KDomledge & SeleDtifie Hems 



A MOXTIILV JOURNAL OF SCIENCE. 



Vol. I. No. 4. 



[new series.] 



MA\', 1904. 



r Entered at "1 

 LStationers' Hall J 



SIXl'ENCH 



Contents and I\'otices. — See Page VII. 



Racdio-Activity OLnd 

 R^QLdium. 



By W. A. Shenstone, F.R.S. 



1. 

 \\ E owe the discovery of radioactivity, and therefore 

 that of radium, to an accident, though the phenomenon 

 itself might almost be said to be a common one. Radio- 

 activity was first noticed by M. H. Becquerel, who, stimu- 

 lated, perhaps, by Rontgen's brilliant discovery of tie 

 X rays, was looking, in 1S96, for yet other new radiations. 



The accident was as follows : There is a salt known as 

 potassium and uranium sulphate which, when it is exposed 

 to sunlight, becomes for a moment self-luminous, and 

 Becquerel was studying this phenomenon photographic- 

 ally. 



His experiment consisted in placing crystals of the 

 salt above photographic plates well protected from light 

 by means of black paper, and then exposing the salt, 

 which was outside the black paper, to the direct light of 

 the sun. When he did this it became evident that some 

 radiation or some emanation was produced which could 

 penetrate the paper, for the photographic film immediately 

 below the salt was so acted upon that when the plate 

 was developed he obtained a silhouette of the crystals 

 more or less like that shown in fig. i, though this par- 

 ticular silhouette was given by a little radium bromide, 

 and not by the uranium salt studied by Becquerel. 



Fig I. — Silliouetti given iv Radium, 



One day, just as everything was ready for an experi- 

 ment, clouds covered the sun, and Becquerel put away 



his plates with the crystals upon them, thinking them 

 spoilt. Several days afterwards liedevcloped the plates and 

 found to his surprise that thesiliioucttes were particularly 

 strong ones. lie found, in short, that the sun was not needed 

 to stimulate the salt; that this latter, without any such 

 stimulant, radiated or emitted something which was able 

 to penetrate black paper and act likr light on a sensitive 

 photographic plate. Me found as the result of further 

 experiments that this was no temporary quality of the 

 salt. It persisted for days and months, and, as he dis- 

 co\-cred subsequently, even for years. I'"urther, the same 

 power was possessed by other uranium salts and by the 

 metal uranium. Anyone who can take photographs can 

 verify all this for himself quite easily. 



The radiations thus discovered by Becquerel are called 

 ■'Becquerel Rays." They resemble the Riintgen rays in 

 many respects, and at one time were regarded as due to 

 Rimtgen rays. Thus, they cause damp dust free air to 

 deposit fog, make air conduct electricity, will pass througii 

 such substances as paper, glass, paraffin, quartz, sulphur, 

 Iceland spar, and thin layers of metal e\en more freely 

 than Rontgen rays, and they cannot be reflected, re- 

 fracted, nor polarized like the waves of which ordinary 

 light is composed. It was found, further, that they are 

 not homogeneous, but consist of several different 

 radiations which can be filtered off from each other as it 

 were, and can then be distinguished by their separate 

 characteristics. 



Bodies which emit these remarkable radiations are 

 said to be " radio-active." As we shall see presently, 

 other metals besides uranium are radio-active, and also 

 the waters of some springs, as, for example, the waters 

 at Bath, and even solid earth. 



Becquerel's great disco\-ery soon proved prolific. It 

 suggested to Madame Sklodowska-Curie the idea that 

 the great radio-activity of specimens of pitchblende, 

 which exceeded that of the uranium present in them, nuist 

 be due to special constituents, and so in her hands and 

 those of others led to the discovery of polonium, radium, 

 and actinium. And the remarkable properties of these 

 new substances in their turn have started new ideas or 

 revived old ones in several departments of science. 



Madame and Monsieur Curie and their colleague, M. 

 Bemont, discovered polonium and radium, and INI . Debierne 

 was the discoverer of a third substance of the same class, 

 actinium. The method of working was to separate the 

 components of the pitchblende, which is a very complex 

 mineral, and to study the radio-active power of each con- 

 stituent. The results obtained with the bismuth and 

 barium from this mineral arrested attention, and presently 

 it was found that the former was associated with the sub- 



