CONSTITUTION OF MATTER — EXJTHERFOED. 177 



COUNTING ATOMS AND ELECTRONS. 



We shall, first of all, consider the method devised by Eiitherford 

 and Geiger for detecting and recording the effects of single alpha 

 particles from radium. At this stage it is unnecessary to enter into 

 details of the nature of the transformations occurring in radioactive 

 matter. It suffices to say here that the atoms of a radioactive sub- 

 stance are unstable and occasionally break up with explosive violence. 

 In many cases the explosion is accompanied by the ejection of a 

 charged body, called the alpha particle, with a velocity of about 10,000 

 miles a second. These alpha particles are known, from other investi- 

 gations, to consist of charged atoms of the rare gas, helium. The 

 presence of these rays is simply shown by the marked phosphorescence 

 they set up in certain substances. I have here a fine glass tube, which 

 was filled about a week ago in Manchester with purified emanation 

 released from about one-fifth of a gram of pure radium. In the 

 interval of its journey across the Atlantic the activity of the emana- 

 tion has decayed to about one-quarter of its original value. The 

 glass walls of the tube are made so thin (about one one-hundredth 

 millimeter) that the alpha rays are able to escape freely into the 

 surrounding air. They produce a small phosphorescence in the walls 

 of the glass tube, which is just visible in the darkened room. On 

 bringing near, however, a screen covered with zinc sulphide, a bril- 

 liant phosphorescence is observed, which increases in intensity as we 

 approach the tube. Similar effects are seen to be produced in this 

 crystal of willemite, while the crystal of kunzite is seen to be trans- 

 lucent and emit a ruddy light. This phosphorescence of zinc sulphide 

 and willemite is due mainly to the alpha rays, and from the present 

 emanation tube about 5,000,000,000 of these particles are projected 

 each second. 



In their passage through air or other gas the alpha particles pro- 

 duce from the neutral molecules a large number of negatively 

 charged particles called ions. The ionization due to the alpha par- 

 ticles can be readily measured by electrical methods, and it can be 

 shown that the effect to be expected from a single alpha particle is 

 much too small to detect except by very refined methods. In order 

 to overcome this difficulty Rutherford and Geiger employed a 

 method of magnifying automatically several thousand times the elec- 

 tric effect due to an alpha particle. The general arrangement of 

 the original apparatus is seen in figure 2. 



A few of the alpha rays from a radioactive substance passed 

 along an exhausted tube E through an opening D covered with thin 

 mica into the detecting tube A B. The latter contained a central 

 insulated electrode B connected with an electrometer, and the pres- 



18618°— SM 1915 12 



