NEWS

THEORY OF ALLOTROPY : ALLO TROPES AND ALLOTROPOIDS.

By MAURICE COPISAROW.

THE term allotropy was first introduced by Berzelius (Fahresb., 1841, xx., II, 13) as denoting the appearance in several states, distinguished from each other by different properties, a definition lacking in precision, owing to the numerous possible interpretations of the "different properties." This vagueness is put forward as a guiding principle by Lothar Meyer (Watts' "Dictionary of Chemistry," 1888, i., 128), who considers that the notion of allotropy is not to be defined with perfect exactness. Allotropy is taken as a term covering the different physical states of matter, and also isomerism, polymerism, and polymorphism (Lehmann, "Zeit. Krystal. Min.," 1877, i., 97; Lowry, Trans. Faraday Soc., 1916, ii., 150), and in this sense might appear, perhaps, to be an unnecessary term without exact meaning in chemical nomenclature. This inclusion of different phenomena, along with Ostwald's and Nernst's definition of allotropy on the basis of energy changes, and Benedick's, Honda's, and Le Chatelier's conception based upon the discontinuity of forms, phases, and properties, is due to the consideration of effects of instead of causes underlying phenomena.

Whilst the close study of various properties of matter consequent to a certain phenomenon may help us in comprehending the phenomenon itself and its relation to other natural manifestations, it is wrong to define a phenomenon by its effects, especially when these effects are anything but specific or characteristic.

A far truer and more productive definition would be based upon the study of the causes of a phenomenon, the unmasking of which should naturally throw much light upon the phenomenon itself and its probable effects.

The possible causes of allotropy may be either-(1) a variation in the intra-molecular structure of elements; or (2) a change in the inter-molecular association or aggrega tion of elements.

Considering that

I. Molecular aggregation subject to the laws of crystallography accounts for polymorphism (Wyroubow, Bull. Soc. Min., 1906, xxvi., 335), and that

II. Allotropic modifications present in most cases con siderable chemical and physical differences-it would appear that allotropy is due to intramolecular differences (Koppel, Naturw. Rund., 1904, xix., 249, 261; Guthrie, Fourn. Roy. Soc. N.S. Wales, 1911, xiv., 318; Oxley, Trans. Faraday Soc., 1916, xi., 129).

We can not only regard allotropy as a function of valency, but also define it as the capacity of an element to exist in forms, differing in the mode of their intra-molecular linkage.

This conception is in full accord with the (1) hypothesis of dynamic allotropy (Smits, Proc. K. Akad. Wetens. Amsterdam, 1910 and onward), as on this basis only a simple space lattice, revealed by the reflection of X-rays from the internal planes of crystals, can be deduced, and (2) the view of different ions existing in solution in presence of two allotropes of an element (Holt, Journ. Soc. Chem. Ind., 1915, 693).

In this sense allotropy, although a function of valency, does not imply isomerism or polymerism, as all allotropes need not necessarily contain the same or a multiple number of atoms in their respective molecules The conception of allotropy as a function of valency ca

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be clearly understood on the basis of the electronic theory of the latter.

According to this theory the valency bonds are considered as having a definite direction. The relative direction of these bonds in the molecule may be influenced by the conditions under which such a molecule is formed. Variation in the direction would give rise to molecular structures differing in the distribution of the electric charges among the constituent atoms. These would be allotropes, and that configuration in which the charges were most symmetrically arranged or neutralised would be the stable form under these conditions (Holt, loc. cit.)

(This conception is closely related, if not identical, with the atomistic hypothesis according to which allotropy is due to changes in the electric and magnetic charges of an atom (Damas, Ann. Chim. Phys., 1831, [2], xlvii., 324 ; Howe, Rep. Brit. Assoc., Sheffield, 1910 p. 562), as intramolecular structures are primarily due to the properties and functions of the atoms).

Recognising allotropy as a function of valency and knowing the valency of an element (maximum in case of elements of variable valency), we should be able to indicate not only the possible constitutional formulæ of allotropes, but also the maximum possible number of allotropes for each individual element. (Without necessarily defining the number of atoms in the molecule, although the constitutional formulæ may in turn throw occasionally much light upon the complexity of the allotropes).

