Feb. 25, 1910 1

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Ir is known that among the new and strongly radio-active bodies polonium was the first to be discovered. Many efforts have been made already to isolate this substance and to characterise it as a chemical element, but in spite of the great activity of the product obtained this result has not yet been achieved. According to the theory of radio-active transformations the quantity of polonium present in radio-active minerals must be very small. According to this theory polonium is looked upon as a descendant of radium, and the relative proportion of these substances in radio-active equilibrium is equal to the ratio of their mean lives. The mean life of radium being about 5300 times greater than that of polonium, and radium being found in pitchblende in about the proportion of o2 grm. per ton, it is seen that the same mineral cannot contain more than about 0.04 mgrm. of polonium per ton. Many problems of supreme importance in radio-activity are connected with the isolation of polonium. This body is an unstable element which apparently represents the last radio-active term in the series derived from radium; we may therefore hope to show the formation of an inactive element derived from polonium. Moreover, polonium giving rise to an emission of a-rays should produce helium, and this production not yet having been observed, it is important to ascertain if there really is in this a fact incompatible with theory.

We have undertaken recently a chemical research with a view of preparing polonium in a concentrated state. This was performed on several tons of residues from the uranium mineral which were at our disposal for this purpose. The mineral was treated with warm strong hydrochloric acid, which has the effect of dissolving polonium almost completely. The solution, which contains no radium, was submitted in a factory to operations having for their object the extraction of its active matter. This treatment, which was done under our direction and which will be described in a more extended memoir, furnished about 200 grms. of a substance having a mean activity about 3500 times that of uranium, and which contained chiefly copper, bismuth, uranium, lead, and arsenic; its activity was due to polonium. We sought to purify this material by treatment in our laboratory.

The

For this purpose the hydrochloric solution was precipitated with ammonia, to remove copper, and the precipitated hydrates were boiled in a solution of soda to dissolve the lead; they were mixed next with a warm solution of ammonium carbonate to dissolve the uranium. All these operations were repeated several times. insoluble carbonates finally obtained were dissolved in hydrochloric acid, and the solution was precipitated with stannous chloride. These operations together were very successful, the original activity being found in the final precipitate in a sufficiently complete manner, which we verified by appropriate weighings.

The precipitate, which weighed about 1 grm., was redissolved, and the hydrochloric solution was precipitated by sulphuretted hydrogen. The sulphides were washed with sodium sulphide, then re-dissolved, and the solution was re-precipitated with stannous chloride; the resulting precipitate weighed a few mgrms. Spectrum analysis, effected on this material, showed the presence of a diversity of elements-mercury, silver, tin, gold, palladium, rhodium, platinum, lead, zinc, barium, calcium, and aluminium; some of these elements being derived from the vessels

85

employed. On attempting to purify the active matter we very difficult to obtain without loss a substance of a more simple composition. Thus, on trying to separate lead by treatment with potash we found that a large part of the polonium passed into solution, although we were able to utilise without danger the same reaction in the presence of elements insoluble under these conditions. From this alkaline solution, polonium can only be re-precipitated by the addition of an alkaline sulphide. The reactions which we always found to be trustworthy are:-Precipitation as sulphide from an acid or alkaline solution, and precipitation with stannous chloride. We also have found that polonium is easily deposited by electrolysis, and this method may be utilised for a quantitative separation when we wish to extract polonium from an acid solution; but at the same time other metals, such as gold, platinum, mercury, &c., are deposited. After many experiments the activity was concentrated in about 2 mgrms. of matter.

The activity was measured by an electric method. For this purpose a known and very small portion of the solution was evaporated on a thin plate of glass, and the absolute value of the saturation current obtained with this plate was measured in an appropriate apparatus. Knowing the value of the charge of a gaseous ion (4'7 x 10-10 E.S. units) and the number of ions produced by an a-particle of polonium along its complete path (about 16x 103), we can calculate the number of a particles emitted per second. Knowing, moreover, the speed of decay of polonium (a reduction to one-half in 140 days) and the number of molecules contained in a molecule-grm. (about 6 x 1023), we can calculate the amount of polonium present, its atomic weight being supposed to be near 200. We also can calculate the amount of helium which is formed in a given time, assuming that each a-particle is an atom of helium.

