Popular Science Monthly/Volume 66/January 1905/Radioactivity and Matter
|RADIOACTIVITY AND MATTER.|
THE discovery of radioactivity has opened to physical and probably also to chemical research a field of extraordinary and peculiar promise. We seem to have a source of energy which flows spontaneously for unlimited time without tangible indication of its source; effects of energy are exhibited which neither in essence nor in phenomena resemble those previously known; substances are presented which seem to be of an entirely new kind, though they resemble our oldest and best-known elements so closely as to make their distinction difficult. The most prominent of these substances is radium, which, in the opinion of its discoverers, may be considered a new chemical element and which has been recognized as such by the International Atomic Weight Committee by giving it a place in their table of atomic weights for 1904.
Much more indefinite are the relations of the other radioactive substances thus far made known. The existence of polonium, discovered by P. and S. Curie, was for a time considered dubious, but seems to awake to new life in the radiotellurium of W. Marckwald. On radiolead voluminous researches have appeared which are especially noteworthy in chemical respects, still they do not yet authorize us to recognize a new element with certainty, even if we disregard the objections raised by F. Giesel against these researches. The same may be said of the actinium of A. Debierne and other radioactive substances, such as are supposed to have been detected in the earths of the cerium and yttrium group.
This uncertainty may be understood if we consider that for the researches thus far made only very small amounts of pure or merely enriched materials have been available, which has made the chemical investigation very difficult. The great fascination of tracing the observed phenomena of radioactivity by means of the sensitive photographic plate, the electroscope and the phosphorescent screen had quite naturally made the investigation of the chemical behavior of the substances in question a secondary matter which, while it has not been entirely overlooked, yet has not received that degree of attention that formerly was bestowed on the determination of the characteristic properties of newly discovered elements.
In the rather simple working up of the raw material containing radium, the treatment of barium has in the main been followed. Subsequently, the utilization of certain differences in solubility has permitted fractional crystallization and we have thus learned of a special spectrum of radium. But it is questionable whether the spectrum reaction can claim a decisive weight. For a substance, which in so minute proportions has been extracted from so complex a material as the residuals of Joachimsthal, the spectrum might readily prove deceptive. There seems to be very little correspondence between the spark-spectrum for radium obtained by Demarcay, and the flame spectrum described by F. Giesel, while the reddish-orange lines in the radium bromide spectrum, as well as the red coloration which this substance imparts to the flame, make it not impossible that some alkaline earths may have remained in the final residue examined. Besides, no radium compounds have been sufficiently investigated, nor have any chemical reactions been detected specifically peculiar to radium. The determination of its atomic weight has been decisive in recognizing radium as a distinct element, especially as this atomic weight was found to be exceptionally high.
Radium lead has been examined chemically more thoroughly, and yet no noteworthy differences have been discovered between its reactions and those of common lead. Only a great difference in atomic weight seemed to exist. The atomic weight of radiolead was determined by ascertaining the amount of sulphuric acid in its sulphate. It was found to contain 41.85 per cent, of SO4, while lead sulphate contains only 31.71 per cent.
Assuming that radiolead is quadrivalent like thorium and uranium, its atomic weight from the above analysis would be 260.2, that of lead only 206.9. But this determination seems of doubtful correctness, because the sulphate obtained has not been heated to a beginning redheat, but has only been dried to 400-420 degrees. At so low a temperature experience has shown it is impossible to remove all adherent sulphuric acid, so that it is not strange that a higher per cent, of sulphuric acid remained. The atomic weight of radiolead can not be deduced from such determinations. Also the supposed discovery of other elements (such as the much hunted for ekamanganese of Mendelejef) can not be established upon the result of such faulty determinations.
Indeed the chemical researches made can not be considered sufficient, however much we may admire the other researches made on the radioactivity of matter. Even the results obtained by fractioning can only be used with extreme caution, because they may lead easily to false conclusions. When the discovery of spectrum analysis had excited the scientific world as it is to-day excited by that of radioactivity, it was seriously supposed that calcium had been decomposed, because the green line of its spectrum had been made to disappear by fractioned precipitations of its chloride, and crystallographers on this ground tried to find reason for the extraordinary richness of the crystal forms of calcite. Similar pardonable errors may also occur in other cases, though we do not mean to assert that such errors have actually been committed in the fractioning of radioactive substances.
