152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



to note that the methods developed long ago for the investigation 

 of the radioactive bodies proper are now every day being applied 

 to study the artificial transformation of elements and to follow the 

 chemical changes involved. I have personally followed with great 

 interest this ever-widening extension of the province of radioactivity, 

 which today embraces so many workers and has already given us a 

 new science in which the reactions occurring in the minute nucleus 

 of an atom can be studied. The opening up of this new territory 

 has only been made possible by the development of new and powerful 

 electric methods of producing intense streams of bombarding particles 

 with high speeds, and by the improvement of the automatic methods 

 of counting swift particles, and by the wide use of that wonderful 

 instrument, the Wilson expansion chamber, to obtain visual evidence 

 of the process of transformation. 



Before discussing the changes in our ideas due to the study of 

 radioactive transformations, we may pause for a moment to consider 

 the prevailing ideas on atoms and their structure just before the 

 discovery of radioactivity (1896) and the proof of the independent 

 existence of the electron (1897). Tlie atomic theory of Dalton had 

 been almost universally accepted as the basis of the interpretation 

 of the facts of chemistry. The work of the chemist for nearly a 

 century had resolved our material world into 80 or more distinct 

 types of atoms or elements, and had shown that the atoms of the 

 elements were stable entities unchangeable by the chemical and 

 physical forces then at our disposal. With increase of knowledge, 

 the old ideas of the alchemists of the transmutation of the elements 

 had been discarded, although it was recognized that one of the main 

 problems of chemistry was to disclose the true relation of the elements 

 and if possible to devise more potent methods capable of changing 

 one element into another. This was well expressed by Faraday, "to 

 decompose the metals, then, to reform them, to change them from 

 one to another, and to realize the once absurd notion of transmuta- 

 tion, are the problems now given to the chemist for solution." To 

 the philosophic mind, the periodic law of Mendeleef was of great 

 significance in indicating that the atoms of the elements were not 

 separate creations but closely related in their ultimate structure, but 

 there was at that time no clue to the underlying meaning of this 

 remarkable relation. It should be recalled that the periodic classifi- 

 cation had been successful in predicting the properties of missing 

 elements, and, indeed, the position of several elements discovered later, 

 after Moseley's generalization, had been indicated correctly. 



While the law of combining proportions did not involve any defi- 

 nite knowledge of the size and structure of atoms, yet the size and 

 weight of the individual atoms had been roughly estimated from 



