DO^ 



SCIENCE PROGRESS 



the disintegrating atoms at enormous velocities. Such large 

 particles cannot travel far without colliding with gas particles, 

 hence their small penetrative power. Their enormous momentum 

 produces the correspondingly great ionising effect actually 

 observed. Rutherford has measured the velocity of a particles 

 from different active elements; it varies from 1-56 x 10^ cms. 

 per second in the case of radium to 2-25 x lo^ cms. per second 

 for thorium C. Light travels at about 30 x 10^ cms. per 

 second. The experiments lead to the conclusion that the sub- 

 stances possessing longest life, viz. uranium and radium, emit 

 particles having the least velocity ; the shorter the life the 

 greater the velocity of the emitted a ray. Rutherford has been 

 able to measure the mass of the particle which carries unit 

 charge of electricity ; it is about twice that of hydrogen. From 

 this he concludes that the a particle consists either of a half 

 atom of helium (atomic weight 4) carr3'ing one electric charge, 

 or a whole atom carrying a double charge. The a rays carry a 

 positive charge — their mass and velocity correspond very closely 

 with those of canal ra^'s ; their close resemblance to these must 

 be held as established. 



The /3 ra3's have a greater range of velocity. The maximum 

 velocity approximates to that of light (200,000 miles per second). 

 Their identity with cathode ra3's has been established on three 

 grounds : {a) the negative electric charge carried b}' both ; {b) 

 the ratio of that charge to the mass — extremely small in both 

 cases — which carries it ; and {c) the fact that a magnetic field has 

 precisely the same effect in both cases. 



a and /3 rays are completel}' absorbed by covering the radio- 

 active matter producing them with a la^'er of lead i cm. in 

 thickness. But even if a la3^er of i inch is used, ra3^s still 

 pass through. They carry no charge of electricit3^, and are not 

 deflected b3' a magnetic field, 3'et they ionise gases, and so affect an 

 electrometer. These are the 7 ra3-s, ether waves, corresponding 

 to X-ra3-s, but of even greater penetrating power. 



All three kinds of ra3^s on impinging on solid particles give 

 rise to secondary rays, as do ordinar3- X-ra3-s. The secondary 

 ra3's also travel in straight lines, and otherwise obey the light 

 laws, but are deflected b3^ a magnetic field. The3^ resemble 

 cathode ra3"S in being electrons, but move with comparatively 

 small velocity. Ultra-violet light allowed to fall on the surface 

 of metals produces a similar phenomenon. 



