12 4

4 7

The calculated percentages are those required for the formula AgC1,Cu, H2O19 + 9H,O, or for the formula AgC12Cu1H-018 + 10H2O on the assumption that the salt retains one molecular proportion of water of hydration after drying at 100°. At any higher temperature, the salt is decomposed and darkened in colour. It is evident, from the proportions of silver and copper found, that the preparation was not quite pure, but the figures, on the whole, seem to confirm the formula, and may be taken as indicating that the radicle of the cuprotartrates is either (C12Cu4H7O18)"" or (C12Cu4H9O19)"", so that of the water indicated in the formula for the potassium salt at least 4H2O are present as water of crystallisation.

The conclusion may thus be drawn, that the cuprotartrates are metallic salts of a definite trivalent radicle (C12H,Cu4O18)" or (C12H,Cu,O19)", and since none of the salts has been obtained with less hydrogen and oxygen than that indicated by the second formula without at the same time undergoing general decomposition, this may be accepted for the present, at any rate, as correct. In this case, the formula of the potassium salt, dried in a vacuum, and of the silver and lead salts dried at 100°, are respectively, K,C12H,CÙО,„,4H2O, Ag ̧C12H2CuО19, and Pbg(C12H9Cu4019)2

9

199

4

Instability of Cuprotartaric Acid.

Some attempts have been made to isolate cuprotartaric acid itself, but so far without success. When a solution of the potassium salt is titrated with a dilute mineral acid, the deep blue colour gradually gives place to the colour of an ordinary copper salt solution, and when this change is complete, or nearly so, a light blue precipitate slowly forms which has the appearance and characters of cupric tartrate. One sample, prepared in this way, was analysed and proved to be slightly impure cupric tartrate. Its formation may be represented by the equation :

KC12H,Cu4019 +5HCl = 3KCl + 3Cu¤ ̧Î ̧Î ̧+CuCl2 + H2O.

4 6

As the precipitate is produced slowly, it is possible that this decomposition is really preceded by the formation of a very unstable cuprotartaric acid, H.C12H,Cu4019, which then reacts with more acid.

3 12

It seemed possible that a solution of the free acid might be obtained without decomposition by suspending the lead salt in water and adding just sufficient dilute sulphuric acid to remove the lead as sulphate, but this also proved unsuccessful. The pale blue solution was proved to contain positive cupric ions, and no complex blue

negative ions, by electrolytic experiments similar to those described at the beginning of this paper.

Sulphuretted hydrogen solution decomposes all the cuprotartrates, precipitating cupric sulphide or a mixture of this with the sulphide of the other metal. The ease with which this occurs is a little surprising, as truly negative metallic radicles do not generally break up in this way. Indeed, in this reaction, the copper of the cuprotartrates shows in marked contrast to, say, the gold of the aurocyanides, yet the electro-negative character of the one is as certain as is that of the other. But it seems to be only the ion that can exist as such, whilst the un-ionised hydrogen cuprotartrate tends to undergo such transformation as to turn the copper back into an ordinary metallic radicle. If this is so, the action of sulphuretted hydrogen, even on the potassium salt, is explained, since a proportion of the hydrogen and cuprotartaric ions must naturally enter into combination, and more must unite to take the place of this when it is transformed.

Kahlenberg (loc. cit.) adduced arguments, based on physical properties, in favour of the view that the excess of alkali in Fehling's solution as usually made, or part of it, is present in the state of combination, and not merely mixed with the blue salt. We have obtained chemical evidence in proof of the truth of this contention, as the addition of alcohol to a solution made from caustic soda and cupric tartrate in the ratio 2NaOH : CuCH40 throws out a strongly alkaline salt which retains this character even after the alcoholic washings have become quite neutral, and contains sodium and copper in the ratio Na: Cu 7:4. We intend to continue this investigation, and in the meantime abstain from any discussion of the probable or possible inner constitution of the negative radicle of the cuprotartrates.

THE UNIVERSITY OF MELBOurne.

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LXX. The Allotropic Modifications of Phosphorus. By DAVID LEONARD CHAPMAN, B.A.

It is generally believed that there are at least three distinct allotropic modifications of phosphorus, namely, the transparent, red, and metallic varieties. The transparent variety may be converted into the red most conveniently by the action of heat; this change takes place with tolerable rapidity, at a temperature of from 240° to 250°, but still more readily at a higher temperature or if traces of certain impurities, such as iodine, are present. The same change may be effected by the

action of sunlight at ordinary temperatures. Pedler (Trans., 1890, 57, 599) has shown that amorphous phosphorus is formed by the action of tropical sunlight, even on a solution of ordinary phosphorus in carbon bisulphide, and that when obtained in this way it is a yellow powder changing gradually to red. Amorphous phosphorus is also sometimes formed at the ordinary temperature in certain chemical reactions; it may be obtained, for instance, by the action of oxalic acid on phosphorus trichloride as a yellow powder, which, on exposure to the air for some time, becomes red. Metallic phosphorus, the supposed third variety, was first obtained by Hittorf (Pogg. Ann., 1865, 126, 193), who heated lead and phosphorus in a sealed tube at a temperature approaching a red heat for 10 hours, and afterwards removed the lead with nitric acid of specific gravity 11. Pedler believes that this third variety is only red phosphorus, but he has left unanswered certain very strong evidence derived from the vapour tension and supporting the view that there are three varieties. Brereton Baker (Phil. Trans., 1888, 179, 571) has shown that vapour is emitted from red phosphorus at a temperature approaching that of boiling mercury, and that below this temperature the phosphorus is unaltered; this was done by introducing one end of an exhausted tube containing red phosphorus into the vapour of boiling mercury, a small quantity of ordinary phosphorus condensing at the cool end of the tube. Pedler has noticed the same fact. The temperature at which metallic phosphorus begins to be converted into vapour is also that of boiling mercury, so that in this particular the two varieties resemble one another.