Attempting to represent elements by structural formulæ on the basis of their vale cies, we find that monovalent elements can exist in one and only one molecular form (disregarding the ionic or nascent state), divalent elements may have two distinct allotropic forms. (Should elements exist in monoatomic molecules the number of modifications will be augmented by one) :

(a) A molecular structure in which both valencies of the atoms are fixed, and

(b) A molecular structure in which some valencies are free.

In the case of trivalent elements two allotropes are possible, being

(a) A saturated molecular configuration, in which all valencies are fixed, and

(b) An unsaturated molecular configuration, in which some valencies are free.

In case of tetra-, penta-, and other polyvalent elements three allotropic forms are possible, being

(a) A rigid molecular form, in which all valencies are fixed;

(b) A rigid molecular form, in which some valencies are free; and

(c) A non-rigid molecular form, in which some valencies are free.

(On the basis of symmetry it is unlikely that a semirigid configuration, in which some atoms are rigidly fixed, whilst others retain the power of free rotation, should occur).

By molecular rigidity the author implies rigidity of atoms constituting the molecule, a rigidity caused by the fixation of the valencies. It appears that whilst an atom linked to two other atoms has the power of free rotation (the valencies being regarded as an axis), an atom linked to three or more atoms has no such power of rotation and can be regarded as rigidly fixed.

This gives the maximum number of distinct allotropes of an element under the most favourable conditions of temperature and pressure, forms possibly but not necessarily occurring in all physical states.

The examination of the chemical and physical properties of any element should enable us to determine the intra-molecular structure of each allotrope.

In trying to develop this hypothesis we must avoid a fallacy so general to many theories; ie, being one-sided

in regarding various phenomena either as identical or totally disconnected with one another.

Continuity, harmonious continuity, is the most striking feature of natural phenomena when viewed as a whole. The examination ef separate most advanced members of any group, series, or species shows discontinuity, but continuity becomes manifest as soon as an effort is made to survey any phenomenon in all its gradations.

This certainly applies to such manifestations as physical states of matter, polymorphism and allotropy. The physical states of matter in their crystalline form, solid and liquid crystals, connect physical states with polymorphism. Polymorphism is, on the other hand, closely connected in many instances with allotropy.

A scrutiny of the possible molecular forms of elements shows that theoretically it is possible for an element to have in certain cases more than one molecular form corresponding to each mode of linkage indicated above; that is, there may occur more than one form of an element with a non-rigid or rigid configuration. But regarding allotropes as the most chemically and physically distinct forms of an element it is obvious that several molecular forms containing a different number of atoms, but all having the power of free rotation, will differ among themselves to a less extent than when compared with a molecular structure of the same element, in which all atoms are rigidly fixed. Thus it follows that valency and the saturation or fixation of the atoms, and not the actual number of atoms, play the predominant part in the determination of allotropes. In this light allotropy becomes the capacity of an element to exist in forms differing in the mode of their intra-molecular linkage.

Molecular forms differing in the number of atoms or distribution of linkages, but all belonging to one and the same type of mode of linkage indicated above, can be termed allotropoids. These molecular forms serving as teh transition or link between polymorphism and allotropy proper can be compared with the cryptoisomeric substances (Pfeiffer, Ber., 1916, xlix., 2426; 1918, li., 554). Each modification of an element, which is an allotropoid in relation to forms belonging to the same mode of linkage is an allotrope, when viewed in connection with modifications belonging to a different type of linkage.

Allotropoids will differ among themselves less in physical and still less in chemical properties than allotropes of the same element.

The problein of discriminating between allotropy and polymorphism, or intra- and inter-molecular structures, comparatively easily solvable in the case of a few advanced characteristic instances, becomes therefore complicated and the available methods uncertain, not only owing to the inert character of some elements, which compels us to draw conclusions from their physical properties only, but also owing to the fact that in some cases we may deal with substances which are simultaneously either (1) allotropes and polymorphic forms, or (2) allotropoids and polymorphic forms.

Thus the physical data may indicate the combined effect of intra- and inter-molecular rearrangements in the element, and as the components, the combination of which gives us the total effect, need not necessarily be magnitades in the same direction, the physical data lose much even of their comparative value.

