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Science Tribune - Article - December 1996

http://www.tribunes.com/tribune/art96/ojas2.htm

Did Becquerel discover radioactivity ?



Tiiu Ojasoo

E-mail : ojasoo@tribunes.com


1996 is - we are told - the centenary year of the discovery of radioactivity by the French Nobel prize-winner, Henri Becquerel (a), and there have been exhibitions, conferences, and articles to honour his memory and achievements. (For more information)

A recent article by P. Jössang (1) has hinted that the inventors of chromatography, who also received the Nobel prize, may have had two less well-known forerunners. They are Tsvett, whose contribution to the beginnings of chromatography at the turn of this century has been acknowledged by historians in the field, and, 50 years earlier, in 1861, a certain Friedrich Goppelsröder who performed an enormous amount of work developing and using chromatography but whose efforts were hardly noticed. Science, it seems, may be perpetual re-discovery !


How Becquerel discovered radioactivity

Becquerel's fame originates in two presentations he made in February and March of 1896 before the French National Academy of Sciences. He reported that a uranium salt which had been exposed to sunlight and placed on a photographic plate wrapped in dark paper had blackened the plate. Wishing to repeat the experiment, but the weather in Paris being too cloudy, he placed his experimental material - uranium crystal and unused photographic plate - in a drawer only to discover a few days later, to his great surprise, that the plate had blackened without prior exposure of the crystal to sunlight.

Is this the full story ?

Recently, a book on the evolution of matter by Gustave Le Bon (b) (2) was brought to my attention. Chapter XIV deals with the 'documents relatifs à l'histoire de la découverte de la dissociation universelle de la matière' (Documents relating to the history of the discovery of the universal dissociation of matter) and here, with some additional data and a few comments, is its main thrust.


What was the current state-of-the-art ?

G. Le Bon points out that to assess the work of someone doing research, one has to evaluate the state-of the-art before and after the question is addressed. His appreciation of the state-of-the-art at the time is as follows.

1 - In 1867, Niepce de Saint-Victor (c) showed that uranium salts could emit radiations for several months in the dark although this demonstration was met by opposition and with indifference at the time and cursorily rejected by Becquerel : 'Niepce n'a pu observer le rayonnement de l'urane parce que l'auteur employait des plaques trop peu sensibles.' (Niepce could not have observed the radiation from uranium because the author used plates that were not sensitive enough).

2 - Roentgen had discovered X-rays in 1895 and it was a presentation of this discovery by Poincaré before the French Academy of Sciences that had inspired Becquerel to study the relationship between fluorescence and X-rays. He decided to study the fluorescence of uranium salts. (Le Bon does not mention this point).

3 - Uranium was thought to emit a kind of invisible light, a thesis that had been put forth by Niepce de Saint Victor, that was upheld by Becquerel, and was to be disproved by Le Bon in 1897 (3).

4 - It was not yet known, as demonstrated by Le Bon in 1897 (3), that metals exposed to light could emit radiations identical to 'uranic' and cathodic rays. On the other hand, it was common knowledge that metals subjected to an electrical current lost their electric charge under the influence of light. Lénard thought that the metal surface turned into dust carrying electrical charges. When informed of Le Bon's results, he pursued the experiments and deflected the cathodic rays with a magnet (4). These experiments were later confirmed by J-J Thompson.

5 - The radioactive emissions were thought to be of an exceptional character and limited to very few compounds. Le Bon thought that they were very widespread in nature ("les propriétés de l'uranium ne seraient donc qu'un cas particulier d'une loi très générale" (5) : the properties of uranium are just a specific case of a very general law) occurring not only under the influence of light, but of heat and of many chemical reactions.

According to Le Bon, therefore, the person who made the first observation was the unpretentious Niepce de Saint-Victor who was disdained during his lifetime and forgotten after his death. Niepce rightly claimed that uranium salts could emit radiations that mark photographic plates in the dark but erroneously considered these radiations a form of captured light, of phosphorescence. His experimental evidence was neglected, but his misconception of the nature of the phenomenon was upheld by the general scientific community.

Le Bon himself published experiments that, unlike those of Becquerel described below, were to be confirmed by future work and were in line with a better understanding of the nature of radioactivity. He admits, however, that he had his own misconceptions and did not distinguish between two very different types of radiation: 1 - infra-red radiation that goes through most non-conducting agents (wood, stone, black paper, ebony...); 2 - radiations emitted by metals under light that he assimilated with cathodic and uranic rays.


When is a discovery not a discovery? (6)

Becquerel apparently spent three years trying to prove that radiations emitted by uranium could, just like light, be polarized, reflected, and refracted. In his reports published in 1896-7 (7), he describes a whole series of experiments which, in his opinion, established the validity of his thesis. ("Cette expérience montre donc à la fois pour les rayons invisibles émis par les sels d'uranium la double réfraction, la polarisation des deux rayons, et leur inégale absorption au travers de la tourmaline". This experiment simultaneously shows, with regard to the invisible rays emitted by uranium salts, the double refraction and polarisation of the two rays, and their unequal absorption through tourmaline (boron aluminium silicate))

On the other hand, Le Bon's work published during this period (1897) considered that these radiations were a new form of energy more closely related to X-rays than to light, an idea Le Bon claims to have been alone to uphold at the time. Indeed, at a physics congress in 1900, Becquerel admits that his own results were irreproducible : "L'expérience sur la polarisation des rayons uraniques n'a pas donné ultérieurement les mêmes résultats....... Les mêmes conclusions négatives ont été observées par M. Rutherford et M. Gustave Le Bon" (8). (The experiment on the polarisation of 'uranic' rays did not subsequently yield the same results.... The same negative conclusions were observed by Mr. Rutherford and Mr. Gustave Le Bon).


