218 



KNOWLEDGE 



[October, 1908. 



pitehblende, traces of which are carried down with the 

 separated urauium salt, and remain with it as an impurity. 

 Now commenced a long, arduous, and painstaking attempt 

 to discover the nature of these substances. Anyone who 

 has attempted to analyse exhaustively a mineral for l-nown 

 substances answering to l-nown chemical reaction can form 

 some faint idea of the diflicidty which the Curies met, and 

 surmounted, in their attempt to isolate the radio-active 

 agent in pitchblende, without any previous knowledge 

 of its chemical properties. First of all, a radio-active 

 substance was separated, which was about 300 times as 

 active as the salts examined by Becquerel. This substance, 

 which resembles bismuth in its chemical behaviour, was 

 tenaed polonium by Mme. Curie, in honour of Poland, her 

 native land. Later on, M. and Mme. Curie, together with 

 M. Bemont, isolated a second substance, which was tenned 

 radium. In 1899 a third substance, which is closely 

 associated with thorium, was isolated ; this substance has 

 been termed actinium. Of these three substances, radium 

 alone has been isolated in a state of purity sufficient to 

 determine its essential properties. It has not been obtained 

 in the form of a metal, but only in that of a salt, such as 

 a bromide, chloride, or nitrate. There appears to be no 

 doubt as to its being a true element ; M. Curie has gone 

 so far as to determine its atomic weight, which is found to 

 be 225. It also has a definite spectrum. '1 he best samples 

 of radium obtained by the Curie are 1,000,000 times as 

 active as the uranium salts originally examined by 

 Becquerel. To understand the difficulty of obtaining an 

 appreciable amount of radium, it must be remembered 

 that as a preliminary step urauium must be removed from 

 the pitchblende; in order to obtain one kilogram (22 lbs.) 

 of radium, it would be necessary to treat 5000 tons of the 

 uranium residues ! After this we can hardly wonder that 

 radium bromide costs about =£1250 jier gramme. As Prof. 

 J. J. Thomson has said, it is more easy to isolate the gold 

 in sea-water than to separate the radium in pitchblende. 



Having obtained a substance so powerfully radio-active 

 as radium, M. and Mme. Curie were in a position to 

 determine the conditions attendmg radio-activity. In the 

 first place it was found that an insulated charged conductor 

 quickly loses its charge if a radio-active substance is 

 sprinkled on it. Further, if a particle of radium is 

 brought near a charged electroscope, the latter is quickly 

 discharged. Now it has been known for many years that 

 ultra-violet light, if allowed to fall on a negatively 

 charged body, will cause a rapid discharge to occur. By 

 a beautiful series of experiments. Prof. J. J. Thomson has 

 shown that ultra-violet light causes small negatively 

 charged particles to be thrown off from bodies on which it 

 falls, and has even determined the mass of these particles, 

 each of which possesses about one-thousandth part of the 

 mass of an hydrogen atom. X-rays, falling on a body, 

 cause negatively charged particles of a similar character to 

 be thrown off. The question now arose as to whether 

 radium rays were essentially similar to either ultra-violet 

 rays or X-rays ? If not, what was their distinguishing 

 characteristic ? 



The answer to this question is derived from the 

 circumstance that the most active portions of radium 

 emanations are deflected when they pass at right angles 

 across a magnetic field, while the i-ays of ultra-violet light, 

 or X-rays, are undeflected under similar conditions. To 

 explain this more fully, let us suppose that a horse-shoe 

 magnet is laid on a photographic plate. Between the 

 magnet poles, stretching from one to the other, are 

 lines of magnetic force, which are said to constitute a 

 magnetic field. These lines will lie, in the main, parallel 

 to the plate. A ray of light, or an " X-ray," incident 

 normally on the plate between tiie ijoles of the magnet, 



will be at right angles to the lines of force. Now, if such 

 a ray is deflected by the magnetic field, the plate, when 

 developed, will exhibit a dark spot at a jiosition different 

 from that which would be produced if the magnet had 

 been absent. In another arrangement, the ray may be 

 allowed to fall on the plate, first, with the magnet absent, 

 anl then, without altering anything else, with a magnet 

 pi iced so that the ray passes between its poles. If the 

 r.iv is deflected by the magnetic field, we shall obtain two 

 sjiots on the developed plate, one due to the deflected, and 

 the other to the undeflected ray. Such deflection has 

 never Ijeen observed with relation to X-rays or light rays, 

 and tliis fortifies us in our belief that X-rays are merely 

 ultra-violet rays of extremely short wave-length. On the 

 other hand, if a particle of radium is placed above a 

 photographic plate, from which it is separated by a sheet 

 of black paper or aluminium, a diffused spot on the de- 

 veloped plate will show the position where the radium rays 

 struck it. The position of this spot differs when a magnet 

 is laid on the plate so that the rays pass between its poles, 

 and when the magnet is absent; thus radium rays are 

 distinguished from X-rays, or light rays, by their capacity 

 to be deflected in a magnetic field. 



It is well known that a conductor traversed by an 

 electrical current will be acted on by a force tending to 

 displace it if it is placed at right angles to the lines of 

 force of a magnetic field ; a practical application of this 

 law has given us the electric motor. It is further 

 generally admitted that a series of electrically charged 

 particles moving along a straight line acts in many respects 

 like an electrical conductor carrying a current. In other 

 words, if we imagine electrified particles to be projected 

 with great velocity along a given straight line, the path 

 of these particles may remain strictly rectilinear if they 

 nowhere cross the lines of force of a magnetic field ; but 

 if, anywhere in their course, they pass between the poles 

 of a strong magnet, their path will there be deflected, and 

 will subsequently be inclined to its original direction. 

 Such a stream of particles would therefore strike different 

 points of a target, according as it passed between the poles 

 of a magnet, or travelled through space free from a 

 magnetic field. These phenomena are so exactly parallel 

 to those observed in connection with radium, that there is 

 little doubt left that radium emanations consist mainly of 

 charged particles shot off from the active substance. 



Up to the present, radium rays have been spoken of as 

 if they were all of one kind. They all, indeed, possess 

 properties which are to a certain extent similar ; but ex- 

 periments enables us to distinguish between at least three 

 kinds of rays. It is found that a great part of the rays 

 emitted by radium are intercepted by a thin screen of 

 aluminium. Those which have passed through this screen 

 can traverse a much thicker screen of the same substance 

 without further loss ; in other words, the rays have been 

 sifted of those which are only slightly penetrative (termed 

 the a rays), while those which possess greater penetrative 

 capacity (termed the ^ rays) have been left. Both the 

 a and S rays are deflected in a magnetic field, though to 

 different extents ; both discharge electrified conductors on 

 which they fall. The a rays, when allowed to fall on a 

 negatively charged conductor, discharge it with great 

 rapidity ; they therefore carry a positive electric charge. 

 Ttie more penetrative (3 rays discharge a positively charged 

 conductor fairly well ; they therefore carry a negative 

 charge. In a magnetic field the a rays are deflected to a 

 smaller extent than the /3 rays ; indeed, it is only quite 

 recently that Prof. Rutherford, using a very strong 

 magnetic field, has succeeded at all in deflecting the a rays. 

 Taken in conjunction with the facts previously dwelt on, 

 this leads to the conclusion that the a ravs are positively 



