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SOME SCIENTIFIC CENTRES.

It was not so with other scientific centres of this series; VI.—THE PHYSICAL LABORATORY AT THE MUSEUM

there there were many discontinuities, here the conD'HISTOIRE NATURELLE.

tinuity is one.

The technical process of gilding due to de la Rive THE Museums d'Histoire naturellep in the beautiful was based upon Becquerel's

observation in 11834 of the founded in 1793, form an institution of acknow- the poles of a pile are immersed in solutions of various ledged eminence; whilst the lectures delivered there metallic salts; that the two solutions needed could be are by the most renowned professors, and on most, kept apart by the use of animal membranes without if not all, branches of the natural sciences. It was preventing the passage of the current, and that with Cardinal Richelieu, as we know, who founded the very feeble currents the deposition of metal is even Jardin des Plantes somewhere about 1626, not long before the establishment of the French Academy by the same great Minister of State.

The physical laboratory in particular of these museums has been the seat of many discoveries and the centre from which has radiated some of the best thought, as well as some of the best work, that has animated the academy and through it the scientific world for three-quarters of a century. It is not often the case with science, nor, indeed, with other branches of learning, that in a single family there should be found for three generations a series of distinguished men of the highest order of intellect who have devoted their lives and best energies to its pursuit and attained to universal fame. More seldom is it, then, that when the lineage is thus preserved unbroken, the members thereof should all be devoted to the one and to the self-same calling. For three generations the Becquerels have occupied in succession the same chair at the same institution, namely, the Museum d'Histoire naturelle in Paris. The number of papers which have been read before the Academie des Sciences by the Becquerels extends to seven or eight hundred.

Henri Becquerel, whose portrait in his laboratory at the Museum d'Histoire naturelle is here reproduced, is, we venture to think, perhaps the most distinguished of his race. His father, Alexander Edmond, is known as the inventor of the phosphoroscope and the author of "La Lumière," a work of great value in its day, whilst his grandfather, Antoine César, was likewise famous for a long series of researches, chiefly on chemical dynamics and electrocapillary phenomena. His electro- Fig. 1.-Prof. Henri Becquerel in his Laboratory at the Paris Museum d'Histoire naturelle. magnetic balance is of historic interest in the development of the galvanometer, and uniform on the surface of the electrode. Although although long since abandoned for practical purposes. rivalled by many others in the application of these

Thus the history of the physical laboratory at the principles, many were the facts and many the methods Museum d'Histoire naturelle may be said to which he announced with rapid succession in laying parallel with the history of the Becquerels, and the the foundations of the art of electro-plating. two to be so closely interwoven that to describe the It was to the study of electrocapillary phenomena, part played by one and the influence exerted by it in which he was the first to observe in 1867, that his later the development and advancement of knowledge is years were devoted. The discovery was a curious one, perhaps equivalent to writing that of the other in detail. the result, if we mistake not, of the deposition of

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metallic copper on a crack in a test-tube containing gates which gradually disintegrate, as radio-active a solution of cupric sulphate, and immersed in another molecules have been found to do,' the change of solution of sodic sulphide. The investigation of this absorption which accompanies fluorescence being due phenomenon was full of interest, and not the least was to the formation of these molecular groups. the suggestion that the deposition of metals in veins The absorption spectrum of crystals exhibits many in rocks is due to the same cause as that which he anomalies, from which Becquerel has extracted a most observed in the broken test-tube.

important principle. If a crystal is composed of two A member of the French Academy from 1829, eight isomorphous substances the molecular elasticity of years before being called to the chair which he filled which varies in different directions, so that the absorp to the end of his life, he was also a corresponding tion varies too, the absorption spectrum will likewise member of the Royal Society, and received from it its vary in different directions, so that it is thus possible greatest honour, the Copley medal, and from the to detect the presence of different substances, since Emperor Napoleon III. the Cross of Commander of the in two isomorphous substances the directions of Legion of Honour. Thus with him there closed a molecular elasticity do not correspond, and therefore chapter, a long, an interesting, and an eventful chapter, the directions of absorption would likewise differ. in the history of the Museums d'Histoire naturelle, Each chemical substance, therefore, affects the direc