Spite, hindsight or truth ?

There can be little doubt that Le Bon must have been rather angry when Becquerel accused him of having 'aucune idée des phénomènes de radio-activité' (no idea of the phenomena of radioactivity) when he - Le Bon - performed his experiments. By returning the compliment and defending his own cause, Le Bon neatly unveils the two sides of the coin and reveals how scientific papers may be re-interpreted with hindsight. In 1896-97, Becquerel tried to prove that the radiations were like those of light but did not succeed. The argument was then turned back to front : he successfully proved that they were not like those of light ! Let us not forget, however, that in the absence of a careful detailed study of the original documents, we cannot know what was the influence of hindsight on Le Bon's interpretations of his own experiments.

Neither Becquerel nor Le Bon coined the word 'radioactivity' which was first used by the Curies after the discovery of radium in 1898. However, it seems that whereas Becquerel may not have been sufficiently aware of the work of Niepce de Saint Victor and might have been looking in the wrong direction in his analogies with light, Le Bon was interpreting the results correctly and laying an intellectual framework for a true understanding of the nature of radioactivity.

This brings us to the question that I submit to you, readers. In a discovery, how much credit should go to the original thinkers - to the Le Bons and others - and how much to those who rediscover an experiment and 'accidentally' make an observation which they interpret wrongly, in terms of the misconceptions of prevalent mainstream thinking, until such time as they fall on an unavoidable stumbling block ?


Le Bon's prediction : the atom bomb

Let us give Le Bon the benefit of the doubt one last time and accept that there is truth and not only hindsight in his chronicles. Le Bon argued that matter could dissociate and understood that the phenomena observed in relation with this dissociation were the manifestation of a new colossal intra-atomic energy from which derived the 'forces of nature, electricity, and solar energy'. In 1903, Pierre Curie had measured the heat given out by 1 g of radium in a calorimeter (100 cal/hour). In 1912, Le Bon wrote : "Le savant qui trouvera le moyen de dissocier instantanément 1 gramme d'un métal quelconque, radium, plomb ou argent, ne verra pas les résultats de son expérience. L'explosion produite sera tellement formidable que son laboratoire et toutes les maisons voisines seront instantanément pulvérisées avec ses habitants". (The scholar who found a way of dissociating instantaneously 1 gram of some metal, whether radium, lead or silver, would not see the results of his experiment. The ensuing explosion would be so great that his laboratory and all the neighbouring houses with their inhabitants would be instantly reduced to dust).

This was over 30 years before the atom bomb !!!



Notes

(a) Becquerel, Henri Antoine (1852-1908). Professor at the French Natural History Museum like his father and grandfather; discovered natural radioactivity in 1896; received the Nobel prize in 1903; became permanent secretary of the French Academy of Sciences.

(b) Le Bon, Gustave (1841-1931). French physician (head of military ambulance division), sociologist and scientist still known in erudite circles for his publications in sociology (L'homme et les sociétés, leur origine et leur histoire (1881); La psychologie des foules (1895); La psychologie du socialisme (1898); La révolution française et la psychologie des révolutions (1912) and many other works especially on the evolution of peoples); largely forgotten as a scientist; wrote several works in the fields of medicine, science, and photography (e.g. Physiologie de la génération de l'homme et des principaux êtres vivants (1868); Les levés photographiques et la photographie en voyage (1888-1889); L'évolution de la matière (1905); L'évolution des forces (1907) etc...); travelled to India in 1884 to study Buddhist monuments.

(c) Niepce de Saint-Victor, Claude (1805-1870). Cousin of Nicephore Niepce who invented photography. Military man; invented a dye process that was adopted by the French War Ministry; when sacked after the revolution, pursued his scientific research on the action of vapours, photography, heliographic engraving, and noted the action of uranium salts on photographic plates without being able to explain it. (Biography taken from : Prevet, François : Morale et métier. La recherche scientifique. Recueil Sirey, Paris).


References

1. Jössang P. The inventor of chromatography (1861) : F. Goppelsröder, professor at the University of Basel. Science Tribune. http://www.tribunes.com/tribune/art96/jose.htm, Oct. 1996.

2. Le Bon G. L'évolution de la matière. Flammarion, Paris, 1912.

3. Le Bon G. C R Acad Sci Paris 5 avril, 1897.

4. Lénard. Erzeugung Kathoden strahlen durch ultra-violette Licht. Vienna Academy of Sciences, October 18, 1899.

5. Comptes rendus 1897, p. 895

6. de Heen. Quel est l'auteur de la découverte des phénomènes dits radio-actifs? Institut de Physique de Liège, 1901.

7. Becquerel H. C R Acad Sci. 1896, pp. 561, 693, 763.

8. Becquerel H. Congrés de physique, 1900, t. III, p. 34.



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