But we must be permitted to ask whether it is really justifiable to assume the presence of a new element in a substance for the sole reason that this substance is radioactive. It would certainly be surprising if every one of the numerous, long known chemical elements, which show radioactivity, were to contain also an elementary associate of almost exactly the same chemical character. Against such a supposition we can mention many reasons, entirely independent of the fact that such a supposition is in conflict with all general experience and the hitherto unshaken idea of the nature of chemical elements. Uranium—one of the elements in question—is certainly a typical element, yet, according to William Crookes, it can be separated into an active and a non-active fraction by merely treating its hydrous nitrate with ether, or by fractional crystallization or by fractional ignition—and at the same time these fractions are said to show no chemical differences whatever. Béla v. Lengyel who doubts the existence of radium, succeeded in making barium active by heating its nitrate with uranylnitrate, and Henry Becquerel prepared a strongly active barium sulphate by precipitating by sulphuric acid a barium chloride solution to winch uranyl chloride had previously been added; in this case the activity of the uranium compound had at the same time diminished. Further K. A. Hofmann and E. Strauss have obtained inactive fractions from active uranium compounds; on the other hand they succeeded in temporarily withdrawing from uranium its activity by means of barium or bismuth compounds. Especially remarkable are the experiments of A. Debierne, which have been conducted in a similar manner with actinium, and by which it was proved that it is easy to make active the compounds of barium so that they scarcely differ from those of radium, except that they do not give a radium spectrum. The active barium is said to stand between barium and radium, its anhydrous chloride is self-luminous like that of radium, and may equally well, by fractional crystallization, be separated in a more active and a less active portion.
By induction alone such remarkable phenomena can hardly be accounted for. The objection raised by F. Giesel against Béla v. Lengyel, namely that the activation of barium first produced by the latter might be due to some radium contained in the uranium salt used, is disproved by the fact stated, that under other conditions the same result even in a much higher degree was obtained.
And what are we to say of the supposed material emanation of radioactive 'elements' of their instability, of their decomposition by splitting off of helium and of the decomposition of the elements themselves? Until now there has not been furnished any demonstration whatever that elements of high atomic weight, to which gold and platinum certainly must be counted, are polymers of elements of lower atomic weights, and that they decompose into such. The idea of a chemical element in the old sense remains still unshaken, and it will require much more thorough chemical experimental research than those produced thus far, to disturb it. On the contrary, it appears from day to day more plainly, that radioactivity seems to be extremely widely distributed, and this observation leads us to the question whether radioactivity may not be simply a purely physical phenomenon, which may be exhibited by matter without in any way modifying its chemical nature, comparable to the magnetism of magnetic iron ore, which, like radioactivity, may be intensified, transferred, apparently destroyed and again called forth, and which, at the same time, also represents a mysterious manifestation of energy, without leading any one for a moment to imagine the existence of another element in magnetic ferro-ferric oxide not existing in the non-magnetic iron oxide. The idea of an elemental material difference was not thought of by any one, when Fr. Heusler succeeded in making magnetic alloys from unmagnetic metals such as manganese, tin, antimony and aluminum. Also in the case of radioactivity we should be free of material differences if the statement of F. Richarz and Rudolf Schenck should remain without valid objection, namely, that oxygen when ozonized becomes radioactive, so as to act upon the photographic plate, to make Sidot's blende (though only this kind of sphalerite) glow intensely and, like radium, to develop heat.
Considering the radium-craze now afflicting the world and affecting non-scientific circles especially, there is something humiliating to the chemist, after six years have elapsed since the discovery of radium, to say no more than that it resembles barium so closely that it can not be distinguished except by its higher atomic weight and its remarkable independent radiation. The chemical individuality of radium remains almost entirely unknown; nevertheless it is constantly sought, especially in those regions where pitchblende deposits occur, and where they already dream of developing radium ores and a future radium industry. So far as known, the occurrence of radioactive substances is tied to that of uranium. This is in the highest degree surprising and can not be understood if we are to assume the presence of special elements. Though the co-occurrence of certain elements is not uncommon and may be explained by their position in the system, their valence, the isomorphism of their compounds and other resemblances, we nowhere meet such an exclusiveness as here would be brought to view. An exception might possibly be made as to the thorium, the radioactivity of which, according to G. F. Barker, is independent of its co-occurrence with uranium. However K. A. Hofmann and F. Zerban contradict this opinion, and F. Zerban believes that he has detected a small amount of uranium in the monazite-sand supposed entirely free from it. The method he used is not quite unobjectionable; it was originally devised (by Laube) for the working of uranium residues and not for analytical purposes.
