384 A CENTURY OF SCIENCE 



cation of the energy of vibration which is permitted to 

 each oscillator. Here both emission and absorption fol- 

 low the classical theory, but the motion of an emitting 

 and absorbing linear oscillator of frequency v is supposed 

 to be stable only for those amplitudes for which the energy 

 of its oscillations is an integral multiple of hv. In order 

 to maintain the energy at these particular values, the 

 oscillator may draw energy from, or deposit surplus 

 energy with, other degrees of freedom which partake 

 neither in emission nor absorption, but act merely as 

 storehouses. 



Photoelectric Effect. When investigating the produc- 

 tion of electromagnetic waves, Hertz had noticed that a 

 spark passed more readily between the terminals of his 

 oscillator when the negative electrode was illuminated by 

 light from another spark. Further investigation by 

 Hallwachs, Elster and Geitel, and others showed that this 

 effect was due to the emission of electrons by a metal 

 exposed to the influence of ultra-violet light. Lenard 

 discovered that the energy with which a negatively 

 charged particle is ejected is entirely independent of the 

 intensity of the light, and further investigation showed 

 it to depend only on the frequency. Einstein suggested 

 that the electrons appearing in this so-called photo-elec- 

 tric effect start from within the metal with an initial 

 energy hv. In passing through the surface a resistance 

 is encountered, however, so he concluded that the energy 

 with which the fastest moving electrons appear outside 

 the metal should be equal to hv less the work done in 

 overcoming this resistance. Recent experiments not 

 only confirm this relation, but provide a most satisfac- 

 tory method of determining the value of h. Millikan 1 * 

 finds it to be 6-57 (10)~ 27 ergs sec., which gives the quan- 

 tum for yellow light a value sixty times as great as the 

 heat energy of a monatomic gas molecule at 0C. That 

 this large amount of energy can be transferred from the 

 incident light to the ejected electron is quite out of the 

 question; it must come from within the atom. In this 

 way some indication is obtained of how vast intra-atomic 

 energies must be. 



Structure of the Atom. The generally accepted model 

 of the atom is that due chiefly to Rutherford. 17 He con- 



