DISCOVERY AND tROPE&TtES OF RADIUM i5l 



the end of the rod. The active matter to be tested is placed on 



the plate A, and the rate at which the gold leaf falls is observed 



by means of a telescope having a E 



scale of divisions in the eye-piece. 



The time taken for the leaf to pass 



between two points on the scale is 



noted by a stop-watch, and the 



average rate of movement per minute 



can be determined. The average rate 



of movement per minute is directly 



proportional to the ionization current 



between the two plates A and B, that 



is to the intensity of the radiation 



emitted by the active substance on 



the lower plate. There is a small 



natural leak due to atmospheric ions, 



for which allowance is always made. 



As to the a-rays some doubt was 

 for a long time experienced as to 

 their nature. But physicists were 

 agreed that they consisted of particles 

 of matter charged with electricity and 

 projected with great velocity. The 

 discovery of helium in association 

 with the minerals showing radio- RUTHERFORD'S ELECTROSCOPE. 

 activity and from which radium was extracted, led Professors 

 Rutherford and Soddy to suggest in 1902 that helium might be 

 a product of the disintegration of the radio-elements. Soon 

 after this Sir William Ramsay and Professor Soddy discovered 

 that helium is contained in the gases, oxygen and hydrogen, 

 which are set free on dissolving radium bromide in water. And 

 later it was found that radium bromide heated in a vacuous 

 tube gives off helium. 



The a-rays, therefore, are believed to consist of atoms of 

 helium positively charged and ejected from the atom of radium 

 with a velocity about T V the velocity of light. 



The /3-rays were discovered in consequence of the fact that 

 they are drawn aside by a magnetic field. If an active preparation 

 is placed in a short narrow lead tube it will cause a fluorescent 

 patch to become visible on a screen coated with a platino- 

 cyanide held above it. If the poles of an electro- magnet placed 



