THE CONSTITUTION OF MATTER 139 



There is one element, however, besides lead, about which interesting 

 evidence has been obtained on this point. Sir J. J. Thomson found by 

 examining the deflection of the positively charged particles produced by 

 an electric discharge through the rare gas, neon, that two elements were 

 present of atomic weights about 20 (neon) and 22. Aston was able by 

 diffusion experiments to separate partially the two components of neon 

 and to show that they differed in density, but failed in attempts to 

 separate them by fractional distillation in charcoal cooled by liquid air. 

 Such results are to be anticipated if neon is a mixture of two isotopes, 

 i. e., elements of identical nuclear charges but different atomic weights. 

 It is obvious that this new point of view will result in a systematic 

 examination of the elements to test for the possible presence of isotopes, 

 and thus give an additional reason for the accurate determination of 

 atomic weights for elements obtained from widely different sources. 



Distribution of Electrons in the Atom 



It is seen that the nucleus theory of the atom offers a simple ex- 

 planation of many important facts which have been brought to light in 

 recent years, and for this purpose it has not been necessary to make 

 any special assumptions as to the actual structure of the nucleus, or of 

 the way in which the external electrons are distributed. The investiga- 

 tion of the latter problem is beset with many difficulties; for an elec- 

 tron is attracted towards the nucleus, and even if it is in orbital motion, 

 it must on the electromagnetic theory lose energy by radiation and ul- 

 timately fall into the nucleus. It appears likely that this difficulty is 

 in reality due to our ignorance of the conditions under which an electron 

 radiates energy. According to the views outlined in this lecture, the 

 hydrogen atom has the simplest possible structure, for it consists of a 

 nucleus of one unit charge and one negative electron. The question 

 naturally arises how such a simple structure can give rise to the com- 

 plex spectrum observed for hydrogen. This problem has been attacked 

 in a series of remarkable papers by Bohr, who concludes that the com- 

 plexity of the spectrum is not due to the complexity of the atomic 

 structure but to the variety of modes in which an electron can emit 

 radiation. Suppose, for example, that a hydrogen atom has lost its 

 negative electron. Bohr supposes that an electron falling towards the 

 positively charged nucleus may occupy temporarily any one of a number 

 of stationary positions fixed relatively to the nucleus. In falling from 

 one stationary state to another, radiation is emitted of a definite fre- 

 quency which is connected with the difference of potential energy E of 

 the electron in the two stationary states by E-h where h is Planck's 

 fundamental constant. On this hypothesis, he has been able to account 

 for the series spectra of hydrogen and to deduce directly from the 

 theory the value of Balmer's constant which plays such an important 



