February 21, 1901] 



NA TURE 



397 



The uranium radiation dissipates with equal rapidity 

 either positive or negative electric charges ; the effect is 

 unequally transmitted through screens of different 

 materials according to their nature and according to the 

 order in which they have been simultaneously interposed. 

 This fact leads to the prediction of the heterogeneity of 

 the radiation. The dissipation of the charges of an 

 electrified body submitted to the uranium radiation takes 

 place through the action of the surrounding gas, which is 

 thereby made conducting ; the air keeps this property 

 during some instants, and if, after having been influenced, 

 it is blown on to an electrified body, the latter is dis- 

 charged. A sphere of uranium remains charged if it is 

 placed in a vacuum ; in air it puts itself in equilibrium as 

 regards potential with the surrounding space. The 

 rate of leak increases with the potential, and, for high 

 potentials, tends to a constant. 



If two conductors are arranged at some centimetres 

 apart, one of which is connected with a source of 

 electricity, and if a piece of uranium is brought near, there 

 is established between the .two conductors a continuous 

 current of electricity. Such are the principal facts that 

 I had observed. 



In 1897 Lord Kelvin and Messrs. Beattie and S. de 

 Smolan varied the conditions of the preceding experi- 

 ments, and showed that the uranium radiation established 

 between two metals in air the same equilibrium as a 

 drop of water which united them. In 1899 Mr. E. 

 Rutherford made it clearly apparent that the conducti- 

 bility set up in gases by uranium was due to a pheno- 

 menon of ionisation identical with that which, according 

 to the experiments of Prof. J. J. Thomson, is provoked 

 by the Rontgen rays. 



In 1898 the discovery of new radiating bodies gave a 

 new and fruitful impulse to these studies. Mme. S. 

 Curie and M. Schmidt had recognised that thorium 

 possesses analogous properties to those of uranium ; 

 then Mme. Curie observed that certain uranium minerals 

 were more active than either metallic uranium or 

 thorium. M. and Mme. Curie concluded from this that 

 there existed other radio-active bodies, and undertook to 

 isolate them. 



I cannot analyse here the chemical part of the fine 

 work of M. and Mme. Curie, who, working with the 

 electrometer as the chemist works with the spectroscope, 

 succeeded in extracting frorp pitch-blende two very 

 active substances : on the one hand, a product contain- 

 ing bismuth and a body which they called polonium ; 

 and, on the other hand, a mixture of barium and another 

 new substance, radium. 



When they had prepared some centigrams of products 

 the activity of which, progressively increasing, became 

 several thousand times greater than that of uranium, 

 M. and Mme. Curie were good enough to give me 

 some milligrams, so that we could thus pursue parallel 

 researches on the new properties. 



M. and Mme Curie recognised, among other impor- 

 tant properties, that these bodies excite the fluorescence 

 of the platinocyanides ; that the salts of radium are 

 spontaneously luminous ; and further, that all bodies 

 receiving the new radiation become radio-active in their 

 turn, but that they gradually lose this property with time. 

 They observed also, as well as Giesel, who prepared mix- 

 tures analogous, but less active, to those of M. and 

 Mme. Curie, that the salts of radium increase spon- 

 taneously in activity for some time after their pre- 

 paration, whilst the activity of polonium salts diminishes. 

 The new radiation produces various chemical actions, 

 alteration of the platinocyanides, violet coloration of 

 glass, production of ozone, &c. 



For my part, I have observed that the polonium 

 radiation does not traverse even black paper ; it is 

 much less penetrating than that of radium, which, 

 moreover, provokes in bodies which it encounters a 



NO. 1634. VOL. 63] 



secondary penetrating radiation which marks a photo- 

 graphic plate in the immediate neighbourhood of the 

 points struck. I have been able to establish also that 

 the intensity of the phosphorescence excited by radium 

 varies as the inverse square of the distance of the 

 excited body from the radiating source ; that the unequal 

 weakening produced by a given screen on the phosphor- 

 escence of different substances furnishes a new proof of 

 the heterogeneity of the exciting radiation ; and, lastly, 

 that the radiation of radium restores the property of 

 becoming phosphorescent by heat to such bodies as fluor 

 spar, leucophane, &c., which had lost this property by a 

 preliminary heating. 



I would here point out the very interestuig researches 

 of M. and Mme. Curie, M. Owens, and of Mr, E. Ruther- 

 ford upon' the penetrating rays of thorium. Mr. E. 

 Rutherford has found that thorium compounds emit, in 

 addition to this ordinary radiation, a very penetrating 

 "emanation" that produces temporary radio-activity in 

 substances in the neighbourhood, if the bodies are all 

 uncharged. With charged conductors the radioactivity 

 is produced on the negatively charged body. The radio- 

 activity can thus be concentrated on the surface, of thin 

 wires, and removed by hydrochloric and sulphuric acids, 

 whose solution, when evaporated, leaves the active portion 

 behind. Thorium may perhaps owe a part of its pro- 

 perties to a new element, actinmm, discovered in 1900 by 

 M. Debierne, and which is as active as radium. 



At the end of 1899 several observers discovered, nearly 

 simultaneously, that the rays from certain radiating 

 substances were deviated by a magnetic field. This was 

 first shown by M. Giesel with preparations of polonium 

 and radium, then by MM. S. Meyer and E. v. Schweidler, 

 who some days later showed the same thing with 

 preparations made by M. Giesel, and then, a little later, 

 without having any knowledge of these observations, I 

 recognised that the radium radiation concentrated itself 

 upon the poles of a non-uniform magnetic field, whilst 

 the radiation of polonium prepared by M. and Mme. 

 Curie is not deviated. The preparation of polonium of 

 M. Giesel was, then, not the same substance as that of 

 M. and Mme. Curie. 



It resulted from these observations that there exists two 

 kinds of radiations, one not capable of deviation and of 

 which the nature is still unknown, the other capable of 

 deviation, which later experiments have identified with 

 the kathode rays. Somewhat later, M. and Mme. Curie 

 recognised that both these rays coexisted in the radium 

 radiation. The non-deviable rays are much less pene- 

 trating than the deviable rays ; the polonium radiation is 

 limited in air to a kind of sheath of some centimetres 

 in thickness. I might add that recently M. Villard has 

 proved the existence in the radium radiation of very 

 penetrating rays which are not capable of deviation, 

 M. Debierne has recognised that actinium emits some 

 deviable rays. 



I have devoted numerous experiments to the study 

 of the deviation of radium in the magnetic field. This 

 radiation is dispersed by the field into rays of different 

 natures, like as light is dispersed by a prism. For each 

 simple radiation, the trajectory in a plane perpendicular 

 to a uniform field is a circumference of radius R, which 

 brings back the radiation to the point of departure. 

 If the radiation makes at the origin an angle a with the 

 axis of the field, the trajectory is a helix rolled on a 

 cylinder parallel to the field and of radius R sin a. The 

 product Hp of the component of the field normal to the 

 displacement at a point, by the radius of curvature of the 

 trajectory in this point is constant, and may serve to 

 characterise each smiple radiation. 



To obtain a beam in which each simple radiation 

 would have a unique trajectory, a radiant source may- 

 be taken of very small diameter, the radiation being 

 received after traversing a narrow gap in a sheet of lead. 