In this way we found that the quantity of polonium obtained would be about o' mgrm.; this quantity is what ought to be found, according to theory, in 2 tons of good pitchblende. Our active matter therefore might contain several per cents of polonium, so that spectrum analysis might be attempted with some chance of success. Many spark spectra were obtained and photographed; unfortunately each of these operations involved a considerable loss

of material.

The appearance of the spectrum is complicated; many elements being present, gold, platinum, mercury, palladium, rhodium, and iridium. Some spectra also showed the presence of alkaline earthy metals, which came probably from the attack of the vessels employed; these were removed by electrolysis. After a careful examination of the different spectra and the identification as complete as possible of known lines by their wave-lengths (records of Exner and Haschek, Watts), or by comparison spectra obtained with the same spectrograph, some lines were left which might be attributed reasonably to polonium. The wave-lengths of these lines are:

definite opinion on the attribution of the lines indicated above. We also may hope to see the spectrum of the element formed at the expense of polonium. According to theory this element should be lead; lead is not entirely absent from our product, but its spectrum is very faint. We have observed that the active substance obtained does not give rise to induced radio-activity, nor to any appreciable emission of penetrating rays. We have observed also an extremely minute disengagement of radium emanation.

A portion of the solution was utilised for the study of the gas disengaged. The solution was introduced into a quartz tube, which itself was placed in an apparatus which could be completely freed from air. The solution disengages much gas; it is easy to observe the continuous formation of gas bubbles, proving the decomposition of water; this decomposition must be attributed to the action of the a-rays of polonium. The gases disengaged are almost totally absorbed by the action of heated copper and oxide of copper, of potash and phosphoric anhydride. The slight gaseous residue was collected and examined by one of us by the method formerly used for the examination of the gases disengaged by actinium and radium (Debierne, Comptes Rendus, 1905, 1909). This residue is sensibly pure helium; its complete spectrum was observed, and the volume measured. The volume was equal to 1.3 cubic mm. at atmospheric pressure, the accumulation having been going on for 100 days. This volume is very near that predicted by theory, which is 1.6 cubic mm. The fact of the production of helium from polonium is therefore established, with the predicted order of magnitude. We propose to make as accurate as possible a determination of this volume, together with experiments on the numeration of the a-particles emitted, so as to obtain the value of the

NEWS

the charcoal, exhausting the system by an air pump, sealing the latter off, and then cooling the charcoal in liquid air. This having been done, the liquid air round the carbon disulphide was replaced by solid carbon dioxide at 78° C.; in these circumstances the carbon disulphide gave off a steady stream of vapour at a tension of about I mm., which was sucked through the discharge and U-tubes at the rate of some 600 ft. a second into the charcoal, by which it was absorbed. When the U-tube was immersed in liquid air, a solid snow of carbon disulphide was deposited in it, about 1 cm. above the level of the liquid air, and nearer to that level a clearly-marked brownish ring gradually made its appearance. When the liquid air was removed from around the U-tube, so that the temperature was permitted to rise, there was an explosion, with production of light, the effect being sometimes sufficiently localised to break the tube, and a dark substance like the polymerised carbon disulphide, described on a previous occasion, was left behind. A very small rise in temperature was sufficient to determine this explosion, which occurred about five seconds after the liquid air had been removed. To show the instability of the new body, a large vessel was placed between the discharge tube and the U-tube, and when this had been filled with the dissociation products, a small portion of its outside surface was cooled by means of a sponge of liquid air. In this way the carbon disulphide and other products were condensed and on evaporation there remained a brown deposit of the polymerised body. If, however, a minute was allowed to elapse before the application of the liquid air no brown deposit was obtainable.