Evidence adduced from Appearance.-Pedler has examined commercial red phosphorus, the yellow variety obtained by the action of light on a solution in carbon bisulphide, and metallic phosphorus under the microscope, and from the facts that they all consist of transparent particles of varying size, that the colour which they present to the naked eye depends on the size of the particles, and that the action of all the varieties on polarised light is the same, he concludes that red and metallic phosphorus are identical, a conclusion which is supported by the following experiments.

Preparation of Metallic Phosphorus.--This substance is formed by dissolving phosphorus in lead at a high temperature, and, according to Hittorf (loc. cit.), separates in microscopic rhombohedra in the lead on cooling. Pedler states that it may be formed more easily by projecting amorphous phosphorus into lead at a somewhat high temperature. Both methods are open to the objection that the amorphous phosphorus may only have mixed with the lead, and this may account for the similarity of properties observed by Pedler in red and metallic phosphorus. In order to obviate the objection, an experi

ment was arranged in such a manner that the lead only came into contact with the vapour of phosphorus. If a substance and its vapour are soluble in a common solvent, both are equally soluble, so that if metallic phosphorus is formed by Hittorf's method, it must also be formed when the vapour alone comes into contact with the lead. A hard glass tube was drawn out as in the diagram. The compartment A contained pure lead, and a plug of glass wool was rammed tightly down against the constriction C. Amorphous phosphorus was introduced into the compartment B, and the tube, after being exhausted and sealed, was placed in an iron tube and heated to dull redness in an inclined position. After cooling, the lead was removed by nitric acid of specific gravity 1·1, and a black powder remained behind; this, when examined on a slide under the microscope, was found to consist of black, opaque masses, which, if lightly pressed against the slide,

separate into exceedingly minute, regular, transparent particles, all of approximately the same size, and so small that it is impossible to assert with confidence what their exact shape is; they have, however, two pairs of parallel edges, and are probably cubes or rhombohedra.

Hittorf states that metallic phosphorus may also be formed by heating amorphous phosphorus at the temperature of boiling phosphorus pentasulphide and condensing the vapour at the temperature of boiling sulphur. I performed a similar experiment. An exhausted tube containing red phosphorus was placed in the vapour of boiling phosphorus pentasulphide. With any liquid of high boiling point, the vapour at the bottom of the vessel is always at a higher temperature than that at the top, unless special precautions are taken, and so in my experiment, after several hours heating, amorphous phosphorus

was deposited at the top of the tube in a yellowish-red layer. The tube was opened under carbon bisulphide to dissolve the ordinary phosphorus, and the layer of red phosphorus on the glass, when examined under the microscope, was found to consist of crystals of the same size and shape as those formed by crystallisation from lead.

Red phosphorus obtained by the action of sunlight, after purification from ordinary phosphorus by carbon bisulphide, and the yellow variety obtained by the action of phosphorus trichloride on oxalic acid, were also examined under the microscope, and were found to consist of similar microscopic particles. The yellow colour of the variety obtained from phosphorus trichloride and oxalic acid is due to the more complete separation of the crystals; these, after exposure to moist air, stick together, and the yellow colour changes to red. In the cases which I have examined, the darker colour of some specimens of red phosphorus was always due to the aggregation of the microscopic rhombohedra, and not to the varying sizes of the crystals.

Commercial red phosphorus consists of irregular masses of varying sizes. When treated with caustic soda or carbon bisulphide, or heated in a vacuum, no alteration in appearance is noticeable. If, however, it is boiled with caustic soda and then with hydrochloric acid, a fine powder rises to the surface of the acid, and, when examined under the microscope, presents the same appearance as the variety known as metallic phosphorus.

The crystalline form of metallic and of red phosphorus is therefore the same. The crystals of red phosphorus, however obtained, are very uniform in size; commercial red phosphorus consists of these microscopic crystals so firmly cemented together by the action of moist air that they form brittle blocks with a metallic lustre, the crystals being probably held together by an oxide or acid of phosphorus. Ordinary phosphorus is not present, as vapour is not given off in a vacuum until a temperature of 358° is reached.

Evidence for the Existence of Two Distinct Varieties of Red Phosphorus.

Hittorf (loc. cit.) has shown that, when amorphous phosphorus is heated for 8 hours in a sealed tube at the temperature of boiling sulphur, the vapour tension at first rises and then falls, and that during the operation the specific gravity of the phosphorus increases. This he explains by assuming that there are two distinct forms of red phosphorus, and that the change from one variety to the other takes place gradually at the temperature of boiling sulphur.

Lemoine (Compt. rend., 1871, 73, 797) examined this point also, and found that when white phosphorus is heated in a closed glass bulb in the vapour of boiling sulphur it is first converted into red, and then