452 



NA TURE 



[March 12, 1903 



of energy, and when stopped by a target naturally consider- 

 able results are produced. 



Now for radiation of any kind there must be acceleration. 

 The greater the acceleration the stronger the radiation. 

 If you want violent radiation take a quickly moving charged 

 bodv, and stop it dead ; which is just what you do in the pro- 

 duction of X-rays, and what is done to some extent by 

 minerals exposed to the kathode rays. These corpuscles have 

 extremely small mass, and so their inertia is extremely 

 small, but a body, no matter how small, moving with the 

 speed of light, must have terrible energy ; thus, by way of 

 illustration, the energy of a gramme of matter (15 grains) 

 travelling at the speed of light would be sufficient to lift 

 the British Navy to the top of Ben Nevis. After the speed 

 of these corpuscles that of bullets is rest in comparison. 

 [Having shown by experiment a vacuum tube containing 

 electrons in motion, the lecturer proceeded.] To show that 

 these are charged particles in motion, it is only necessary 

 to show that they have the properties of a current, that is, 

 that they are amenable to magnetism — such as that of an 

 ordinary steel magnet — and what you see going on in the 

 tube is the nearest approach you have to seeing electricity. 

 In that tube electricity is as isolated and as separated as 

 we can ever hope to have it. 



All electrical phenomena seem to depend upon these elec- 

 trons. In the case of gaseous conduction what we observe 

 is the flying of the particles — the bullet method or electric 

 particles in free flight. When we deal with liquid conduc- 

 tion it is the slow travelling charges moving, but retarded 

 or loaded with the atom of matter, having to convey the 

 atom of matter with it ; hence they travel very slowly, 

 the atoms jostle and have to work their way through the 

 rest of the material, and instead of going something like 

 1000 miles a second they go more like an inch an hour ; 

 *t depends upon what gradient of potential is applied. That 

 I call the bird-seed method, meaning that the charge is 

 carried as a bird carries a seed, the bird and seed travelling 

 together until they arrive at an electrode, when the electron 

 is dropped.' In the case of solid conductors or metals the 

 atoms cannot move along as they do in the liquid, they 

 can only vibrate a little, are fixed, rigid, crystallised into 

 their places. So when the electrons travel it must be be- 

 cause they are handed on from one to the next ; each receives 

 one and passes it on, not necessarily the same one ; and 

 this may be called the fire-bucket method. 



A word more about radiation. If conduction is explicable 

 in this way, how is radiation to be explained? Until quite 

 recently radiation has been a puzzle. Atoms of matter 

 vibrate; radiation is waves in the ether. Hence it used to 

 be thought, and it did not seem puzzling at that time, that 

 vibrating atoms of matter could generate waves in the ether 

 just as a bell can generate waves in the air. The method 

 by which light is generated was not clearly understood, but 

 it was thought to be something analogous to the production 

 of sound by a tuning fork or bell. But certain experiments 

 made by me at Liverpool showed that matter and ether 

 are disconnected from one another — that matter alone can- 

 not generate these waves. It becomes necessary to assume 

 that it is not matter which is vibrating so much as the 

 charge on the matter — that radiation is caused not by the 

 atom itself, but by the electron which it carries. It is 

 during the accelerative period that radiation occurs. If 

 the atom simply carries a charge along there is no radiation. 

 There is nothing visible in the kathode rays as long as they 

 are travelling with uniform speed and direction ; it is when 

 they are accelerated, started or stopped, or curved, that 

 radiation occurs. The electron instead of simply vibrating 

 might be revolving round the atom like a satellite ; that 

 would be centripetal acceleration, which is just as effective 

 as longitudinal acceleration. 



But if radiation is due to an orbital motion of an electric 

 charge, it ought to be amenable to a magnetic field ; every 

 motion of an electron constitutes an electric current, and 

 electric currents are amenable to a magnet. A source of 

 light put between the poles of a magnet ought to show some 

 difference. Faraday tried many experiments in this direc- 

 tion and failed, because the appliances available in his day- 

 would not show it. Nowadays, with a Rowland grating, 

 the spectrum is much better defined, and a few years ago 



Zeeman, of Amsterdam, was able to see the difference when 

 light is magnetised. 