In the course of our experiments a curious effect of the rays was observed. The polonium was kept dry in a small quartz capsule. This capsule was found to be cracked in a large number of places under the substance. The production of these cracks may be attributed to electric discharges.

An abundant disengagement of ozone was noticed generally in the neighbourhood of the substance. - Comptes Rendus, cl., pp. 386-389, February 14, 1910.

THE CHANGE OF CARBON DISULPHIDE INTO A GASEOUS PRODUCT CONDENSABLE AND EXPLOSIVE ABOUT THE TEMPERATURE OF LIQUID AIR.* By Sir JAMES DEWAR, F.R.S., and Dr. H. O. JONES.

THE authors pointed out that, just as the dissociation of carbon dioxide, at a high temperature or under the influence of the electric discharge in a vacuum tube, yielded a mixture in equilibrium of CO, O, and CO2, so carbon disulphide, subjected at a low temperature and slight tension to the silent electric discharge, might be expected to dissociate into a mixture of CS, S, and CS2. A necessary condition of the experiment, however, would be the rapid removal of the products formed. To this end a bulb containing about 5 cc. of carbon disulphide was connected to an electrodeless discharge tube, which in turn was sealed on to a U-tube and a charcoal receiver. The carbon disulphide was then frozen solid by immersing the bulb in liquid air; in that state it had practically no tension, and it was possible to free the apparatus completely from moisture and extraneous gases by heating

* A Paper read before the Royal Society, February 10, 1910.

By Sir EDWARD THORPE, C.B., LL.D., F.R.S., and

ARTHUR GORDON FRANCIS, B.Sc., F.I.C.

IN the Bakerian Lecture for 1907, "On the Atomic Weight of Radium (Proc. Roy. Soc., A, lxxx., 298), one of us described a rapid and effective method of separating the clear supernatant solution above a precipitate, say, of silver chloride or silver bromide, which obviated the necessity of

employing any of the ordinary methods of filtration and otherwise greatly simplified the manipulative process. As the contrivance was found convenient in practice and seemed to admit of a high degree of accuracy, we have thought it worth while to make use of it in a re-determination of the atomic weight of strontium which should seek to conform to the standard of precision prescribed by modern procedure in atomic weight estimations.

Determinations of the atomic weight of strontium have been made by the following: :

1816 Stromeyer (Schweig Journ., xix., 228; also Meyer and K. Seubert's "Atomgewichte," p. 123). Rose (Pogg. Ann., viii., 189).

1816

1843

1845

1858

1859

1894

1895 1905

Salvétat (Comptes Rendus, xvii., 318).

Pélouze (Comptes Rendus, xx., 1047; and Liebig's Ann., 1845, lvi., 204).

Marignac (Liebig's Ann., cvi., 168; and N. Arch.

Phys. Nat., i., 209).

Dumas (Ann. Chim. Phys., 1859, lv., 191; and Liebig's Ann., cxiii., 34).

Richards (Proc. Amer. Acad., xxx., 369; xl., 603).

With the exception of those of Richards these estimations have no claim to great accuracy, and are therefore only of historical interest.

Richards's final values as re-calculated by us when Ag= 107.880, Cl=35'460, and Br=79'916 are as follows:

* A Paper read before the Royal Society, January 13, 1910.

Mean.

II. 87.627 87.629

III.

87.631

..

I. II.

The mean of the whole is Sr = 87.62, which is the value adopted in the last Report (1910) of the International Committee on Atomic Weights.

We have sought to establish the atomic weight of strontium by means of the ratios :

I. 2Ag: SrBr2. II. 2AgBr SrBr2. III. 2Ag: SrCl2.

IV. 2AgCl SrCl2.

Ratio IV. has already been used by Rose; Ratio III. by Pélouze, Marignac, Dumas, and Richards; Ratios I. and II. by Richards alone.

Preparation and Purification of Materials.