The knowledge of the maximum possible number of allotropes, serving as a guiding limitation, augmented by the chemical and physical properties of the modifications should be sufficient in most cases in identifying the allotropes of an element. In cases where owing to the inertness of an element we have to depend upon the physical factor only, as many physical properties as possible are to be taken into consideration, as no single feature or property is sufficiently characteristic as to serve as a decisive indication of allotropy.

The functional connection of physical constants with the periodic system of atomic weights and volume of elements make it very necessary, if any generalisations are to be

made, to have the experimental conditions for the determination of physical constants specific for every element with special relation to the absolute melting and transition points of every modification of an element.

Differences in crystalline symmetry are indefinite as a criterion or definition of allotropy owing to the possibility of substances belonging either to different crystalline systems or different classes of the same system. The observation that the higher the crystalline symmetry of an element the smaller is the number of atoms constituting its molecule (Relgers, Zeit. Phys. Chem., 1894, xiv., 1), and consequently the smaller is its tendency to allotropy (Barlow and Pope, Journ. Chem. Soc., 1906, lxxxix., 1741) is of an empirical character, and can hardly serve as a guiding principle in distinguishing between allotropic from polymorphic forms, this being specially so as crystalline structures may depend at least as much upon molecular aggregation as on the intra-molecular configuration of the atoms.

The sudden changes in and the discontinuity of properties can be accepted as an indication of allotropy, but with reserve, not only owing to the difficulty of establishing such discontinuity (Honda, Journ. Iron and Steel Inst., 1915,; Science Rep., Tokio, 1915, [IV.], No. 3, 261), but also owing to the fact that such changes accompany both allotropy and polymorphism, and if there is any difference it is in the degree not the kind of change.

The thermal changes so characteristic of the most pronounced allotropic transformations of the non metallic elements, though often very useful as an indication, are not completely to be relied upon, as all thermal measurements for metals give decidedly lower values than for nonmetals, and considerable changes of volume, hardness, porosity, electromotive forces, electrical resistance, specific heat, and thermoelectric power are not necessarily accompanied by great heat changes.

Considerable heat changes may be taken as a positive indication of allotropic rearrangement, but small beat changes are hardly safe as a negative proof, the more so as the heat evolution actually recorded may be the sum of two or more values or reactions, some exothermic and others endothermic.

As mentioned above, the allotropes of metals are more difficult to isolate and distinguish than those of the nonmetals owing to their inert character. But even in cases where no separate allotropic modifications were isolated, (1) the dependence of such physical properties as density, specific heat, expansion, electrical conductivity, and the melting, solidification, and transition points upon the thermal history of the specimen, and (2) the variation of the chemical behaviour with the past history of the metal -are best explained on the basis of the allotropy hypothesis.

The occlusion of hydrogen by palladium and palladium black, the etching of iron by nitric acid, and the solubility of carbon in a and y iron can be quoted as such extreme

cases.

The constitutional formulæ of allotropes developed on the basis of valency and the chemical and physical properties of each modification, although not actually solving the interesting problem of molecular complexity, offer us substantial aid in this respect, in so far as they often indicate the minimum molecular complexity satisfying the requirements of the formula..

HIS MAJESTY THE KING, on the recommendation of the President of the Board of Trade, has been pleased to award the Silver Medal for Gallantry in Saving Life at Sea to each member of the crew of the Danish s.s. Mary, which rescued Mr. Harry George Hawker and Commander Kenneth F. Mackenzie-Grieve, R. N., in the North Atlantic on May 19. The Board of Trade, with approval of His Majesty, have also awarded pieces of plate to the master of the ship and to the person in charge of the rescuing boat, and sums of money to the rest of the boat's crew.

CHEMICAL June 6, 1919

}

By EDWARD J. BRADY.

THIS note is a brief description of a precision pressure gauge of high sensibility and great accuracy recently constructed at this Laboratory.

In principle it is virtually a large diameter U-tube (6 cm. diam.) filled with some transparent non-volatile, non-oxidising liquid such as kerosene or paraffin oil. In each leg is placed a hollow brass float. The two floats are connected together with a fine silk thread which passes downwards around a light graduated wheel with jewel bearings immersed in oil at the bottom. The position of the wheel, which is provided with a vernier, is read through a glass window in one end of the box that forms the bottom of the gauge.

Laboratory workers having to do with the accurate measurement of small gas pressure, either plus or minus, are greatly in need of an instrument similar to the one to be described.

