7 6 2 D O C . 4 8 1 P H Y S I C S A N D T H E E S S E N C E O F T H I N G S combinations of whole numbers according to a certain scheme, led to the idea that solid matter is composed of equal particles (molecules), which, when found in a crystalline state, organize themselves in regular networks. These were the first dis- coveries, indirectly acquired, of the fine structure of matter. The size of atoms, which was deduced for the first time from the kinetic theory of heat, was as yet completely unknown.[2] When, in the mid-nineteenth century, the validity of the energy conservation principle, and in particular the quantitative transformability of mechanical work into heat, were recognized, the idea that this must be understood as an irregular mo- tion of molecules also gained ground. A century earlier some mathematicians had tried to explain the pressure of gases against the walls of containers, claiming that the gas molecules moved like swarms of mosquitoes, colliding with the walls of the container. But this kinetic theory of gases only came into its own a century later, when empirical knowledge of the phenomena of heat became more advanced. The theory provided not only a numerical relation between pressure and heat content, between the conductivity of heat and viscosity (resistance that opposes mobility), as well as an explanation of the strange phenomenon that the resistance and heat conductivity of a gas are independent of its density—facts that were confirmed af- ter they were predicted by this theory—but also allowed us to know the real size and mass of molecules and atoms. This is, to my mind, the greatest triumph of the mechanics of Galileo and Newton. Using what the senses perceive as its point of departure, it allows us to obtain deep, irrefutable knowledge of the fine structure of matter that will remain forever hidden to the crude capabilities of human senses. Theoretical physics based on the mechanics of Galileo and Newton will, in a cer- tain sense, endure forever, which does not mean that it is the final stage of our knowledge. From the beginning it was difficult to create a usable theory of light that would explain all experiments. Newton’s corpuscular theory did not provide an explanation of the phenomena of refraction and the interference of light. Huygens’s wave theory—far superior because of its explanatory results—had the disadvan- tage of having to depend on the postulate of a kind of matter, inaccessible to the senses, the ether, which should have been mechanically analogous to the solid ob- jects perceivable by the senses and which, at the same time, can be penetrated by those senses. Further, electricity was a serious stumbling block for the mechanical concept of the universe. Electricity should also have been a kind of matter, but the laws that appeared to rule its reciprocal effects did not fit into Newton’s scheme. The frame that held Newton’s system together was broken by Faraday and Max- well’s theory of electricity and its experimental confirmation by H. Hertz.[3] Mass and its long-distance effects could no longer be considered fundamental realities of physics.