202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



electrons (hydrogen nuclei) and negative electrons, and that atoms 

 are purely electrical structures. 



There can be little doubt that conditions have existed in the past 

 in which these electrons have combined to form the atoms' of the 

 elements, and it may be quite possible under the very intense elec- 

 trical disturbances which may exist in hot stars that the process 

 of combination and dissociation of atoms still continues. 



In these lectures I have tried to give an idea of some modern 

 views of the structure of the atoms and of the great variety of 

 new and pov.^erful methods which have been applied to the attack 

 of this problem in recent years. We have seen that a heavy atom 

 is undoubtedly a complex electrical system consisting of positively 

 and negatively charged particles in rapid motion. The general 

 evidence indicates that each atom contains at its center a massive 

 charged nucleus or core of very small dimensions surrounded by a 

 cluster of electrons, probably in rapid motion, which extend for 

 distances from the center very great compared with the diameter 

 of the nucleus. Such a view affords a reasonable and simple ex- 

 planation of many important facts obtained in recent years, but so 

 far only a beginning has been made in the attack on the detailed 

 structure of atoms — that fundamental problem which lies at the 

 basis of physics and chemistry. 