The way in which the differential pressure enters in the formulæ for both the Venturi meter and the Pitot tube may be represented by P=(f) Q2, where P is the pressure that must be read and Q is the quantity of material passed. It will be seen that both of the methods become in creasingly inaccurate at low velocity, because of the difficulty of measuring small pressures accurately. This gauge ought to increase the usefulness of both the Venturi and the Pitot tube.

A modification of the gauge with an opening to receive the horizontal velocity pressure of the wind might be used as an anemometer.

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the pressure whose small variation is to be measured. It is then withdrawn, when the telescope and scale can be used as before.

The glass wiodow may be made a section of a cylinder, so that all rays from the scale would pass normally through the glass, eliminating refraction effects, but this was not found necessary in practice.

The instrument as constructed has twin hollow brass floats and a brass wheel below. The bearings are jewelled, with very little friction. The friction may easily be made as near zero as is possible by making the wheel of aluminium and so proportioning it that the weight of the wheel in the oil used is just equal to the buoyancy of the two floats.

Even with the brass wheel having some weight on the bearings, the system has a very definite zero. The motion of the mirror for small displacements is almost critically damped.

The instrument may be dismantled by inserting two small wire hooks through the hose connections at the top to support the floats while the oil flows out at the bottom.

A gauge constructed as above described has been in use at this laboratory and has given entire satisfaction. When not in use a small piece of rubber tubing is connected from one inlet to the other to keep the dust out. With an ordinary U-gauge an accumulation of dirt invariably appears at the point where the menisci are read; with this gauge, however, the graduations on both the wheel and the vernier are immersed in the oil and always appear bright and clean.-Journal of the Franklin Institute, clxxxvii., No. 4.

ELECTROLYSIS OF SOLUTIONS OF THE RARE EARTHS.*

By L. M. DENNIS and B. J. LEMON.

The diameter of the wheel was so chosen that a pressure of one inch of distilled water exerted on a paraffin oil, such as kerosene, produces a motion of the periphery of the wheel of 10 divisions. By the aid of the vernier this precision can be increased.

These cylinders are made of glass, so that the gauge can be read approximately and from a distance like an ordinary U-gauge with the inch mark on the scale between the two cylinders.

Its use as a zero reading instrument capable of indicating pressure differences of less than 10-3 inches of water, will now be described.

Upon the little shaft that carries the graduated wheel there is fastened a small mirror, arrangeď to face toward the glass window in the lower position. About 4 feet in front of the gauge there is mounted a telescope and scale. This can only be used for comparatively small variations in pressure, say less than 2/10". However, the telescope and scale may be used to measure small differences at pressures of, say, 2"-3" and 4", by moving the mirror around on the shaft, so that it has the proper aspect when the gauge is subjected to the above pressures.

This is facilitated by a sma.. bent movable wire, projecting through a stuffing box in the side. It is pushed in sufficiently to interfere with the rotation of the mirror until the float assumes the position corresponding with

• Note from the Physical Laboratory of the United Gas Improvement Company, U.S.A.

Historical.

aqueous solutions of their salts. Edgar F. Smith (Ber., 1880, xiii., 751) stated that "didymium is not precipitated, either as nitrate or acetate, although a partial precipitation takes place at the positive pole." Brauner (Monatsh., 1882, iii., 1) electrolysed a solution of didymium acetate using platinum electrodes. There formed on the negative pole a pale red crystalline crust which contained didymium and acetic acid. Solutions of the nitrate and of the sulphate of didymium gave similar products. Brauner's object in these experiments was to ascertain whether a superoxide would appear on the positive pole. No such formation was noted, and he did not pursue the subject further. In a brief article entitled "Electrolysis of Solutions of the Rare Earths" (Zeit. Anorg. Chem., 1893, iii., 60) Krüss makes the following statement:-"A solution of a chloride of a rare earth behaves upon electrolysis like a solution_of_an_hydroxide in dilute hydrochloric acid; chlorine and bydrogen are set free at the electrodes, the solution loses more and more hydrochloric acid, and as the amount of the solvent diminishes, the hydroxide of the earth is precipitated in increasing amount. In this manner the rare earths can be removed from chloride solutions of mixtures of the earths, the amount thus precipi. tated depending upon the strength of the current and the duration of the electrolysis. It is to be expected that those bases which are the weakest toward hydrochloric acid will first be precipitated as hydroxides as soon as a part of the hydrochloric acid is decomposed by the electrolysis. The stronger bases will remain in solution as the more stable chlorides. In order to remove the hydrochloric acid uniformly from all parts of the solution

From the Journal of the American Chemical Society, xxxvii. No. 1.

