THE CONSTITUTION OF MATTER 115 



changes of energy in chemical reactions without the assumption of any 

 definite theory of the constitution of matter. This tendency advanced 

 so far that text-hooks of chemistry were written in which the word 

 atom or molecule was taboo, and chemistry was based instead on the 

 law of combination in multiple proportion. At that time, it did un- 

 doubtedly appear that there was little, if any, hope of finding a concrete 

 proof of the validity of the atomic hypothesis, or of detecting by its 

 effects a single atom of matter or a single electron, for it was known 

 that the smallest fragment of matter visible under a high power micro- 

 scope must still contain many millions, or even billions, of atoms. 



The march of science has, however, been so rapid in this direction 

 that we have been able in recent years to show in a definite and con- 

 crete way the independent existence of atoms and also of electrons in 

 rapid motion. 



Counting Atoms and Electrons 



We shall first of all consider the method devised by Rutherford 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 substance are un- 

 stable 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 investigations 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 emanation has decayed to about one 

 quarter of its original value. The glass walls of the tube are made so 

 thin — about 1/100 millimeter — that the alpha rays are able to escape 

 freely into the surrounding ^ir. They produce a small phosphor- 

 escence in the walls of the glass tube which is just visible in the dark- 

 ened room. On bringing near, however, a screen covered with zinc 

 sulphide, a brilliant phosphorescence is observed which increases in in- 

 tensity 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 

 translucent and emit a ruddy light. This phosphorescence of zinc sul- 

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

 present emanation tube about 5,000,000,000 of these particles are pro- 

 jected 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 



