Ixviii PROCEEDINGS. 



Tn the case of radium two kinds of particles are projected 

 with great velocity,, a particles of atomic dimensions, and /3 

 particles of the same order of size as the corpuscles detected by 

 Sir Joseph Thomson, or a thousand times smaller than an atom 

 of hydrogen. The question at once arises, how is the detection of 

 such minute masses of matter possible? The answer is that these 

 particles are electrically charged and that the possibility of follow- 

 ing their movements is thereby almost infinitely increased. Sir 

 Joseph Thomson in his recent presidential address to the British 

 Association at Winnipeg gave a very striking illustration of this 

 fact. The smallest quantity of unelectrified matter ever detected 

 is, probably, a trace of neon amounting to half a millionth of a 

 cubic centimeter. This small quantity would contain about ten 

 million million molecules. "Now", to quote the words of the 

 address, "the population of the earth is estimated at about fifteen 

 hundred millions, so that the smallest number of molecules of 

 neon we can identify is about 7000 times the population of the 

 earth. In other words, if we had no better test for the existence 

 of a man than we have for that of an unelectrified molecule we 

 should come to the conclusion that the earth is uninhabited." 

 On the other hand when molecules are electrified the presence of 

 only three or four of them in a cubic centimeter can be detected. 

 It was by taking advantage of this fact that Rutherford and 

 his co-workers have recently been able to detect by a special 

 electric method the entrance of a single a particle into a vessel 

 prepared for its reception. The detection was effected by the 

 impulse given to an electrometer needle on the entrance of the 

 particle. The experiment was so arranged that each a particle 

 emitted could be counted by continuing the impulses of the needle. 

 The next step was to allow these a particles to be projected in 

 vast numbers into a suitable receiver previously exhausted. It 

 was found that helium gas accumulated in the receiver. Xow it 

 may be shown that, if there are such things as molecules at all, 

 the molecule and atom of helium must be identical, that is, it is 

 what is called a monatomic gas. It follows that the a particles 

 expelled from radium are helium atoms or molecules and hence 

 v, r e have what seems direct experimental proof of the* existence of 

 tVie atom. 