87

"heads" and "tails were taken off from alternate series. As the strontium salt is readily soluble, it is very necessary to free each crop of crystals from the mother-liquor as far as practicable, and this was effected in a small centrifugal apparatus. After 30 series, involving 160 re-crystallisations, about 107 grms. of purified material was obtained. This was still further purified by fractional precipitation from aqueous solution by means of pure hydrochloric acid, and the crystals so obtained were dried centrifugally as before. The process was repeated eight times, and four portions were eventually obtained. These were separately dissolved in water, filtered under cover, and the solutions mixed with 1 cc. of pure hydrochloric acid, and evaporated to dryness under a glass shade. The mass was then dried at 200° in an oven heated by an alcohol flame. Fractions II. and III. were employed for the determinations. Strontium Bromide.-500 grms. of commercially pure strontium bromide was systematically re-crystallised and "centrifuged" in precisely the same manner as the chloride. In all there were 39 series involving 209 crystallisations. After the 21st, 27th, 33rd, and 39th series the crystals of the middle fractions were "whirled " until dry, dissolved in water, the solutions filtered into a platinum vessel, and evaporated to dryness under a glass shade as before. The four portions were dried at 160° to 170° in an oven heated by an alcohol flame. The fraction obtained from the 39th series was assumed to be the purest, and it was preferred therefore for the determinations.

The purified and dried fractions of the two salts were

The silver employed was prepared by reducing silver chloride (obtained from the purest commercial silver nitrate) by a boiling solution of potash and lactose, repeatedly washing the metallic silver with dilute aqueous ammonia and hot water, and fusing it when dry, in small quantities at a time, on a block of lime, the buttons being allowed to solidify in a reducing flame. The buttons, which were of varying size, were scrubbed with a clean stiff brush and shaken together in a bottle with successive portions of dilute nitric acid to remove any trace of adhering lime, and afterwards etched with nitric acid of specific gravity 1.2. After being again washed with pure water and dried, they were sorted into three sizes for convenience of weighing. Some of the larger buttons were rolled between silver rollers into thin foil, which was again washed and etched. The silver was preserved in bottles kept in a desiccator containing fused potash.

As regards its purity, we have assumed that it was of the same character as that employed by one of us in connection with the determination of the atomic weights of gold (Fourn. Chem. Soc., 1887, li., 569), and which had been prepared in precisely the same manner. This silver was estimated by the late Chief Assayer of the Mint, Sir William Roberts-Austen, after a series of very careful trials, to contain o'99994 grm. of real silver per grm. Exactly the same value is obtained from our present determinations if we compare the aggregate amount of silver chloride formed from the known weight of the silver used with the amounts calculated from Ag 107.88 and Cl 35:46, which are considered by the International Committee as the most probable values of these atomic weights.

Preparation of the Strontium Salts.

Strontium Chloride.-About 500 grms. of commercially pure crystallised strontium chloride, obtained from Kahlbaum, was subjected to a systematic re-crystallisation from water, on the same principle as that adopted in the isolation of radium chloride (Bakerian Lecture, 1907, loc. cit.). The series consisted of five portions each, and

phosphoric oxide until wanted for use.

The specific gravities of the fused salts, obtained_by displacement of paraffin oil, were:-SrCl2, 3'051; SrBr2, 4'201. Richards (loc. cit.) obtained 4:216 for the bromide, and adopted Schröder's value of 3.05 for the chloride.

A special laboratory was set apart for the determinative work, and this had attached to it a dark room in which the manipulation of the silver halides was done. The balance was a new 10-cm. short beam instrument, fitted with stirrup pans, made by Oertling, and was sensitive at its maximum load to 1/50th mgrm. To check tremor it was mounted in the determinations of the atomic weight of radium. The on packets of filter-paper in the same manner as that used weights of gilded brass, also by Oertling, were carefully standardised, the sensibility of the balance being determined for varying loads up to 100 grms. The object to be weighed was invariably tared by a precisely similar vessel of as far as possible the same weight and dimensions, and both vessels were uniformly treated in exactly the same way. The weighings were made by vibrations, observed through a lens, the zero-point being ascertained before and The corrections added to each grm. after each weighing.

