530 



NATURE 



[January 9, 19 13 



selves (Fig'. 3). The paths, it should be explained, are 

 ^hown starting parallel from a common line instead ot 

 radiating from a point. Mr. Wilson's picture shows 

 that I have somewhat exaggerated the deflections to 

 which I wished to direct atten- 

 tion ; but otherwise the agree- 

 ment is satisfactory. 



The results which 

 I would emphasise 

 are these, that an 

 atom of helium can 

 and does sometimes 

 move at a rate com- 

 parable with that of 

 light, and when en- 

 dowed with that 

 speed can penetrate 

 other atoms with 

 ease. 



Before leaving the 

 a ray, there is one 

 other point I should 

 like to mention. If 

 it can be that de- 

 particle are so rare, 

 we find ourselves 

 with Rutherford 

 due to a force 



111 



approaching in some cases to that of light. When it 

 moves so fast it only ionises occasionally, so that its 

 fog track is fainter. On these slides, some 3 ray 

 tracks are clearly shown (Figs, g and 6) ; some are 

 quite straight and are due to rays of high velocity, 

 others show much bending, and these are made by 

 particles which have lost their great speed, and are 



P. 



we consider how 

 flections of the o 

 and yet so sharp, 

 driven to consider 

 that the deflection i 



exerted from a very small centre or 

 central core within the atom, backed 

 by all the mass of the atom. It is 

 only when the flying a particle tries 

 to pass very close to this centre that 

 a noticeable deflection is produced. We 

 may picture to ourselves the electrons 

 belonging to the atom as revolving about this central 

 c ore, which we must then take to be electrically posi- 

 tive, just as the planets move about th? sun. When 

 an a or particle penetrates an 

 atom and is deflected it is the 

 central core that is in the main 

 responsible; electron 

 satellites are of no ac- 

 count. .A. rough analogy 

 is to be found in the 

 motion of a comet 

 through the solar system. 

 When a deflection 

 takes place we may ex- 

 pect a recoil of the atom 

 in which it occurs. In 

 some of the illustrations 

 you will observe that 

 there is a slight enlarge- 

 ment of the track at its 

 beginning (Fig. 4). This 

 may well be the recoil of 

 the radio-active atom 

 from which the a. particle 

 has been ejected. W'e 

 have for some time been 

 familiar with this recoil 

 effect, which has been 

 made the basis of certain 

 important electrical 

 methods of radio-active 

 investigation. It is very 



Fig. 4.— A con.plele or.iy iltfesting tO See a Well- 



(rom radium einanaiiuii. marked little spur On One of the 

 a ray tracks in Fig. 2, just where 

 we should expect to find the effects of an atom ot 

 oxygen or nitrogen recoiling from its effort to turn 

 the helium atom out of its path. 



A & ray does not leave such an obvious track. It 

 is the single electron moving with velocity very closely 

 NO. 2254, VOL. 90] 



nd 3-Rays from i 



knocked hither and thither by collision with the atoms 

 of the air. It is to be remembered that the & particle 

 is many thousands of times lighter than the o particle. 

 Now we come to the third type of rays emitted by 

 the radio-active substances, the 7 ray, which is the 

 same in kind as the Rontgen rav. 



The fog apparatus show's no tracks which can be 

 directly assigned to such rays. When the & and 7 

 rays act together, only 5 ray tracks are found. When 

 a stream of X-rays passes through the chamber the 

 result is such as is shown in the figure (Fig. 7), a mass 

 of short, tortuous tracks originating within the path of 

 the X-rays, and ending indiscriminately inside or out- 



