246 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 193 



erties such as the composition of the himinous rays they are capable 

 of emitting under definite conditions, for instance, when they are 

 heated to incandescence. 



There is one point which we must note. In order to develop in 

 a satisfactory manner this theory of an atom similar to a solar sys- 

 tem, Bohr had to introduce a strange idea which he united with the 

 theory of quanta earlier developed by Planck. I have just indi- 

 cated that in the experiments where we can follow the motion of an 

 electron, the latter behaves like a small corpuscle of very small mass 

 and that its motion can be predicted by the laws of classical me- 

 chanics. But let us take into consideration the movements of an 

 electron in trajectories of very small dimensions — motions which we 

 can not follow in detail in experiments but which we needs must 

 imagine in developing certain theories and explaining certain facts. 

 Planck found by his calculations that such movements could not be 

 predicted only from the laws of classical mechanics. To be more defi- 

 nite, among all the motions predictable in ordinar}^ mechanics, only 

 certain ones could be followed by the electron ; these privileged ones 

 having been designated " quantized orbits." In his theory of a solar- 

 modeled system, Bohr found it necessary to add this idea of Planck, 

 for in the atoms the trajectories of the planetary electrons are 

 necessarily of very small dimensions. He found that the electrons 

 could execute quantized orbits only; in a manner this circumstance 

 gave the key to all the atomic properties. 



Let us summarize briefly what we have said. The study of the 

 properties of matter has led physicists to think of it as formed of 

 little corpuscles — electrons and protons. Various assemblages of 

 these corpuscles form the atoms of the 92 simple elements; from 

 these are formed the composite bodies; in turn, from the latter is 

 made all matter, which therefore is composed of corpuscles of known 

 mass and charge and which in their movements obey the laws of 

 classical mechanics. 



However, when we consider movements taking place on a very 

 small scale, the corpuscles seem incapable of making most of the 

 movements which the laws of classical mechanics would authorize. 

 They can take only certain quantized paths. This odd circumstance 

 warns us that it is doubtless insufficient to consider the ultimate ele- 

 ments of matter, the electrons for example, as being merely small 

 corpuscles. This manner of considering the electrons must be modi- 

 fied and completed in a way suggested by certain light phenomena. 

 Accordingly we must turn our thoughts to some phenomena 

 concerning light. 



Light which comes to our eyes from the sun or the stars has 

 traversed immense space where matter is absent. Light passes with- 

 out difficulty through a vacuum and, differently from sound, is not 



