. In the presence of phosphates, e.g. in the treatment of monazite or xenotime, the precipitate of oxalates should be ignited to the oxides, these dissolved in acid, and a second precipitation with oxalic acid effected; this treatment is necessary to remove phosphoric acid completely.

Preliminary examination of the earth mixture.

—Before a method of separation can be decided upon, some knowledge of the composition of the mixture to be treated must be obtained. The nature of the mineral used for the extraction will, as a rule, afford useful information. It is known that in some minerals the cerium group, in others the yttrium group, predominates more or less completely; certain minerals, also, are known to be rich in elements of one or another subgroup. An approximate knowledge of the relative proportions of the cerium, terbium, and yttrium groups will be afforded by a rough double sulphate separation; thorium, zirconium, and scandium come down with the cerium earths. For approximate separation, Urbain[185] proposes the use of the ethylsulphates. The yttrium elements can be quickly separated in an approximate manner by fractional precipitation of the hydroxides with magnesia. The successive fractions obtained by these methods are examined spectroscopically; from the results, the composition of each, and so of the original mixture, may be roughly deduced.

The Spectrum Examination

In no department of chemistry have the methods of spectrum analysis proved of more value than in the field of the rare earths. They provide the chemist with a means of following and controlling his processes of separation which is far more delicate and decisive than the older method of determining the equivalent weight. Whilst the examination of emission spectra, and especially of arc spectra, is of decisive value in every case, it has the disadvantage of requiring delicate and complicated apparatus and great experimental skill; wherever possible, therefore, the examination of the absorption spectra is preferred, though this is useful only for a few of the elements, and varies considerably with the conditions employed.

The Absorption Spectra.

—Absorption in the visible region of the spectrum is observed only with those rare earth compounds which are coloured, and is of value, therefore, chiefly for identification in the case of praseodymium and neodymium among the cerium elements, and of erbium among the yttrium metals; these give characteristic absorption bands, even in dilute solution. The absorption spectra of the rare earth compounds are highly characteristic, the bands being well defined and sharply bounded, whereas coloured compounds of the common elements show general absorption, or at best diffuse bands, under the same conditions.

In observing an absorption spectrum, the light from a Nernst lamp, or incandescent burner, is passed through a layer of a suitable solution of the coloured compound, of known concentration and thickness, and after collimation is analysed by a suitable prism; the spectrum is observed by a telescope in the ordinary way. Where accurate readings are not required, as, for example, in testing for the presence or absence of a particular element, the position of the bands may be read to a sufficient degree of accuracy by means of a scale, the image of which is adjusted to coincide with the spectrum as seen through the eyepiece; but in mapping a spectrum accurately, more refined methods must of course be used. The photographic method, in which a photograph of the spectrum is taken on a plate which bears, for purposes of measurement, a comparison spectrum of known lines, is very convenient for examining the absorption in the violet and ultraviolet regions.

The intensity, and to some extent also the position, of bands in an absorption spectrum may vary considerably, according to the conditions employed. Of the various factors which must be considered, the concentration of the solution, the thickness of the layer used, the nature of the solvent, and of the acid radicle, and the presence of other earths are the most important. The concentration of the solution, and the thickness of the layer, which together constitute the Optical Density, must be so adjusted that the absorption is neither too strong nor too weak; in the first case the sharp bands tend to merge into broad diffusion areas, and details are obscured, whilst in the second case the presence of coloured compounds which do not show strong absorption bands may be overlooked.

The nature of the acid radicle has considerable influence on the position of the absorption maxima, the general rule being that the bands are shifted towards the red end of the spectrum as the molecular weight of the compound used increases. Naturally, also, the nature of the solvent has an important effect, all the usual phenomena which must be considered in the measurement of the physical properties of substances in solution coming into play; electrolytic dissociation, hydration, dissociation and the formation of complexes, for example, are all important factors. The presence of colourless earths has also been found to cause important differences. It follows, therefore, that for the chemist, the absorption spectra can be considered as a valuable aid only in detecting the presence or absence of the three elements which give the strongest and most characteristic absorption bands, viz. praseodymium, neodymium, and erbium, and that conclusions regarding the quantitative composition of mixtures must be drawn with the utmost caution.

The Emission Spectra: Spark Spectra.

—The factors which tend to limit the value of the absorption spectra for analytical purposes, for the most part disappear when the emission spectra are employed. In the case of the spark spectra, indeed, great differences are observed according to the conditions and method of experiment; but the arc spectra are practically invariable under all conditions, and hence they constitute the ultimate test in all cases. The spark spectra are observed when one terminal—the cathode—of an induction coil is embedded in the oxides to be examined, and the discharge then passed. The discharge is also frequently passed between platinum poles partly immersed in a strong solution of a salt of the element under examination; a form of apparatus very suitable for this method of observation has been described by Sir W. Crookes.[186] The spectra so obtained are in a high degree characteristic, but they vary very considerably with the form and dimensions of the coil, the length and cross-section of the wires, the potential difference employed, and so on. An entirely new spectrum also is obtained in many cases by mere reversal of the current; under these conditions, a phosphorescent appearance is observed, the spectrum of which—reversed spark spectrum of de Boisbaudran—has been found in many cases to resemble the cathode luminescence spectra of Crookes.