August 2, 1906] 



NA TURE 



323 



years ago, I visited liini there, for, a hundred years before, 

 it was the dwelling-house of the great Cuvier. Here Henri 

 Becquercl's father and grandfather — men renowned through- 

 out the world for their discoveries in mineralogy, electricity, 

 and light — had worked, and here he had himself gone 

 almost daily from his earliest childhood. Many an experi- 

 ment bringing new knowledge on the relations of light and 

 electricity had Henri Bccquerel carried out in that quiet 

 old-world place before the day on which, about twelve 

 vears ago, he made the experimental inquiry, Does uranium 

 give off penetrating rays like Riintgen's rays? He wrapped 

 a photographic plate in black paper, and on it placed and 

 left lying there for twenly-four hours some uranium salt. 

 He had placed a cross, cut out in thin metallic copper, 

 under the uranium powder, so as to give some shape to 

 Ihe photographic print should one be produced. It was 

 produced. Penetrating rays were given off by the uranium : 

 ihe black paper was penetrated, and the form of the copper 

 cross was printed on a dark ground. The copper was also 

 penetrated to some extent by the rays from the uranium, so 

 that its image was not left actually white. Only one step 

 more remained before Becquerel made his great discovery. 

 It was known, as I stated just now, that sulphide of 

 i-.Tlcium and similar substances become phosphorescent 

 'vhen exposi-d to sunlight, and lose this phosphorescence 

 after a few hours. Becquerel thought at first that perhaps 

 the uranium acquired its power similarly by exposure to 

 light; but very soon, by experimenting with uranium long 

 kept in the dark, he found that the emission of penetrating 

 rays, giving photographic effects, was produced spon- 

 taneously. The emission of rays by this particular frag- 

 ment of uranium has shown no sign of diminution since 

 this discovery. The emission of penetrating rays by 

 uranium was soon found to be independent of its phos- 

 phorescence. Phosphorescent bodies, as such, do not emit 

 penetrating rays. Uranium compounds, whether phos- 

 phorescent or not, emit, and continue to emit, these pene- 

 trating rays, capable of passing through black paper and 

 metallic copper. Ihey do not derive this property from 

 the action of light or any other treatment. The emission 

 of these rays discovered by Becquerel is a new property 

 of m.itler. It is called " radio-activity," and the rays are 

 called Becquerel rays. 



From this discovery by Becquerel to the detection and 

 separation of the new element radium is an easy step in 

 thought, though one of enormous labour and difficulty in 

 practice. Prof. Pierre Curie (whose name I cannot mention 

 without expressing the grief with which we all heard in 

 ."Vpril last of the sad accident by which his life was taken) 

 and his wife, Madame Sklodowski Curie, incited by 

 Becquerel 's discovery, examined the ore called pitch-blende 

 which is worked in mines in Bohemia and is found also 

 in Cornwall. It is the ore from which all commercial 

 uranium is extracted. The Curies found that pitch-blende 

 has a radio-activity four times more powerful than that 

 of metallic uranium itself. They at once conceived the 

 idea that the radio-activity of the uranium salts examined 

 by Becquerel is due not to the uranium itself, but to 

 another element present with it in variable quantities. 

 This proved to be in part true. The refuse of the first 

 processes by which in the manufacturer's works the 

 uranium is extracted from its ore, pitch-blende, was found 

 to contain four times more of the radio-active matter than 

 does the pure uranium. By a long series of fusions, solu- 

 tions, and crystallisations the Curies succeeded in " hunt- 

 ing down," as it were, the radio-active element. The first 

 step gave them a powder mixed with barium chloride, and 

 having 2000 times the activity of the uranium in which 

 Becquerel first proved the existence of the new property — 

 radio-activity. Then step by step they purified it to a con- 

 dition 10,000 times, then to loo.ooo times, and finally to 

 the condition of a crystalline salt having 1,800,000 times 

 the activity of Becquerel's sample of uranium. The purifi- 

 cation could go no further, but the extraordinary minute- 

 ness of the quantity of the pure radio-active substance 

 obtained and the amount of labour and time expended in 

 preparing it may be judged from the fact that of one 

 ton of the pitch-blende ore submitted to the process of 

 purification only the hundredth of a gram — the one-seventh 

 of a grain — remained. 



