6i8 SCIENCE PROGRESS 



from 80' to 20 absolute), and plotted them against atomic 

 weight. The atomic heats, ranging from 6*82 for Caesium to 

 0*03 for diamond, then reveal a definite periodic variation 

 resembling the Lothar Meyer curve for atomic volumes in 

 the solid state. If experiments were made between the boiling 

 point of liquid hydrogen and that of liquid helium, the atomic 

 specific heats would probably be all nearly equal and very 

 small. 



Turning to theories on the structure of the atom, Bohr's work 

 claims attention ; it is a mathematical treatment of Rutherford's 

 nucleus atom, which has been very successful. Bohr's atom 

 gives a theoretical account of the line spectra of the elements, 

 especially those of the relatively simple hydrogen and helium 

 atoms. It is interesting as referring the discontinuities of wave- 

 length in the line spectrum of a gas back to the discontinuities 

 of energy emission postulated by the theory of quanta. As a 

 result of experiments on the scattering of a particles by matter 

 Rutherford in 191 1 put forward the theory that the atom 

 consists of a central positive nucleus, of dimensions very small 

 compared to the atomic radius, in which practically all the mass 

 of the atom is concentrated, this nucleus being surrounded by 

 a distribution of electrons making the atom neutral as a whole. 

 The number of electrons was deduced to be about half the 

 atomic weight; it is now considered likely that it is the "atomic 

 number " already mentioned. To get the spectral lines which 

 would be emitted by such an atom Bohr makes the assumption 

 that the electrons are rotating round the nucleus in circular 

 orbits ; there is no energy emitted when the electrons are 

 rotating steadily in a stationary state. An electron can, how- 

 ever, pass from one such stationary state to another, changing 

 the radius of its orbit ; during this transition a homogeneous 

 radiation is emitted, whose frequency v is determined from the 

 change of energy between the two stationary states by the 

 equation E = hv, where h is Planck's constant, and E is the 

 energy change. The amount of energy emitted is thus always 

 a whole quantum, and a further assumption as to the connection 

 between the total energy of formation of the system and the 

 frequency of rotation of the electron in the system formed leads 

 to the conclusion that the angular momentum of any electron 



round the nucleus is an entire multiple of the quantity — ; in 



