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TEE POPULAR SCIENCE MONTHLY 



cules can be directly inferred from those 

 of the Brownian particles. 



Much more spectacular is the evidence 

 afforded by the cathode rays developed 

 in Crooke's tubes. In these tubes there 

 is emitted from the cathode a stream of 

 luminosity which has very remarkable 

 properties especially that of being de- 

 flected by a magnet. This shows that 

 the luminous ray is composed of mater- 

 ial particles in motion and charged with 

 electricity. Just to what extent these 

 particles consist of ordinary matter and 

 in how far merely of the electric charge 

 is more or less problematical, but many 

 physicists consider the moving entities 

 as atoms of electricity and this also ap- 

 pears to be Sir Ernest Eutherford's 

 view. These particles are now usually 

 called electrons and they have been 

 identified with the so-called beta rays 

 emitted by radioactive products. 



Eadium results from the degenera- 

 tion of uranium, though there are inter- 

 mediate products, and radium itself 

 likewise gives rise to a series of radioac- 

 tive products differing from one 

 another. In each of these cases of de- 

 generation, the process is similar. 

 Radium decomposes with the emission of 

 two sorts of rays called the alpha rays 

 and the beta rays. The alpha rays are 

 neither more nor less than atoms of the 

 gas helium, long since known to exist in 

 the sum by its spectrum, and more re- 

 cently detected in a uranium ore. The 

 beta particles are identical with the 

 electrons which form the cathode rays. 

 The alpha particles are expelled from 

 the radium at a tremendous velocity, but 

 this is far exceeded by the velocity of 

 the beta Tays. Sir Ernest Eutherford 

 and his colleagues in radiological inves- 

 tigation have succeeded not only in de- 

 termining the identity of the alpha par- 

 ticles with helium, but also in establish- 

 ing the relative size of the electrons and 

 the atoms of helium. The mass of the 

 beta particles is only about one seven- 

 thousandth part the size of the helium 

 atom, and most of the heating effect of 

 radium is due to the energy of the 

 larger alpha particles. 



So far has the analysis of these 

 products progressed, and so delicate is 

 the apparatus devised for the study, 

 that Eutherford and Geiger have actu- 

 ally succeeded in making either alpha 

 or beta particles one by one give rise 

 to electrical discharges and light in 

 such a way that the number of either 

 kind of particles emitted per second 

 from a given mass of radioactive mat- 

 ter can be counted. The most efficient 

 apparatus for this purpose is called the 

 string electrometer, so designed as to 

 give a record consisting of small notches 

 on a continuous line. It is like the 

 record of a chronograph and in fact the 

 instrument may be considered as a chron- 

 ograph. Of this record the notches pro- 

 duced by the alpha particles greatly ex- 

 ceed in depth those given by the beta 

 particles, and thus the rate at which 

 each is given off can be studied with 

 the utmost accuracy on a permanent 

 record. 



In a popular description of this kind 

 it is difficult to convey an idea of the 

 extraordinary sensitiveness of the ap- 

 paratus devised, and none at all of the 

 genius which was requisite to its devel- 

 opment, but perhaps enough has been 

 said to show that the most carefully 

 hidden secrets of the ultra-microscopic 

 structure of matter are now subject to 

 scrutiny, and that before long many of 

 its features will be fairly well under- 

 stood. Sir Ernest concluded his lec- 

 ture by an illustration of the number 

 of atoms contained in a cubic centi- 

 meter of helium. It was something like 

 this. If one hundred million people 

 were to undertake to count these atoms, 

 each person enumerating four per sec- 

 ond day and night, the tale would be 

 complete in a couple of thousand years. 



THE STRUCTURE OF THE ATOM 



Sir Ernest Eutherford's second 

 lecture dealt with the problems of the 

 structure of the atom and the bear- 

 ings of recent researches on this subject. 

 The lecture was most eloquent and left 

 the audience in a condition of the great- 

 est enthusiasm which they testified by 