The amount of radium in pitch-blende is one ten- 

 millionth per cent. ; rarer than gold in sea-water. The 

 marvel of this story and of all that follows consists largely 

 in the skill and accuracy with which our chemists and 

 physicists have learnt to deal with such infinitesimal quanti- 

 ties, and the gigantic theoretical results which are securely 

 posed on this pin-point of substantial matter. 



The Curies at once determined that the minute quantity 

 of colourless crystals they had obtained was the chloride 

 of a new metallic element with the atomic weight 225, to 

 which they gave the name radium. The proof that radium 

 is an element is given by its " sign-manual " — the spectrum 

 which it shows to the observer when in the incandescent 

 state. It consists of six bright lines and three fainter lines 

 in the visible part of the spectrum, and of three verj' 

 intense lines in the ultra-violet (invisible) part. A very 

 minute quantity is enough for this observation ; the lines 

 given by radium are caused by no other known element in 

 heaven or earth. They prove its title to be entered on the 

 roll-call of elements. 



The atomic weight was determined in the usual way by 

 precipitating the chlorine in a solution of radium chloride 

 by means of silver. None of the precious element was lost 

 in the process, but the Curies never had enough of it to 

 venture on any attempt to prepare pure metairic radium. 

 This is a piece of extravagance no one has yet dared to 

 undertake. Altogether the Curies did not have more than 

 some four or five grains of chloride of radium to experiment 

 with, and the total amount prepared and now in the hands 

 of scientific men in various parts of the world probably does 

 not amount to more than sixty grains at most. When 

 Prof. Curie lectured on radium four years ago at the Royal 

 Institution in London he made use of a small tube an inch 

 long and of one-eighth inch bore, containing nearly the 

 whole of his precious store, wrenched by such determined 

 labour and consummate skill from tons of black shapeless 

 pitch-blende. On his return to Paris he was one day demon- 

 strating in his lecture room with this precious tube the 

 properties of radium when it slipped from his hands, broke, 

 and scattered far and wide the most precious and magical 

 powder ever dreamed of by alchemist or artist of romance. 

 Every scrap of dust was immediately and carefully collected, 

 dissolved, and re-crystallised, and the disaster averted with 

 a loss of but a minute fraction of the invaluable product. 



Thus, then, we have arrived at the discovery of radium — 

 the new element endowed in an intense form with the new 

 property " radio-activity " discovered by Becquerel. The 

 wonder of this powder, incessantly and without loss, under 

 any and all conditions pouring forth by virtue of its own 

 intrinsic property powerful rays capable of penetrating 

 opaque bodies and of exciting phosphorescence and acting 

 on photographic plates, can perhaps be realised when we 

 reflect that it is as marvellous as though we should dig up 

 a stone which without external influence or change, con- 

 tinually poured forth light or heat, manufacturing both in 

 itself, and not only continuing to do so without appreciable 

 loss or change, but necessarily having always done so for 

 countless ages whilst sunk beyond the ken of man in the 

 bowels of the earth. 



Wonderful as the story is, so far it is really simple and 

 commonplace compared with what yet remains to be told. 

 I will only barely and abruptly state the fact that radio- 

 activity has been discovered in other elements, some very 

 rare, such as actinium and polonium ; others more abundant 

 and already known, such as thorium and uranium, though 

 their radio-activity was not known until Becquerel's 

 pioneer-discovery. It is a little strange and no doubt 

 significant that, after all, pure uranium is found to have a 

 radio-activity of its own and not to have been altogether 

 usurping the rights of its infinitesimal associate. 



The wonders connectad with radium really begin when 

 the experimental examination of the properties of a few 

 grains is made. What I am saying here is not a 

 .systematic, technical account of radium ; so I shall venture 

 to relate some of the story as it impresses me. 



Leaving aside for a moment what has been done in re- 

 gard to the more precise examination of the rays emitted 

 by radium, the following astonishing facts have been found 

 out in regard to it : (i) If a glass tube containing radium 

 is much handled or kept in the waistcoat pocket, it pro- 



NO. 1918 VOL. 74] 



