1908] on Recent Researches in Radio-activity. 35 



stance. On account of their importance in radio-active phenomena, 

 I shall devote some little attention to the behaviour of these rays. 

 The work of Bragg and Kleeman, of Adelaide, first gave us a clear 

 idea of the nature of the absorption of these rays by matter. The a 

 particles from a very thin film of any simple kind of radio-active 

 matter are all projected at an identical speed, and lose their power of 

 ionising the gas or of producing phosphorescence or photographic 

 action after they have traversed exactly the same distance, which may 

 conveniently be called the " range " of the a particle. Now every 

 product emits a particles at an identical speed among themselves, but 

 different from every other product. For example, the swiftest a 

 particles from the radium family, viz. that from radium 0, travels 

 7 cm. in air under ordinary conditions before it is stopped, while that 

 from radium itself is projected at a slower speed, travelling only 

 3 • 5 cm. We may regard the a particle as a projectile travelling so 

 swiftly that it plunges through every molecule in its path, producing 

 positively and negatively charged ions in the process. On an average, 

 an a particle before its career of violence is stopped breaks up about 

 100,000 molecules. So great is the kinetic energy of the a projectile 

 that its collisions with matter do not sensibly deflect it, and in this 

 respect it differs markedly from the ^ particle, which is apparently 

 easily deflected by its passage through matter. At the same time, 

 there is undoubted evidence that the direction of motion of some of 

 the a particles is slightly changed by their passage through matter. 



The sudden cessation of the ionising power produced by the a 

 particle after traversing a definite distance of air has been shown by 

 Bragg to be a powerful method of analysis of the number of a-ray 

 products present in a substance. For example, suppose the amount 

 of ionisation in the gas produced by a narrow pencil of a rays is 

 examined at varying distances from the radium. At a distance of 

 7 cm. there is a sudden increase in the amount of ionisation, for at 

 this distance the a particles from radium C enter the testing vessel. 

 There are again sudden changes in the ionisation at distances of 

 4*8 cm., 4*3 cm., and 3 '5 cm. These are due to the rays from the 

 radium A, the emanation and radium itself respectively entering the 

 testing vessel. The a-ray analysis thus discloses four types of a rays 

 present in radium in equilibrium — a result in conformity with the 

 more direct analysis. This method allows us to settle at once whether 

 more than one a-ray product is present in a given radio-active 

 material. For example, an analysis by Hahn by this method of the 

 radiation from the active deposit of thorium has disclosed the 

 existence of two a-ray products instead of one as previously supposed. 

 We can consequently gain information on the complexity of radio- 

 active material, even though no chemical methods have been found 

 to separate the products concerned. The range of the a particle from 

 each product is a definite constant which is characteristic of each 

 product, 



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