It is just fifty-five years since Walter Bagehot wrote his Physics and Politics, a very suggestive book in its day. He began the first chapter of this book with a reference to “the sudden acquisition of much physical knowledge” which had marked the second half of the nineteenth century, and declared it his purpose to show the bearing of these new ideks upon the political conceptions of mankind. That purpose he, fulfilled with much ingenuity, pointing out the various lines along which the advance in natural science seemed to suggest modifications in the old theories of the state and of government.

This was only a half-century ago; yet the new physics of Bagehot's day has already growh old. Its basic concepts have been turned inside out and upside down. Its laws relating to the indestructibility of mass and the conservation of energy have been radically amended. Even a generation ago the atom was held to be the ultimate and indivisible unit in the composition of the universe. It was the basis upon which the scientists of the nineteenth century built up an inclusive set of laws and principles relating to the structure of all creation. No one had ever seen an atom, but its existence could be postulated and its properties were held to be knowable.