July 23, 1920] 



SCIENCE 



69 



variety of tlie human race, but that one 

 Chinaman was just like another. Analogy 

 would lead us to doubt whether all the mole- 

 cules even of water are alike. Could they be 

 examined individually and in detail, marked 

 differences would probably be found. 



The case is similar with respect to the. 

 atoms, although the number of varieties of 

 atoms seems to be limited while the number 

 of varieties of molecules does not. Our in- 

 formation with respect to atoms is largely 

 statistical. But even so, the chemists are 

 recognizing the isotopes of the various ele- 

 ments, and certainly two varieties of lead 

 are now known where previously we had but 

 one; illustrating beautifully the principle 

 that differences and individuality tend to 

 grow with increasing acquaintance. 



When we descend one more step in the 

 scale of the physical imits and reach the 

 electrons, we are so remote from our own 

 position in the scale and oiu- acquaintance 

 with these units is so far from being intimate 

 that it is not surprising that we regard all 

 positive electrons as being alike, and all 

 negative electrons as being alike. We seem 

 to have reached that ultimate simplicity for 

 which the mind is always seeking. Nor is 

 our information with respect to the electron 

 entirely statistical, for Millikan has performed 

 the amazing feat of measuring their electrical 

 charges one at a time, and finds that in this 

 resi)ect they actually are measurably alike. 

 So far then as we think of the electron as 

 possessing the single property of the electrical 

 charge we are justified in assuming that they 

 are all alike. The human mind, however, is 

 incurably si)eculative, and few of us, I fancy, 

 would be willing to admit that this is their 

 only property, or that the electrons really are 

 all identical, or that the electron is not still 

 further resolvable into smaller units. 



Since the beginning of the present century 

 the physicists have been very busy with the 

 atom. The phenomena of radioactivity and 

 of the S-rays have led them along a brilliantly 

 lighted path in their exploration of its in- 

 terior, and they have supplied us with verbal 

 pictiu-es of considerable clearness. The elec- 



trical charge of a positive electron is numer- 

 ically equal to the electi-ical charge of a nega- 

 tive electron, but its mass is nearly two thou- 

 sand times greater while its diameter is only 

 one two-thousandths as great. If we could 

 apply the ordinary notions of density to these 

 statements we should have to say that the 

 density of a positive electron is ten million 

 million times the density of a negative 

 electron, although its electrical charge is 

 equal. But the ordinary notions of density 

 perhaps do not apply. 



If we accept the picture that a hydrogen 

 atom consists of a negative electron moving 

 in a circular orbit about a positive electron, 

 we have so far as relative sizes and distances 

 are concerned a veritable planetary system, 

 except that the diameter of the satellite is 

 two thousand times the diameter of the pri- 

 mary, for their distances apart are relatively 

 as great as between the sun and I^eptune. 

 The nucleus of a helium atom has two free 

 positive electrical charges and two negative 

 satellites; lithium has three, and so on; there 

 is a chemical element for each integral mul- 

 tiple up to 92 which belongs to the element 

 uranium, with perhaps a half dozen gaps in 

 the entire series; and furthermore, there is 

 no chemical element which does not fit into 

 the series. We have therefore a complete 

 ordering of the chemical elements upon a 

 purely numerical basis, which makes intelli- 

 gible the periodic law of these elements which 

 has been long known by the chemists on the 

 basis of their chemical properties. 



Notwithstanding the brilliant achievements 

 of the physicists in their work with the atom 

 their analysis is by no means completed. 

 Many fascinating questions remain to be 

 answered. Por example, are all of the ele- 

 ments merely hydrogen atoms locked together 

 in a very tight embrace, and if so will a suffi- 

 ciently violent bombardment separate them? 

 Rutherford's success in obtaining hydrogen 

 from nitrogen by a bombardment with a-par- 

 ticles is certainly suggestive. If the answer 

 is to be in the affirmative, what is the nature 

 of this embrace? How do the electrons, 

 positive and negative, arrange themselves? 



