The first section concerns bodies as they appear to us and as we perceive them directly by means of our eyes. By further research we have learnt that these bodies are built out of very small particles which we call atoms. Even though these atoms cannot be seen directly with the eye, yet we still have quite certain proofs of their existence and of many of their properties. The whole of our knowledge concerning the atomic structure of matter has been discussed in the second section. The work of the last few decades has allowed us to penetrate still further into the inner structure of matter. The structure of the atom itself is examined in the third section. Finally, there exists a whole series of phenomena, which have been explained on the assumption that, in addition to ponderable matters, there is something still else which fills all interstellar space, a medium which is called the aether. The phenomena which take place in this medium are discussed in the last section of the book.