Edmond Becquerel, although a pupil of his father tion of propagation and of absorption. and for a considerable time his assistant at the If the directions of absorption do not coincide with museums, did not teach there, and, indeed, as Sir the optic axes, it is due to the presence of different William Crookes has said in his obituary notice of isomorphous substances in the crystal. The absorp. him in the Proceedings of the Royal Society, of which tion spectrum of each substance remains different and he was a foreign member, it may “be remarked in its own particular direction, whilst that of refracthat though he had early distinguished himself by tion is the resultant effect. By this contrivance the scientific works of high value, and as the son of an composition of crystals has been examined and after. eminent and much respected Academician he was not wards confirmed by chemical means, whilst in many without influence, yet none of the great scientific instances the presence of substances in quantities too establishments of his country offered him an appoint-minute for the chemist to notice has been detected by ment.” He finally, however, secured a permanent this elegant method of analysis. position at the Conservatoire des Arts et Métiers, and But the most striking work that has issued from there the abilities so long latent had full play, and Becquerel's laboratory relates to the radio-activity of manifested themselves by the success of his subsequent matter. Of this great discovery, separating as it does career.

the ideas of this century from those of the last, so much At the death of his father, in 1878, he succeeded to has been written, upon their far-reaching importance, the chair of physics at the museum, and this im- so many ideas have been discussed in these columns, portant position he continued to hold until his death in that to discourse upon them here would be but vain 1892. Brought up as he was in a scientific atmo- repetition of all that has been said before; yet, parasphere, he evidently inherited from his father his doxical though it may seem, it is unquestionably the

acute power of observation," and that “infinite work of all works that most definitely separates, and capacity of taking pains which seems to be the

at the same time most closely unites, the two sciences essential characteristic of the Newtons, the Faradays, of physics and chemistry, whilst it brings into prominand the Darwins, and, in short, of all the great leaders ence what may appropriately be called a new scienceof science."

that of radio-activity--a science which neither physics Since 1892 Henri Becquerel has been professor at nor chemistry can claim within its old province, and the Museum d'Histoire naturelle, and has continued yet neither can disclaim, nor would it very readily do those studies which his ancestors in days gone by so if it could. pursued with ardour and with success, not the less What is the influence which these laboratories have marked, although perhaps, on the whole, notwith- exerted and exert? We may ask, what is the instanding their brilliant achievements, less fruitful in fluence of the Royal Institution ? Is it not to be revealing that knowledge which was to come; for by measured by the work which has been done there and his memoirs on the radio-activity of matter Henri by the ideas which have been scattered from those Becquerel has given to the world of science the results great fountains of thought-if they can be measured ? of a very remarkable series of researches.

How many youthful imaginations, how many There are four methods of studying the infra-red enthusiastic aspirations have been aroused within parts of the spectrum : the thermopile, as employed those venerable halls, of the Becquerels as of the by Tyndall and others, the radiomicrometer of Boys, Davys, the Faradays, and the Tyndalls ? Parisian the bolometer as used by Langley, and the phos- lecturers are savants, philosophers, and orators. For phorescent screen of Becquerel. After exposure to the although the Teuton regards the gift of eloquence violet rays, and if the screen is subjected to the action (we hope it is his own) as the gift to be designated as of the infra-red, the phosphorescence becomes so intense gab,” the southerner or the Celt thinks it indis. that the energy, accumulated is radiated so rapidly pensable in the expression of a clear mind and of a that the parts thus acted upon become quite dark great soul, at once saturated with thoughts and the relatively to the other parts of the screen.

Thus a

grandeur of its subject; and in France this counts for map of the infra-red can be produced and studied at more than it usually does among us. leisure so long as the phosphorescence of the screen These lectures are a source of inspiration to the lasts, or, indeed, photographs of the screen thus multitude as well as to the grave, and their importance affected may be taken. The effect is due most prob- cannot be overrated. ably to heat, and is therefore a case of thermolumin- Having said thus much of the laboratories of the escence. Under the influence of heat the collisions

Museum d'Histoire naturelle, we may perhaps be perbetween molecules become more frequent and more mitted to add a word as to the central figure in this violent, and the energy absorbed from the more re- centre of scientific thought, of M. Henri Becquerel; frangible rays, and stored up in the substance, by from none need we expect greater freedom, greater some means at present not very clearly understood, is ease, or kindlier consideration. The brief summary once more yielded up to the æther and radiated away. The energy is stored up in unstable molecular aggre

British Association and Electrician, 1900-02; and Phil. Mag., 1901

Phil. Trans., 1897

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