The material thus far used for obtaining radium and other radioactive substances has mainly been the residue left by the K. K. Austrian Uran works after extracting uranium from pitchblende. This residue remains after pitchblende has been roasted, ignited with soda and niter, and extracted with water and dilute sulphuric acid. It consists mainly of gangue, silica, ferric oxide, basic iron sulphate and lead sulphate, but contains also some bismuth and silver. It amounts to 40 per cent, of the uranium ore worked, and we may estimate that in the 50 years of their existence the uranium works have dumped 150 to 200 tons of this residue. It is most difficult to estimate the total amount of radioactive substances in this dump, but the amount of radium will count only by grams, that of radiotellurium only by fractions of a gram.
The residue referred to possesses 4.5-fold the activity of metallic uranium. According to S. Curie, 1,000 kilos of the residue yielded 10 to 20 kilos crude sulfates of 30 to 60 activity, and these again 8 kilos of barium chloride containing radium also of 60 activity. This seems to prove that the radioactivity does not keep pace with the concentration of the barium compounds, but lags far behind. A reliable measure of the radium-amount is not furnished. Even a preparation of 3,500 activity must have consisted mainly of barium chloride, since it yielded 140 for the atomic weight which is only slightly in excess of that of barium. As to the activity of the purest radium chloride, we find no statements, but it is said that the best radium preparations have an activity of 50,000 to 100,000 times that of uranium.
Now arises the question: in what form of combination is radium contained in pitchblende, and in the final solution of the residue? In the latter it is certainly as sulphate, probably also in the former, since heavy spar is most commonly associated with pitchblende. Since now the solubility of the sulphates of the metals of the alkaline earths diminishes with the increase of the atomic weight, radium sulphate ought to be the most insoluble of all in this group. From this and from the isomorphism of barium and radium compounds, we must conclude that the heavy spar associated with pitchblende really must carry the radium of the Joachimsthal ores. But William Crookes has been unable to find any radioactivity in any of the heavy spars examined by him, and even if he did not specially examine heavy spar from Joaehimsthal, the above result would be strange, since all other experience of the co-occurrence and mutual replacement of the elements would preclude that radium, in contradistinction to calcium and strontium, would be prevented from association with natural barium compounds, if it really were an element closely resembling barium. But if radioactive heavy spar occurs, its radium contents would pass into the barium chloride manufactured therefrom, and in that case it would not be difficult to separate it by fractional crystallizations on a large scale, however minute the radium contents might be. The experiment made by S. Curie with 50 kilos of commercial barium chloride has, however, given a negative result.
But even if radium for reasons at present unknown should exclusively follow the occurrence of uranium, it ought to occur in relatively wide diffusion at least in the Erzgebirge. For even outside of the deposits of pitchblende, such as those of Joachimsthal or of Johanngeorgenstadt, Schneeberg, Annaberg and Freiberg, one finds in the region specified frequently uranium-carrying rocks, especially granites, the uranium contents of which are indicated by an incrustation of calcium uranite acting on the sensitive plate and often visible in the crushed rocks used in the streets. It therefore appears not impossible to find in that region, where also bismuth is widely diffused, a new material which might successfully be worked for the extraction of radioactive substances. By producing radium in the pure state in larger quantities and by the thorough study of its chemical behavior we might possibly not only attain knowledge of the essence of radioactivity, but also place the existence of radioactive elements of individual character and definite chemical behavior beyond doubt.
- Translated from Berichte der Deutschen Chemischen Gesellschaft. The author, best known by this discovery of the rare element Germanium, died Oct. 8, 1904, at Dresden.—G. D. H.