58 SCIENCE PROGRESS 



property of " ionising," or rendering " electrically conducting M 

 any gas through which they pass. The effect is produced by 

 the shattering of some of the gas molecules by the radium rays : 

 the extent of the ionisation may be measured by means of an 

 electroscope. 



It was soon found that, of the three kinds of rays, one 

 (called the alpha rays) was readily absorbed by a few sheets of 

 paper ; another (the beta rays) was only stopped by a thick 

 sheet of metal such as lead ; while a third (the gamma rays) 

 was still more penetrating. 



In the alpha rays we were introduced to a new type of atoms 

 which possesses two features quite out of the common ; the 

 first, an electric charge (of positive sign) ; the other, an enor- 

 mous speed — some 12,000 miles a second. The alpha rays 

 have also proved to consist of atoms of helium, and their energy 

 is such that their impact against a fluorescent screen is attended 

 by a visible flash of light, as in the spinthariscope of Sir William 

 Crookes. Such experiments provided the first means of count- 

 ing individual atoms. The expulsion of the alpha ray, it should 

 be added in conclusion, is responsible for the recoil of the re- 

 mainder of the radium atom, just as a gun recoils after a shot is 

 fired out of it. 



The beta ray was early identified with the electron to be the 

 tiniest entity of which we have knowledge. It carries a negative 

 charge of electricity and its mass amounts to no more than 

 about 1/1800 of that of the atom of hydrogen. The ray con- 

 sists of a considerable number of groups of a variety of speeds, 

 each group of rays moving with the same speeds. These speeds 

 are inconceivably prodigious, the highest approaching within 

 \ per cent, of that of light — 1 86,000 miles a second. It requires 

 about half an inch of lead to stop particles such as these. 



The third great group of rays — the gamma rays — is identical 

 in kind with X rays, which have lately been shown to be an 

 extreme form of ultra-violet light. They are almost certainly 

 conditioned by the expulsion of the beta rays, and as a con- 

 sequence the radium gives out a " spectrum " of gamma rays, 

 each of the component parts corresponding to one of the group 

 of beta rays. Some gamma rays are less penetrating than X 

 rays ; but others are much more penetrating, and will in fact 

 pass readily through six inches of lead. Against such rays it is 

 almost impossible to obtain protection ; but, happily for 