It often falls to men of genius to predict a great deal 

 more than their generation can realise. A theory had been 

 given by * sundry people, including Fitzgerald, Larmor, 

 Lorentz, and others. Perhaps the theory has been given 

 more completely by Lorentz than by anyone else. It was 

 an interesting case of prophetic prediction. They predicted 

 that the effect observed by Zeeman would follow if light 

 were due to revolving electrons. Time only permits me to 

 indicate the explanation. It comes near to astronomy, and, 

 indeed, it had been worked out six years before by Dr. 

 Johnstone Stoney on astronomical principles. He had fully 

 worked out the perturbations, but had not suggested that 

 they would be caused by a magnet. But Larmor and the 

 others did. They perceived that on applying to an orbit or 

 circular current a strong magnetic field, that orbit will 

 tend to be deflected ; the effect of a magnetic field in 

 general is a deflecting force. But as the circulating elec- 

 tron has inertia, the application of a deflecting force will 

 not make it simply obey the force that is applied, but will 

 make it move sideways, like any planetary orbit or a 

 spinning top. A precessional motion is set up. Anything 

 spinning that has inertia does not obey the force but moves 

 at right angles. Thus the revolving electron will not, when 

 the force is applied immediately, set itself normal to the 

 field, but will go round the magnetic lines in a precessional 

 motion ; and that precessional motion will analyse the 

 original lines of the spectrum into three. [Here the lecturer 

 gave an illustrative experiment, and proceeding, pointed 

 out that when the polarisation of the lines is examined, the 

 vibrations are precisely as predicted.] It was further 

 found that by the amount of separation of these lines a 

 calculation could be made of what the magnitude of the 

 electric charge was in relation to the inertia of the revolving 

 portions of matter. It was thus found that the radiating 

 particles have just the same inertia and just the same 

 charge as the particles in the kathode rays. All the 

 known phenomena connected with conduction and radiation 

 are allied to these very small particles — the same inertia, 

 the same electric charge, and the same kind of velocities, 

 the mass being something like the thousandth part of a 

 hydrogen atom. 



Passing over chemical affinity and cohesion, the lecturer 

 proceeded to discuss other phenomena which are due to 

 these small particles. These particles, in order to give rise 

 to visible radiation, revolve with terrific velocity. The 

 number of vibrations which constitute visible light is from 

 400 to 800 million million times per second ; and although 

 it is no great distance round an atom, yet these particles 

 have to go at very high speed ; hence, naturally, some 

 of them occasionally fly off. This will occur from various 

 causes ; they will fly off under the action of ultra-violet 

 light, and so give rise to leakage of negative electricity. 

 But there are certain substances which will emit these 

 particles without any stimulus, and the first discovered was 

 uranium. Although there may be aluminium or other 

 screen between a piece of uranium and a photographic plate, 

 something will penetrate through to the photographic plate. 

 This constituted the discovery, by Becquerel, of the radio- 

 activity of substances. In the researches of Dr. Russell 

 various substances were found to possess this quality 

 of giving out something on their own account. But the 

 subject has gone ahead very far and fast. The most 

 important developments have been made by Monsieur and 

 Madame Curie in France, discovering polonium and radium, 

 which latter has the properties possessed by uranium in a 

 most extraordinary degree. The rays given off by these 

 substances are of extraordinary interest ; they have mar- 

 vellous penetrating powers and are very intense, more intense 

 than the X-rays given by a Rontgen tube. Radium rays 

 will not only penetrate a foot of aluminium or wood, but 

 they will penetrate three-eighths of an inch of lead, and 

 then be as strong as are the rays from uranium. The full 

 mechanism of the giving off of this great amount of radia- 

 tion has still to be further investigated. It is a kind of 

 electric evaporation, an emission of particles ; this seems 

 clear. There are three kinds of radiation, (1) particles 

 which are readily slopped by obstacles, absorbable rays ; 



NO. 1 741. VOL. 67] 