of the substance weighed were:

Silver (sp. gr. 10.5)
Silver chloride (sp. gr. 5'59)
Silver bromide (sp. gr. 6'47)
Strontium chloride (sp. gr. 3'05)
Strontium bromide (sp. gr. 4°2)

-0.000031 +0'000070

+0'000041

+0'000249

+0.000141

Guye and Zachariades (Comptes Rendus, 1909, cxlix., 593) have recently drawn attention to the fact already pointed out by Marignac in 1843, that if the substances weighed are in the state of fine powder, the ordinary method of reducing weights to a vacuum gives a correction in excess of the truth. As the materials actually weighed in our determinations were invariably fused, and were usually in compact masses, we think that the error due to this circumstance may be considered as practically negligible.

Determination of the Ratios SrBr2: Ag2 and SrBr2: 2AgBr, An unweighed quantity of the purified and dried strontium bromide, about 2 grms. in amount, was placed in a platinum boat contained in a silica tube provided with ground-in connections, and after displacing the air with dry

nitrogen, gradually heated, first in the stream of nitrogen and then in a stream of dry hydrogen bromide, until the bromide fused. The fused salt was allowed to solidify in a current of dry nitrogen, which was then displaced by dry air, and the bromide transferred whilst still warm, under conditions which precluded the absorption of atmospheric moisture, to a tared stoppered 50 cc. flask, and the exact weight of the anhydrous strontium bromide ascertained with all known precautions. The strontium bromide was next dissolved in pure recently boiled water, the solution acidulated with nitric acid, and a determinate quantity of silver, usually about 1 to 2 mgrms. in excess, dissolved in nitric acid, was added to it under non-actinic light, and the excess of silver then ascertained by titration by the Gay

solution of pure nitric acid of known strength, the bulbs containing water were placed in position, and the solution of the metal started and maintained by a gentle heat. The clear solution of the silver nitrate was then introduced drop by drop into the solution of the strontium bromide, kept in gentle rotation, through the tap, lubricated with pure water, and then the bulb-tube was turned on its axis so that its contents were emptied into the burette, and the wash-water was transferred to the strontium solution. whole apparatus was then repeatedly rinsed with successive small quantities of water, and the washings delivered by means of the tap into the solution of the strontium salt. After being well shaken the flask was allowed to stand in the dark room for about eighteen hours before the titration

The

&

Lussac method using a solution of strontium bromide of known strength. After the titration was finished, the clear supernatant liquid was removed by the method already referred to, and the precipitate of silver bromide washed, dried, fused, and weighed. As regards details, the hydrogen bromide was made by dropping concentrated hydrobromic acid solution on a mixture of sand, red phosphorus, and phosphoric oxide, free from phosphorous oxide, and was further dried by passing through tubes containing a mixture of red phosphorus and pure phosphoric oxide. The object of adding sand to the mixture was to moderate the violence of the production of the hydrogen bromide.

The fused strontium bromide was invariably colourless, and its solution in water was perfectly clear, and was neutral to litmus and phenolphthalein.

The weighed quantity of silver was brought into the burette seen in the figure together with a slight excess of a

was proceeded with. tions were used.

For the final adjustment two solu

1. A solution of strontium bromide, made by dissolving 0.2640 grm. of the pure fused salt in 500 cc. of water at 16.5°; I cc. of this solution contained therefore o'000528 grm. SrBr2.

2. A solution of silver nitrate made by dissolving o'2338 grm. of silver in a slight excess of nitric acid and diluting the liquid to 500 cc. at 16.5°; I cc. of this solution contained o'0004676 grm. Ag. Hence, the solutions were very nearly equivalent. The solutions as used were delivered from small (5 cc.) burettes carefully calibrated.

The excess of silver (1 to 2 mgrms.) in the strontium solution was now ascertained in the usual manner, the operations being carried out in non-actinic light in the dark room. As the end-point was reached care was taken to allow ample time for the appearance of the last traces of