234 ANNTTAL REPORT SMITHSONIAN INSTITUTION, 19 30 



all these abstractions in describing natural phenomena. The prog- 

 ress of physical chemistry was probably set back many years by the 

 failure of the chemists to take full advantage of the atomic theory 

 in describing the phenomena that they observed. The rejection of 

 the atomic theory for this purpose was, I believe, based primarily 

 upon a mistaken attempt to describe nature in some absolute manner. 

 That is, it was thought that such concepts as energy, entropy, tem- 

 perature, chemical potential, etc., represented something far more 

 nearly absolute in character than the concept of atoms and molecules, 

 so that nature should preferably be described in terms of the former 

 rather than the latter. We must now recognize, however, that all 

 of these concepts are human inventions and have no absolute inde- 

 pendent existence in nature. Our choice, therefore, can not lie be- 

 tween fact and hypothesis, but only between two concepts (or 

 between two models) which enable us to give a better or worse 

 description of natural phenomena. By better or worse we mean, 

 approximately, simpler or more complicated, more or less convenient, 

 more or less general. If we compare Ostwald's attempts to teach 

 chemistry without the use of the atomic theory with a good modern 

 course based upon the atomic theory, we get an understanding of 

 what should be meant by better or worse. 



The more recent advances in atomic theory which have resulted 

 from the development of the quantum theory and which have given 

 us our present knowledge of atomic structure, afford us interesting 

 applications of the new methods of thought, first introduced into 

 physics and chemistry by the relativity theory. 



The older atomic and molecular theories of the chemists took on 

 more definite form through the development of the kinetic theory 

 of gases, and through the electron theory and the study of radio- 

 activity developed to a point where the atom is conceived of as con- 

 sisting of a definite number of electrons revolving around the nucleus. 

 The atom ceased to be indestructible and was no longer the smallest 

 particle of matter which could take part in a chemical reaction. 

 The nucleus, rather than the atom, became characteristic of the chem- 

 ical elements. The chemical properties of the atom, however, de- 

 pended upon the number and arrangement of electrons. 



Bohr, in 1913, developed a marvelous new theory of the atom by 

 combining Planck's quantum theory with a relative theory of the 

 nuclear atom. He evolved several new quantitative mathematical 

 relationships with new concepts such as energy levels, quantum states, 

 etc., and showed how the spectra of elements could be explained 

 in terms of these new concepts. He also gave a mechanical model 

 consisting of electrons revolving in orbits about the nucleus accord- 

 ing to laws which were partly classical and partly inconsistent with 