CHEMICAL NEWS, June 6, 1319

that we owe also to the kindness of Dr. Miner. The earths of the yttrium group were obtained from the crude rare earth oxalates from xenotime; the mixture that was used in the experiments showed strong absorption bands of erbium, and rather faint bands of europium, samarium, and holmium. The didymium bands were very faint. Apparatus.-The first electrolysis was performed in a small crystallising dish and stationary electrodes of sheet platinum were used. On electrolysing a slightly acid solution of the nitrates of the didymium group, no precipitation appeared until the free nitric acid had been broken down by the current. As soon as this had been effected, however, a separation of the hydroxides of the earths appeared at the cathode, and the internal resistance of the cell rapidly increased. Upon interrupting the current and examining the contents of the cell it was found that a portion of the hydroxides had separated in flocculent form resembling aluminium hydroxide, and also that that part of the cathode which was immersed in the solution was heavily covered with a dense granular precipitate of the hydroxides. It was this coating of the

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surface of this mercury cathode was kept clean ty violently agitating it by jets of air, the air being conducted into the mercury through a glass tube with three outlets. The anode consisted of a piece of platinum wire 0.76 mm. in diameter. The source of current was a set of eight storage cells, an ammeter and variable resistance were placed in series with the cells and a voltmeter was connected in shunt with the cell. The aqueous solution of the rare earths that was to be subjected to electrolysis was placed in the cell upon the surface of the mercury. I. Separation of Lanthanum from the Other Earths of the Didymium Group.

A neutral solution of the nitrates of the rare earths neodymium, praseodymium, lanthanum, and samarium, that contained 50 grms. of the oxides of the earth in one litre of the solution, was placed upon the mercury cathode in the cell, the mercury was set in agitation by a blast of air, the current was turned on, and the voltage was gradually stepped up to a point where fairly rapid precipitation of the hydroxides took place. The voltage was

cathode surface that interfered with the passage of the current and caused marked fluctuations in the voltage.

It was thus evident that no definite voltage could be maintained for any considerable length of time unless the cathode surface could be kept free from deposit of the hydroxides. To attain this end, a rotating cathode of copper in the form of a wheel 15 cm. in diameter and 2.5 cm. in thickness was next employed. This was mounted on a horizontal shaft of the same material and was set in such a position that the wheel dipped into the solution of the nitrates of the earth to a depth of three centimetres. An anode of platinum was used. A copper scraper was fastened in such position that it rested against the edge of the cathode wheel, and it was hoped that by means of this arrangement the deposit of hydroxides upon the cathode might rapidly be removed, and the voltage in this manner be kept sufficiently constant to permit of fractional decomposition of the different nitrates in solution. The apparatus did not give satisfactory results, and after thorough trial it was abandoned, and a mercury cathode was employed. The mercury was placed in a glass cell about 12 cm. in diameter and 13 cm. high, the layer of mercury being about three centimetres deep. The

maintained constant at this value, 9 volts, throughout tle series of fractionations, and this voltage was used in the three fractional electrolyses described below. The first fractional electrolysis was of six hours' duration. The current was then turned off, the hydroxides were allowed to settle, the clear supernatant liquid was siphoned off, and the remaining solution was separated from the precipitate by filtration through a Büchner funnel. The filtration was then returned to the cell and was again electrolysed. In this manner eleven fractions were obtained. The hydroxides of each fraction were first thoroughly washed with hot water, were then dissolved in hydrochloric acid, and the small amount of mercury that they were found to contain was removed by double treatment with hydrogen sulphide. The rare earths in the solution were then precipitated from a slightly acid olution with oxalic acid, were thoroughly wasehd, and were then ignited to the oxides. The atomic weights of the rare earth elements were determined by igniting one portion of the oxalate to oxide and determining the oxalic acid in another portion by titration with potassium perThe details of this run manganate. are shown in Table I.