116 



SCIENCE 



[N. S. Vol. XL. No. 1021 



a new load on the atom. The conditions 

 which the atoms have to explain may in- 

 deed be written down, but to do so is 

 merely to make a complete index for all 

 books on physics and chemistry in the 

 widest sense. 



3. In the early days of the kinetic theory 

 of gases, now well established in its broad 

 outlines, it was sufSeient to regard the 

 atom as a perfectly elastic sphere, and it is 

 about a generation ago^ that leading sav- 

 ants were triumphantly determining the 

 effective radius as about 10"^ cm. (a con- 

 venient shorthand for the hundred mil- 

 lionth of a centimeter). 



The discovery of electrons as the cathode 

 rays of an electric discharge in an ex- 

 hausted tube, and as the beta rays of 

 radium, opened up new regions.^ It ap- 

 pears that negative electricity consists of 

 electrons with their accompanying but un- 

 explained effects in the ether. Electrons 

 in motion produce magnetic fields. Their 

 effective mass is about one eighteen hun- 

 dredth part of that of a hydrogen atom, 

 and their effective radius one hundred 

 thousandth. The greatest known speed of 

 electrons nearly approaches that of light. 



The Zeeman effect, or separation of a 

 single line in the spectrum by suitable 

 magnetic fields, into two or more lines 

 proved conclusively that the vibrations of 

 negative electrons in the atom are the cause 

 of the disturbances in the ether which we 

 know as light. 



4. The first scheme of an electronic atom, 

 propounded by Sir Joseph Thomson, was 

 a sphere of positive electricity, of unde- 



1 Young proved this in 1805, but his work was 

 forgotten, until Eayleigh called attention to it in 

 1890 (Phil. Mag., XXX., 474). 



2 It is remarkable how little the general public 

 has shared in this advance. In Montreal there were 

 eleven thousand people witnessing a wrestling 

 match while few availed themselves of an invita- 

 tion to meetings and discussions of the Royal So- 

 ciety. 



fined character within which revolved con- 

 centric rings of electrons in the same plane. 

 There necessarily followed the simplicity 

 of circular motion under a force to the 

 center, proportional to the distance between 

 the electron and the center of the atom. 



5. Previous to this Lord Eayleigh had 

 called attention to a serious anomaly. In 

 a train of waves of a periodic character, 

 the electric intensity E varies as the sine 

 of nt, where t is the time and 2iT/n is the 

 period. As the equations involve the sec- 

 ond differential of E, it appears inevitable 

 that the square of n should appear in the 

 law for spectral series. As a matter of 

 fact there appears not the square of n, but 

 n itself. It is desirable to be more explicit. 

 If parallel light from a luminous source 

 passes through a slit and a prism, together 

 with suitable lenses, then the eye or photo- 

 graphic plate can detect a number of 

 bright lines forming the spectral images of 

 the slit for different colors, provided that 

 the light is from luminous mercury vapor 

 or hydrogen, or some such source. Many 

 of these lines have been found to belong to 

 one or more series crowding together to- 

 wards the violet end. Balmer and Rydberg 

 have found that the general type of form- 

 ula for their frequency n is 



' = ^»&-i)' 



where N^ is a universal constant called 

 Rydberg's number, the same in value for 

 all electrons of all atoms; and a and 6 are 

 whole numbers or integers. "We shall refer 

 later to the importance of Eydberg's con- 

 stant and of this magnificent generalization. 

 The trouble to which Eayleigh referred 

 was first faced by Eitz in a startling man- 

 ner. He imagined that there were inside 

 the atom, placed end to end, a number of 

 small magnets with an electron constrained 

 to move in a circular path around the line 

 of magnets. With this hypothesis he was 



