Philosophical Foundations of Physics - Chapters 23 through 26 of this 1966 work. - http://www.marxists.org/reference/subject/philosophy/works/ge/carnap.htm
Empiricism, Semantics, and Ontology - Paper by Rudolf Carnap, first published in Revue Internationale de Philosophie 4 (1950): 20-40. - http://www.ditext.com/carnap/carnap.html