This historical monograph (the Russian original 1985 was not reviewed) is an account of the programme of a field synthesis in physics developed in the years from 1905 (advent of special relativity) to the early 1930s (advent of quantum field theory). The programme was restricted to unification of electromagnetism with gravity in terms of classical field theory in a curved spacetime and therefore was gradually abandoned after 1932 in the light of successes of quantum theory. The author presents in detail the unified theories of Nordström, Weyl, Eddington, Kaluza and Klein and numerous theories proposed by Einstein. It is stressed that these theories, contrary to the widely held view, were not completely fruitless. They were precursors of the modern quantum gauge field theories which inherited from them a number of important concepts.
The monograph presents the first complete historical description of the subject. It is addressed to all physicists and mathematicians interested in the history of unification in physics.