Septembeb 3, 1909] 



SCIENCE 



301 



law of distribution of the a particles about 

 the average value. 



Quite apart from the importance of 

 studjang radioactive changes, the radia- 

 tions from active bodies provide very val- 

 uable information as to the effects pro- 

 duced by high velocity particles in 

 traversing matter. The three types of 

 radiation, the a, p and y rays, emitted from 

 active bodies, differ widely in character 

 and their power of penetration of matter. 

 The a particles, for example, are completely 

 stopped by a sheet of notepaper, while the 

 y rays from radium can be easily detected 

 after traversing twenty centimeters of 

 lead. The differences in the character of 

 the absorption of the radiations are no 

 doubt partly due to the difference in type 

 of the radiation and partly due to the dif- 

 ferences of velocity. 



The character of the effects produced by 

 the a and p particles is most simply studied 

 in gases. The a particle has such great 

 energy of motion that it plunges through 

 the molecules of the gas in its path, and 

 leaves in its train more than a hundred 

 thousand ionized or dissociated molecules. 

 After traversing a certain distance, the a 

 particle suddenly loses its characteristic 

 properties and vanishes from the ken of 

 our observational methods. It no doubt 

 quickly loses its high velocity, and after its 

 charge has been neutralized becomes a 

 wandering atom of helium. The ionization 

 produced by the a particle appears to con- 

 sist of the liberation of one or more slow 

 velocity electrons from the molecule, but in 

 the case of complex gases there is no doubt 

 that the act of ionization is accompanied 

 by a chemical dissociation of the molecule 

 itself, although it is difficult to decide 

 whether this dissociation is a primary or 

 secondary effect. The chemical dissocia- 

 tion produced by a particles opens up a 

 wide field of investigation, on which, so far, 

 only a beginning has been made. 



The p particle differs from the a particle 

 in its much greater power of penetration 

 of matter, and the very small number of 

 molecules it ionizes compared with the o 

 particle traversing the same path in the 

 gas. It is very easily deflected from its 

 path by encounters with the gas molecules, 

 and there is strong evidence that, unlike 

 the a particle, the p particle can be stopped 

 or entrapped by a molecule when traveling 

 at a very high speed. 



When the great energy of motion of the 

 a particle and the small amount of energy 

 absorbed in ionizing a single molecule are 

 taken into consideration, there appears to 

 be no doubt that the a particle, as Bragg 

 pointed out, actually passes through the 

 atom, or rather the sphere of action of the 

 atom which lies in its path. There is, so to 

 speak, no time for the atom to get out of 

 the way of the swiftly moving a particle, 

 but the latter must pass through the atomie 

 system. On this view, the old dictum, no 

 doubt true in most cases, that two bodies; 

 can not occupy the same space, no longer- 

 holds for atoms of matter if moving at a 

 sufficiently high speed. 



There would appear to be little doubt 

 that a careful study of the effects produced' 

 by the a or p particle in passing through 

 matter will ultimately throw much further 

 light on the constitution of the atom itself. 

 Work already done shows that the char- 

 acter of the absorption of the radiations is 

 intimately connected with the atomie 

 weights of the elements and their position 

 in the periodic table. One of the most 

 striking eft'eets of the passage of j8 rays 

 through matter is the scattering of the j8" 

 particles, i. e., the deflection from their- 

 rectilinear path by their encounters -with 

 the molecules. It was for some time- 

 thought that such a scattering could not 

 be expected to occur in the case of the a 

 particles in consequence of their mucb 

 greater mass and energy of motion. The 



